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Borregaard J, Davis EJ, Bentsen GS, Schleier-Smith MH and Sørensen AS (2017), "One- and two-axis squeezing of atomic ensembles in optical cavities", New Journal of Physics., Sep, 2017. Vol. 19(9), pp. 093021.
Abstract: The strong light–matter coupling attainable in optical cavities enables the generation of highly squeezed states of atomic ensembles. It was shown by Sørensen and Mølmer (2002 Phys. Rev. A 66 022314) how an effective one-axis twisting Hamiltonian can be realized in a cavity setup. Here, we extend this work and show how an effective two-axis twisting Hamiltonian can be realized in a similar cavity setup. We compare the two schemes in order to characterize their advantages. In the absence of decoherence, the two-axis Hamiltonian leads to more squeezing than the one-axis Hamiltonian. If limited by decoherence from spontaneous emission and cavity decay, we find roughly the same level of squeezing for the two schemes scaling as ##IMG## [http://ej.iop.org/images/1367-2630/19/9/093021/njpaa8438ieqn1.gif] {NC} where C is the single atom cooperativity and N is the total number of atoms. When compared to an ideal squeezing operation, we find that for specific initial states, a dissipative version of the one-axis scheme attains higher fidelity than the unitary one-axis scheme or the two-axis scheme. However, the unitary one-axis and two-axis schemes perform better for general initial states.
BibTeX:
@article{1367-2630-19-9-093021,
  author = {J Borregaard and E J Davis and G S Bentsen and M H Schleier-Smith and A S Sørensen},
  title = {One- and two-axis squeezing of atomic ensembles in optical cavities},
  journal = {New Journal of Physics},
  year = {2017},
  volume = {19},
  number = {9},
  pages = {093021},
  url = {http://stacks.iop.org/1367-2630/19/i=9/a=093021}
}
Das S, Elfving VE, Faez S and Sørensen AS (2017), "Interfacing Superconducting Qubits and Single Optical Photons Using Molecules in Waveguides", Phys. Rev. Lett.., Apr, 2017. Vol. 118, pp. 140501. American Physical Society.
Abstract: We propose an efficient light-matter interface at optical frequencies between a single photon and a superconducting qubit. The desired interface is based on a hybrid architecture composed of an organic molecule embedded inside an optical waveguide and electrically coupled to a superconducting qubit placed near the outside surface of the waveguide. We show that high fidelity, photon-mediated, entanglement between distant superconducting qubits can be achieved with incident pulses at the single photon level. Such a low light level is highly desirable for achieving a coherent optical interface with superconducting qubit, since it minimizes decoherence arising from the absorption of light.
BibTeX:
@article{PhysRevLett.118.140501,
  author = {Das, Sumanta and Elfving, Vincent E. and Faez, Sanli and Sørensen, Anders S.},
  title = {Interfacing Superconducting Qubits and Single Optical Photons Using Molecules in Waveguides},
  journal = {Phys. Rev. Lett.},
  publisher = {American Physical Society},
  year = {2017},
  volume = {118},
  pages = {140501},
  url = {https://link.aps.org/doi/10.1103/PhysRevLett.118.140501},
  doi = {10.1103/PhysRevLett.118.140501}
}
Borregaard J, Sørensen AS, Cirac JI and Lukin MD (2017), "Efficient quantum computation in a network with probabilistic gates and logical encoding", Phys. Rev. A., Apr, 2017. Vol. 95, pp. 042312. American Physical Society.
Abstract: An approach to efficient quantum computation with probabilistic gates is proposed and analyzed in both a local and nonlocal setting. It combines heralded gates previously studied for atom or atomlike qubits with logical encoding from linear optical quantum computation in order to perform high-fidelity quantum gates across a quantum network. The error-detecting properties of the heralded operations ensure high fidelity while the encoding makes it possible to correct for failed attempts such that deterministic and high-quality gates can be achieved. Importantly, this is robust to photon loss, which is typically the main obstacle to photonic-based quantum information processing. Overall this approach opens a path toward quantum networks with atomic nodes and photonic links.
BibTeX:
@article{PhysRevA.95.042312,
  author = {Borregaard, J. and Sørensen, A. S. and Cirac, J. I. and Lukin, M. D.},
  title = {Efficient quantum computation in a network with probabilistic gates and logical encoding},
  journal = {Phys. Rev. A},
  publisher = {American Physical Society},
  year = {2017},
  volume = {95},
  pages = {042312},
  url = {https://link.aps.org/doi/10.1103/PhysRevA.95.042312},
  doi = {10.1103/PhysRevA.95.042312}
}
Dellantonio L, Das S, Appel J and Sørensen AS (2017), "Multipartite entanglement detection with nonsymmetric probing", Phys. Rev. A., Apr, 2017. Vol. 95, pp. 040301. American Physical Society.
Abstract: We show that spin-squeezing criteria commonly used for entanglement detection can be erroneous if the probe is not symmetric. We then derive a lower bound on squeezing for separable states in spin systems probed asymmetrically. Using this we further develop a procedure that allows us to verify the degree of entanglement of a quantum state in the spin system. Finally, we apply our method for entanglement verification to existing experimental data, and use it to prove the existence of tripartite entanglement in a spin-squeezed atomic ensemble.
BibTeX:
@article{PhysRevA.95.040301,
  author = {Dellantonio, Luca and Das, Sumanta and Appel, Jürgen and Sørensen, Anders S.},
  title = {Multipartite entanglement detection with nonsymmetric probing},
  journal = {Phys. Rev. A},
  publisher = {American Physical Society},
  year = {2017},
  volume = {95},
  pages = {040301},
  url = {https://link.aps.org/doi/10.1103/PhysRevA.95.040301},
  doi = {10.1103/PhysRevA.95.040301}
}
Hu J, Vendeiro Z, Chen W, Zhang H, McConnell R, Sørensen AS and Vuletić V (2017), "Strictly nonclassical behavior of a mesoscopic system", Phys. Rev. A., Mar, 2017. Vol. 95, pp. 030105. American Physical Society.
Abstract: We experimentally demonstrate the strictly nonclassical behavior in a many-atom system using a recently derived criterion [E. Kot et al., Phys. Rev. Lett. 108, 233601 (2012)] that explicitly does not make use of quantum mechanics. We thereby show that the magnetic-moment distribution measured by McConnell et al. [Nature (London) 519, 439 (2015)] in a system with a total mass of 2.6×10^5 atomic mass units is inconsistent with classical physics. Notably, the strictly nonclassical behavior affects an area in phase space 10^3 times larger than the Planck quantum ℏ.
BibTeX:
@article{PhysRevA.95.030105,
  author = {Hu, Jiazhong and Vendeiro, Zachary and Chen, Wenlan and Zhang, Hao and McConnell, Robert and Sørensen, Anders S. and Vuletić, Vladan},
  title = {Strictly nonclassical behavior of a mesoscopic system},
  journal = {Phys. Rev. A},
  publisher = {American Physical Society},
  year = {2017},
  volume = {95},
  pages = {030105},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.95.030105},
  doi = {10.1103/PhysRevA.95.030105}
}
Mahmoodian S, Lodahl P and Sørensen AS (2016), "Quantum Networks with Chiral-Light-Matter Interaction in Waveguides", Phys. Rev. Lett.., Dec, 2016. Vol. 117, pp. 240501. American Physical Society.
Abstract: We propose a scalable architecture for a quantum network based on a simple on-chip photonic circuit that performs loss-tolerant two-qubit measurements. The circuit consists of two quantum emitters positioned in the arms of an on-chip Mach-Zehnder interferometer composed of waveguides with chiral-light–matter interfaces. The efficient chiral-light–matter interaction allows the emitters to perform high-fidelity intranode two-qubit parity measurements within a single chip and to emit photons to generate internode entanglement, without any need for reconfiguration. We show that, by connecting multiple circuits of this kind into a quantum network, it is possible to perform universal quantum computation with heralded two-qubit gate fidelities ℱ∼0.998 achievable in state-of-the-art quantum dot systems.
BibTeX:
@article{PhysRevLett.117.240501,
  author = {Mahmoodian, Sahand and Lodahl, Peter and Sørensen, Anders S.},
  title = {Quantum Networks with Chiral-Light-Matter Interaction in Waveguides},
  journal = {Phys. Rev. Lett.},
  publisher = {American Physical Society},
  year = {2016},
  volume = {117},
  pages = {240501},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.117.240501},
  doi = {10.1103/PhysRevLett.117.240501}
}
Iakoupov I, Ott JR, Chang DE and Sørensen AS (2016), "Dispersion relations for stationary light in one-dimensional atomic ensembles", Phys. Rev. A., Nov, 2016. Vol. 94, pp. 053824. American Physical Society.
Abstract: We investigate the dispersion relations for light coupled to one-dimensional ensembles of atoms with different level schemes. The unifying feature of all the considered setups is that the forward and backward propagating quantum fields are coupled by the applied classical drives such that the group velocity can vanish in an effect known as “stationary light.” We derive the dispersion relations for all the considered schemes, highlighting the important differences between them. Furthermore, we show that additional control of stationary light can be obtained by treating atoms as discrete scatterers and placing them at well-defined positions. For the latter purpose, a multimode transfer matrix theory for light is developed.
BibTeX:
@article{Iakoupov2016,
  author = {Iakoupov, Ivan and Ott, Johan R. and Chang, Darrick E. and Sørensen, Anders S.},
  title = {Dispersion relations for stationary light in one-dimensional atomic ensembles},
  journal = {Phys. Rev. A},
  publisher = {American Physical Society},
  year = {2016},
  volume = {94},
  pages = {053824},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.94.053824},
  doi = {10.1103/PhysRevA.94.053824}
}
Kyriienko O and Sørensen AS (2016), "Continuous-Wave Single-Photon Transistor Based on a Superconducting Circuit", Phys. Rev. Lett.., Sep, 2016. Vol. 117, pp. 140503. American Physical Society.
Abstract: We propose a microwave frequency single-photon transistor which can operate under continuous wave probing and represents an efficient single microwave photon detector. It can be realized using an impedance matched system of a three level artificial ladder-type atom coupled to two microwave cavities connected to input-output waveguides. Using a classical drive on the upper transition, we find parameter space where a single photon control pulse incident on one of the cavities can be fully absorbed into hybridized excited states. This subsequently leads to series of quantum jumps in the upper manifold and the appearance of a photon flux leaving the second cavity through a separate input-output port. The proposal does not require time variation of the probe signals, thus corresponding to a passive version of a single-photon transistor. The resulting device is robust to qubit dephasing processes, possesses low dark count rate for large anharmonicity, and can be readily implemented using current technology.
BibTeX:
@article{Kyriienko2016,
  author = {Kyriienko, Oleksandr and Sørensen, Anders S.},
  title = {Continuous-Wave Single-Photon Transistor Based on a Superconducting Circuit},
  journal = {Phys. Rev. Lett.},
  publisher = {American Physical Society},
  year = {2016},
  volume = {117},
  pages = {140503},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.117.140503},
  doi = {10.1103/PhysRevLett.117.140503}
}
Lin Y, Gaebler JP, Reiter F, Tan TR, Bowler R, Wan Y, Keith A, Knill E, Glancy S, Coakley K, Sørensen AS, Leibfried D and Wineland DJ (2016), "Preparation of Entangled States through Hilbert Space Engineering", Phys. Rev. Lett.., Sep, 2016. Vol. 117, pp. 140502. American Physical Society.
Abstract: We apply laser fields to trapped atomic ions to constrain the quantum dynamics from a simultaneously applied global microwave field to an initial product state and a target entangled state. This approach comes under what has become known in the literature as “quantum Zeno dynamics” and we use it to prepare entangled states of two and three ions. With two trapped Be9+ ions, we obtain Bell state fidelities up to 0.990+2−5; with three ions, a W-state fidelity of 0.910+4−7 is obtained. Compared to other methods of producing entanglement in trapped ions, this procedure can be relatively insensitive to certain imperfections such as fluctuations in laser intensity.
BibTeX:
@article{Lin2016,
  author = {Lin, Y. and Gaebler, J. P. and Reiter, F. and Tan, T. R. and Bowler, R. and Wan, Y. and Keith, A. and Knill, E. and Glancy, S. and Coakley, K. and Sørensen, A. S. and Leibfried, D. and Wineland, D. J.},
  title = {Preparation of Entangled States through Hilbert Space Engineering},
  journal = {Phys. Rev. Lett.},
  publisher = {American Physical Society},
  year = {2016},
  volume = {117},
  pages = {140502},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.117.140502},
  doi = {10.1103/PhysRevLett.117.140502}
}
Sørensen HL, Béguin J-B, Kluge KW, Iakoupov I, Sørensen AS, Müller JH, Polzik ES and Appel J (2016), "Coherent Backscattering of Light Off One-Dimensional Atomic Strings", Phys. Rev. Lett.., Sep, 2016. Vol. 117, pp. 133604. American Physical Society.
Abstract: We present the first experimental realization of coherent Bragg scattering off a one-dimensional system—two strings of atoms strongly coupled to a single photonic mode—realized by trapping atoms in the evanescent field of a tapered optical fiber, which also guides the probe light. We report nearly 12% power reflection from strings containing only about 1000 cesium atoms, an enhancement of 2 orders of magnitude compared to reflection from randomly positioned atoms. This result paves the road towards collective strong coupling in 1D atom-photon systems. Our approach also allows for a straightforward fiber connection between several distant 1D atomic crystals.
BibTeX:
@article{Soerensen2016,
  author = {Sørensen, H. L. and Béguin, J.-B. and Kluge, K. W. and Iakoupov, I. and Sørensen, A. S. and Müller, J. H. and Polzik, E. S. and Appel, J.},
  title = {Coherent Backscattering of Light Off One-Dimensional Atomic Strings},
  journal = {Phys. Rev. Lett.},
  publisher = {American Physical Society},
  year = {2016},
  volume = {117},
  pages = {133604},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.117.133604},
  doi = {10.1103/PhysRevLett.117.133604}
}
Reiter F, Reeb D and Sørensen AS (2016), "Scalable Dissipative Preparation of Many-Body Entanglement", Phys. Rev. Lett.., Jul, 2016. Vol. 117, pp. 040501. American Physical Society.
Abstract: We present a technique for the dissipative preparation of highly entangled multiparticle states of atoms coupled to common oscillator modes. By combining local spontaneous emission with coherent couplings, we engineer many-body dissipation that drives the system from an arbitrary initial state into a Greenberger-Horne-Zeilinger state. We demonstrate that using our technique highly entangled steady states can be prepared efficiently in a time that scales polynomially with the system size. Our protocol assumes generic couplings and will thus enable the dissipative production of multiparticle entanglement in a wide range of physical systems. As an example, we demonstrate the feasibility of our scheme in state-of-the-art trapped-ion systems.
BibTeX:
@article{Reiter2016,
  author = {Reiter, Florentin and Reeb, David and Sørensen, Anders S.},
  title = {Scalable Dissipative Preparation of Many-Body Entanglement},
  journal = {Phys. Rev. Lett.},
  publisher = {American Physical Society},
  year = {2016},
  volume = {117},
  pages = {040501},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.117.040501},
  doi = {10.1103/PhysRevLett.117.040501}
}
Borregaard J, Zugenmaier M, Petersen JM, Shen H, Vasilakis G, Jensen K, Polzik ES and Sorensen AS (2016), "Scalable photonic network architecture based on motional averaging in room temperature gas", Nat Commun., Apr, 2016. Vol. 7 Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved..
Abstract: Quantum interfaces between photons and atomic ensembles have emerged as powerful tools for quantum technologies. Efficient storage and retrieval of single photons requires long-lived collective atomic states, which is typically achieved with immobilized atoms. Thermal atomic vapours, which present a simple and scalable resource, have only been used for continuous variable processing or for discrete variable processing on short timescales where atomic motion is negligible. Here we develop a theory based on motional averaging to enable room temperature discrete variable quantum memories and coherent single-photon sources. We demonstrate the feasibility of this approach to scalable quantum memories with a proof-of-principle experiment with room temperature atoms contained in microcells with spin-protecting coating, placed inside an optical cavity. The experimental conditions correspond to a few photons per pulse and a long coherence time of the forward scattered photons is demonstrated, which is the essential feature of the motional averaging.
BibTeX:
@article{Borregaard2016,
  author = {Borregaard, J. and Zugenmaier, M. and Petersen, J. M. and Shen, H. and Vasilakis, G. and Jensen, K. and Polzik, E. S. and Sorensen, A. S.},
  title = {Scalable photonic network architecture based on motional averaging in room temperature gas},
  journal = {Nat Commun},
  publisher = {Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.},
  year = {2016},
  volume = {7},
  note = {Article},
  url = {http://dx.doi.org/10.1038/ncomms11356}
}
Das S, Grankin A, Iakoupov I, Brion E, Borregaard J, Boddeda R, Usmani I, Ourjoumtsev A, Grangier P and Sørensen AS (2016), "Photonic controlled-phase gates through Rydberg blockade in optical cavities", Phys. Rev. A., Apr, 2016. Vol. 93, pp. 040303. American Physical Society.
Abstract: We propose a scheme for high-fidelity photonic controlled-phase gates using a Rydberg blockade in an ensemble of atoms in an optical cavity. The gate operation is obtained by first storing a photonic pulse in the ensemble and then scattering a second pulse from the cavity, resulting in a phase change depending on whether the first pulse contained a single photon. We show that the combination of a Rydberg blockade and optical cavities effectively enhances the optical nonlinearity created by the strong Rydberg interaction and makes the gate operation more robust. The resulting gate can be implemented with cavities of moderate finesse, allowing for highly efficient processing of quantum information encoded in photons. As an illustration, we show how the gate can be employed to increase the communication rate of quantum repeaters based on atomic ensembles.
BibTeX:
@article{Das2016,
  author = {Das, Sumanta and Grankin, Andrey and Iakoupov, Ivan and Brion, Etienne and Borregaard, Johannes and Boddeda, Rajiv and Usmani, Imam and Ourjoumtsev, Alexei and Grangier, Philippe and Sørensen, Anders S.},
  title = {Photonic controlled-phase gates through Rydberg blockade in optical cavities},
  journal = {Phys. Rev. A},
  publisher = {American Physical Society},
  year = {2016},
  volume = {93},
  pages = {040303},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.93.040303},
  doi = {10.1103/PhysRevA.93.040303}
}
Pedersen KGL, Andersen BM, Bruun GM and Sørensen AS (2015), "Using superlattice potentials to probe long-range magnetic correlations in optical lattices", Phys. Rev. A., Dec, 2015. Vol. 92, pp. 063633. American Physical Society.
Abstract: We previously proposed [K. G. L. Pedersen, B. M. Andersen, G. M. Bruun, O. F. Syljuåsen, and A. S. Sørensen, Phys. Rev. A 84, 041603 (2011)] a method to utilize a temporally dependent superlattice potential to mediate spin-selective transport and thereby probe long- and short-range magnetic correlations in optical lattices. Specifically, this can be used for detecting antiferromagnetic ordering in repulsive fermionic optical lattice systems, but more generally it can serve as a means of directly probing correlations among the atoms by measuring the mean value of an observable, the number of double occupied sites. Here we provide a detailed investigation of the physical processes that limit the effectiveness of this “conveyer belt method.” Furthermore, we propose a simple way to improve the procedure, resulting in an essentially perfect (error-free) probing of the magnetic correlations. These results shows that suitably constructed superlattices constitute a promising way of manipulating atoms of different spin species as well as probing their interactions.
BibTeX:
@article{Pedersen2015,
  author = {Pedersen, Kim G. L. and Andersen, Brian M. and Bruun, Georg M. and Sørensen, Anders S.},
  title = {Using superlattice potentials to probe long-range magnetic correlations in optical lattices},
  journal = {Phys. Rev. A},
  publisher = {American Physical Society},
  year = {2015},
  volume = {92},
  pages = {063633},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.92.063633},
  doi = {10.1103/PhysRevA.92.063633}
}
Fresta L, Borregaard J and Sørensen AS (2015), "Elementary test for nonclassicality based on measurements of position and momentum", Phys. Rev. A., Dec, 2015. Vol. 92, pp. 062111. American Physical Society.
Abstract: We generalize a nonclassicality test described by Kot et al. [Phys. Rev. Lett. 108, 233601 (2012)], which can be used to rule out any classical description of a physical system. The test is based on measurements of quadrature operators and works by proving a contradiction with the classical description in terms of a probability distribution in phase space. As opposed to the previous work, we generalize the test to include states without rotational symmetry in phase space. Furthermore, we compare the performance of the nonclassicality test with classical tomography methods based on the inverse Radon transform, which can also be used to establish the quantum nature of a physical system. In particular, we consider a nonclassicality test based on the so-called filtered back-projection formula. We show that the general nonclassicality test is conceptually simpler, requires less assumptions on the system, and is statistically more reliable than the tests based on the filtered back-projection formula. As a specific example, we derive the optimal test for quadrature squeezed single-photon states and show that the efficiency of the test does not change with the degree of squeezing.
BibTeX:
@article{Fresta2015,
  author = {Fresta, Luca and Borregaard, Johannes and Sørensen, Anders S.},
  title = {Elementary test for nonclassicality based on measurements of position and momentum},
  journal = {Phys. Rev. A},
  publisher = {American Physical Society},
  year = {2015},
  volume = {92},
  pages = {062111},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.92.062111},
  doi = {10.1103/PhysRevA.92.062111}
}
Borregaard J, Kómár P, Kessler EM, Lukin MD and Sørensen AS (2015), "Long-distance entanglement distribution using individual atoms in optical cavities", Phys. Rev. A., Jul, 2015. Vol. 92, pp. 012307. American Physical Society.
Abstract: Individual atoms in optical cavities can provide an efficient interface between stationary qubits and flying qubits (photons), which is an essential building block for quantum communication. Furthermore, cavity-assisted controlled-not (cnot) gates can be used for swapping entanglement to long distances in a quantum repeater setup. Nonetheless, dissipation introduced by the cavity during the cnot may increase the experimental difficulty in obtaining long-distance entanglement distribution using these systems. We analyze and compare a number of cavity-based repeater schemes combining various entanglement generation schemes and cavity-assisted cnot gates. We find that a scheme, where high-fidelity entanglement is first generated in a two-photon detection scheme and then swapped to long distances using a recently proposed heralded controlled-Z (cz) gate, exhibits superior performance compared to the other schemes. The heralded gate moves the effect of dissipation from the fidelity to the success probability of the gate thereby enabling high-fidelity entanglement swapping. As a result, high-rate entanglement distribution can be achieved over long distances even for low cooperativities of the atom-cavity systems. This high-fidelity repeater is shown to outperform the other cavity-based schemes by up to two orders of magnitude in the rate for realistic parameters and large distances (1000 km).
BibTeX:
@article{Borregaard2015,
  author = {Borregaard, J. and Kómár, P. and Kessler, E. M. and Lukin, M. D. and Sørensen, A. S.},
  title = {Long-distance entanglement distribution using individual atoms in optical cavities},
  journal = {Phys. Rev. A},
  publisher = {American Physical Society},
  year = {2015},
  volume = {92},
  pages = {012307},
  url = {http://link.aps.org/doi/10.1103/PhysRevA.92.012307},
  doi = {10.1103/PhysRevA.92.012307}
}
Tighineanu P, Sørensen AS, Stobbe S and Lodahl P (2015), "Unraveling the Mesoscopic Character of Quantum Dots in Nanophotonics", Phys. Rev. Lett.., Jun, 2015. Vol. 114, pp. 247401. American Physical Society.
Abstract: We provide a microscopic theory for semiconductor quantum dots that explains the pronounced deviations from the prevalent point-dipole description that were recently observed in spectroscopic experiments on quantum dots in photonic nanostructures. The deviations originate from structural inhomogeneities generating a large circular quantum current density that flows inside the quantum dot over mesoscopic length scales. The model is supported by the experimental data, where a strong variation of the multipolar moments across the emission spectrum of quantum dots is observed. Our work enriches the physical understanding of quantum dots and is of significance for the fields of nanophotonics, quantum photonics, and quantum-information science, where quantum dots are actively employed.
BibTeX:
@article{Tighineanu2015,
  author = {Tighineanu, P. and Sørensen, A. S. and Stobbe, S. and Lodahl, P.},
  title = {Unraveling the Mesoscopic Character of Quantum Dots in Nanophotonics},
  journal = {Phys. Rev. Lett.},
  publisher = {American Physical Society},
  year = {2015},
  volume = {114},
  pages = {247401},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.114.247401},
  doi = {10.1103/PhysRevLett.114.247401}
}
Rajitha K. V, Dey TN, Das S and Jha PK (2015), "Microwave-controlled efficient Raman sub-harmonic generation", Opt. Lett.., May, 2015. Vol. 40(10), pp. 2229-2232. OSA.
Abstract: We propose a novel scheme for efficient Raman sub-harmonic generation in a system of inhomogeneously broadened atomic vapor with atoms in a closed three-level &x39B;-configuration. The key feature underlying Raman sub-harmonics generation is microwave-induced hyperfine coherence between the two lower level metastable states. We show explicitly how the amplitude and phase of the microwave field generate controllable spectral amplitudes of Raman sub-harmonics. Our scheme is viable in currently available atomic vapor cells thereby opening the way for efficient sub-harmonic generation in standard experimental setups.
BibTeX:
@article{V15,
  author = {Rajitha K. V. and Tarak N. Dey and Sumanta Das and Pankaj K. Jha},
  title = {Microwave-controlled efficient Raman sub-harmonic generation},
  journal = {Opt. Lett.},
  publisher = {OSA},
  year = {2015},
  volume = {40},
  number = {10},
  pages = {2229--2232},
  url = {http://ol.osa.org/abstract.cfm?URI=ol-40-10-2229},
  doi = {10.1364/OL.40.002229}
}
Borregaard J, Kómár P, Kessler E. M, Sørensen A. S and Lukin M. D (2015), "Heralded Quantum Gates with Integrated Error Detection in Optical Cavities", Phys. Rev. Lett.., Mar, 2015. Vol. 114, pp. 110502.
Abstract: We propose and analyze heralded quantum gates between qubits in optical cavities. They employ an auxiliary qubit to report if a successful gate occurred. In this manner, the errors, which would have corrupted a deterministic gate, are converted into a nonunity probability of success: once successful, the gate has a much higher fidelity than a similar deterministic gate. Specifically, we describe that a heralded, near-deterministic controlled phase gate (CZ gate) with the conditional error arbitrarily close to zero and the success probability that approaches unity as the cooperativity of the system, C, becomes large. Furthermore, we describe an extension to near-deterministic N-qubit Toffoli gate with a favorable error scaling. These gates can be directly employed in quantum repeater networks to facilitate near-ideal entanglement swapping, thus greatly speeding up the entanglement distribution.
BibTeX:
@article{Borregaard:2015,
  author = {Borregaard, J. and Kómár, P. and Kessler, E.M. and Sørensen, A.S. and Lukin, M.D.},
  title = {Heralded Quantum Gates with Integrated Error Detection in Optical Cavities},
  journal = {Phys. Rev. Lett.},
  year = {2015},
  volume = {114},
  pages = {110502},
  doi = {10.1103/PhysRevLett.114.110502}
}
Komar P, Kessler EM, Bishof M, Jiang L, Sørensen AS, Ye J and Lukin D (2014), "A quantum network of clocks", Nature Physics., AUG, 2014. Vol. 10(8), pp. 582-587.
Abstract: The development of precise atomic clocks plays an increasingly important role in modern society. Shared timing information constitutes a key resource for navigation with a direct correspondence between timing accuracy and precision in applications such as the Global Positioning System. By combining precision metrology and quantum networks, we propose a quantum, cooperative protocol for operating a network of geographically remote optical atomic clocks. Using nonlocal entangled states, we demonstrate an optimal utilization of global resources, and show that such a network can be operated near the fundamental precision limit set by quantum theory. Furthermore, the internal structure of the network, combined with quantum communication techniques, guarantees security both from internal and external threats. Realization of such a global quantum network of clocks may allow construction of a real-time single international time scale (world clock) with unprecedented stability and accuracy.
BibTeX:
@article{Komar:2014,
  author = {Komar, P. and Kessler, E. M. and Bishof, M. and Jiang, L. and Sørensen, A. S. and Ye, J. and Lukin, D.},
  title = {A quantum network of clocks},
  journal = {Nature Physics},
  year = {2014},
  volume = {10},
  number = {8},
  pages = {582-587},
  doi = {10.1038/NPHYS3000}
}
Tighineanu P, Andersen ML, Sørensen AS, Stobbe S and Lodahl P (2014), "Probing Electric and Magnetic Vacuum Fluctuations with Quantum Dots", Phys. Rev. Lett.., Jul, 2014. Vol. 113, pp. 043601. American Physical Society.
Abstract: The electromagnetic-vacuum-field fluctuations are intimately linked to the process of spontaneous emission of light. Atomic emitters cannot probe electric- and magnetic-field fluctuations simultaneously because electric and magnetic transitions correspond to different selection rules. In this Letter we show that semiconductor quantum dots are fundamentally different and are capable of mediating electric-dipole, magnetic-dipole, and electric-quadrupole transitions on a single electronic resonance. As a consequence, quantum dots can probe electric and magnetic fields simultaneously and can thus be applied for sensing the electromagnetic environment of complex photonic nanostructures. Our study opens the prospect of interfacing quantum dots with optical metamaterials for tailoring the electric and magnetic light-matter interaction at the single-emitter level.
BibTeX:
@article{Tighineanu:2014,
  author = {Tighineanu, P. and Andersen, M. L. and Sørensen, A. S. and Stobbe, S. and Lodahl, P.},
  title = {Probing Electric and Magnetic Vacuum Fluctuations with Quantum Dots},
  journal = {Phys. Rev. Lett.},
  publisher = {American Physical Society},
  year = {2014},
  volume = {113},
  pages = {043601},
  url = {http://link.aps.org/doi/10.1103/PhysRevLett.113.043601},
  doi = {10.1103/PhysRevLett.113.043601}
}
Kessler EM, Komar P, Bishof M, Jiang L, Sørensen AS, Ye J and Lukin MD (2014), "Heisenberg-Limited Atom Clocks Based on Entangled Qubits", Phys. Rev. Let.., MAY 15, 2014. Vol. 112(19)
Abstract: We present a quantum-enhanced atomic clock protocol based on groups of sequentially larger Greenberger-Horne-Zeilinger (GHZ) states that achieves the best clock stability allowed by quantum theory up to a logarithmic correction. Importantly the protocol is designed to work under realistic conditions where the drift of the phase of the laser interrogating the atoms is the main source of decoherence. The simultaneous interrogation of the laser phase with a cascade of GHZ states realizes an incoherent version of the phase estimation algorithm that enables Heisenberg-limited operation while extending the coherent interrogation time beyond the laser noise limit. We compare and merge the new protocol with existing state of the art interrogation schemes, and identify the precise conditions under which entanglement provides an advantage for clock stabilization: it allows a significant gain in the stability for short averaging time.
BibTeX:
@article{Kessler:2014,
  author = {Kessler, E. M. and Komar, P. and Bishof, M. and Jiang, L. and Sørensen, A. S. and Ye, J. and Lukin, M. D.},
  title = {Heisenberg-Limited Atom Clocks Based on Entangled Qubits},
  journal = {Phys. Rev. Let.},
  year = {2014},
  volume = {112},
  number = {19},
  doi = {10.1103/PhysRevLett.112.190403}
}
Manzoni MT, Reiter F, Taylor JM and Sørensen AS (2014), "Single-photon transistor based on superconducting systems", Phys. Rev. B., MAY 12, 2014. Vol. 89(18)
Abstract: We present a scheme for a single-photon transistor which can be implemented with only minor modifications of existing superconducting circuits. The proposal employs a three-level anharmonic ladder atom, e. g., a transmon qubit, placed in a cavity to mimic a Lambda-type atom with two long-lived states. This configuration may enable a wide range of effects originally studied in quantum optical systems to be realized in superconducting systems, and in particular allow for single-photon transistors. We study analytically and numerically the efficiency and the gain of the proposed transistor as a function of the experimental parameters, in particular of the level anharmonicity and of the various decay and decoherence rates. State-of-the-art values for these parameters indicate that error probabilities of similar to 1% and gains of the order of hundreds can be obtained.
BibTeX:
@article{Manzoni:2014,
  author = {Manzoni, Marco T. and Reiter, Florentin and Taylor, Jacob M. and Sørensen, Anders S.},
  title = {Single-photon transistor based on superconducting systems},
  journal = {Phys. Rev. B},
  year = {2014},
  volume = {89},
  number = {18},
  doi = {10.1103/PhysRevB.89.180502}
}
Bagci T, Simonsen A, Schmid S, Villanueva LG, Zeuthen E, Appel J, Taylor JM, Sørensen A, Usami K, Schliesser A and Polzik ES (2014), "Optical detection of radio waves through a nanomechanical transducer", Nature., MAR 6, 2014. Vol. 507(7490), pp. 81-85.
Abstract: Low-loss transmission and sensitive recovery of weak radio-frequency and microwave signals is a ubiquitous challenge, crucial in radio astronomy, medical imaging, navigation, and classical and quantum communication. Efficient up-conversion of radio-frequency signals to an optical carrier would enable their transmission through optical fibres instead of through copper wires, drastically reducing losses, and would give access to the set of established quantum optical techniques that are routinely used in quantum-limited signal detection. Research in cavity optomechanics(1,2) has shown that nanomechanical oscillators can couple strongly to either microwave(3-5) or optical fields(6,7). Here we demonstrate a room-temperature optoelectromechanical transducer with both these functionalities, following a recent proposal(8) using a high-quality nanomembrane. A voltage bias of less than 10 V is sufficient to induce strong coupling(4,6,7) between the voltage fluctuations in a radio-frequency resonance circuit and the membrane's displacement, which is simultaneously coupled to light reflected off its surface. The radio-frequency signals are detected as an optical phase shift with quantum-limited sensitivity. The corresponding half-wave voltage is in the microvolt range, orders of magnitude less than that of standard optical modulators. The noise of the transducer-beyond the measured 800 pV Hz^-1/2 Johnson noise of the resonant circuit-consists of the quantum noise of light and thermal fluctuations of the membrane, dominating the noise floor in potential applications in radio astronomy and nuclear magnetic imaging. Each of these contributions is inferred to be 60 pV Hz^-1/2 when balanced by choosing an electromechanical cooperativity of similar to 150 with an optical power of 1 mW. The noise temperature of the membrane is 300 K divided by the cooperativity. For the highest observed cooperativity of 6,800, this leads to a projected noise temperature of 40 mK and a sensitivity limit of 5 pV Hz^-1/2. Our approach to all-optical, ultralow-noise detection of classical electronic signals sets the stage for coherent up-conversion of low-frequency quantum signals to the optical domain(8-11).
BibTeX:
@article{Bagci:2014,
  author = {Bagci, T. and Simonsen, A. and Schmid, S. and Villanueva, L. G. and Zeuthen, E. and Appel, J. and Taylor, J. M. and Sørensen, A. and Usami, K. and Schliesser, A. and Polzik, E. S.},
  title = {Optical detection of radio waves through a nanomechanical transducer},
  journal = {Nature},
  year = {2014},
  volume = {507},
  number = {7490},
  pages = {81-85},
  doi = {10.1038/Nature13029}
}
Schmid S, Bagci T, Zeuthen E, Taylor JM, Herring PK, Cassidy MC, Marcus CM, Villanueva LG, Amato B, Boisen A, Shin YC, Kong J, Sørensen AS, Usami K and Polzik ES (2014), "Single-layer graphene on silicon nitride micromembrane resonators", Journal of Applied Physics., FEB 7, 2014. Vol. 115(5)
Abstract: Due to their low mass, high quality factor, and good optical properties, silicon nitride (SiN) micromembrane resonators are widely used in force and mass sensing applications, particularly in optomechanics. The metallization of such membranes would enable an electronic integration with the prospect for exciting new devices, such as optoelectromechanical transducers. Here, we add a single-layer graphene on SiN micromembranes and compare electromechanical coupling and mechanical properties to bare dielectric membranes and to membranes metallized with an aluminium layer. The electrostatic coupling of graphene covered membranes is found to be equal to a perfectly conductive membrane, without significantly adding mass, decreasing the superior mechanical quality factor or affecting the optical properties of pure SiN micromembranes. The concept of graphene-SiN resonators allows a broad range of new experiments both in applied physics and fundamental basic research, e. g., for the mechanical, electrical, or optical characterization of graphene. (c) 2014 AIP Publishing LLC.
BibTeX:
@article{Schmid:2014,
  author = {Schmid, Silvan and Bagci, Tolga and Zeuthen, Emil and Taylor, Jacob M. and Herring, Patrick K. and Cassidy, Maja C. and Marcus, Charles M. and Villanueva, Luis Guillermo and Amato, Bartolo and Boisen, Anja and Shin, Yong Cheol and Kong, Jing and Sørensen, Anders S. and Usami, Koji and Polzik, Eugene S.},
  title = {Single-layer graphene on silicon nitride micromembrane resonators},
  journal = {Journal of Applied Physics},
  year = {2014},
  volume = {115},
  number = {5},
  doi = {10.1063/1.4862296}
}
Lin Y, Gaebler JP, Reiter F, Tan TR, Bowler R, Sørensen AS, Leibfried D and Wineland DJ (2013), "Dissipative production of a maximally entangled steady state of two quantum bits", Nature., DEC 19, 2013. Vol. 504(7480), pp. 415+.
Abstract: Entangled states are a key resource in fundamental quantum physics, quantum cryptography and quantum computation(1). Introduction of controlled unitary processes-quantum gates-to a quantum system has so far been the most widely used method to create entanglement deterministically(2). These processes require high-fidelity state preparation and minimization of the decoherence that inevitably arises from coupling between the system and the environment, and imperfect control of the system parameters. Here we combine unitary processes with engineered dissipation to deterministically produce and stabilize an approximate Bell state of two trapped-ion quantum bits (qubits), independent of their initial states. Compared with previous studies that involved dissipative entanglement of atomic ensembles(3) or the application of sequences of multiple time-dependent gates to trapped ions(4), we implement our combined process using trapped-ion qubits in a continuous time-independent fashion (analogous to optical pumping of atomic states). By continuously driving the system towards the steady state, entanglement is stabilized even in the presence of experimental noise and decoherence. Our demonstration of an entangled steady state of two qubits represents a step towards dissipative state engineering, dissipative quantum computation and dissipative phase transitions(5-7). Following this approach, engineered coupling to the environment may be applied to a broad range of experimental systems to achieve desired quantum dynamics or steady states. Indeed, concurrently with this work, an entangled steady state of two superconducting qubits was demonstrated using dissipation(8).
BibTeX:
@article{Lin:2013,
  author = {Lin, Y. and Gaebler, J. P. and Reiter, F. and Tan, T. R. and Bowler, R. and Sørensen, A. S. and Leibfried, D. and Wineland, D. J.},
  title = {Dissipative production of a maximally entangled steady state of two quantum bits},
  journal = {Nature},
  year = {2013},
  volume = {504},
  number = {7480},
  pages = {415+},
  doi = {10.1038/Nature12801}
}
Huck A, Witthaut D, Kumar S, Sørensen AS and Andersen UL (2013), "Large Optical Nonlinearity of Surface Plasmon Modes on Thin Gold Films", Plasmonics., DEC, 2013. Vol. 8(4), pp. 1597-1605.
Abstract: We investigate the optical nonlinear effects of a long-range surface plasmon polariton mode propagating on a thin gold film. These effects may play a key role in the design of future nanophotonic circuits as they allow for the realization of active plasmonic elements. We demonstrate a significant enhancement of the transmission on a timescale below a millisecond as well as a phase shift exceeding 2pi already for modest peak powers of 150 mW. On the contrary, slow effects suppress the transmission on a millisecond timescale.
BibTeX:
@article{Huck:2013,
  author = {Huck, Alexander and Witthaut, Dirk and Kumar, Shailesh and Sørensen, Anders S. and Andersen, Ulrik L.},
  title = {Large Optical Nonlinearity of Surface Plasmon Modes on Thin Gold Films},
  journal = {Plasmonics},
  year = {2013},
  volume = {8},
  number = {4},
  pages = {1597-1605},
  doi = {10.1007/s11468-013-9576-0}
}
Reiter F, Tornberg L, Johansson G and Sørensen AS (2013), "Steady-state entanglement of two superconducting qubits engineered by dissipation", Phys. Rev. A., SEP 16, 2013. Vol. 88(3)
Abstract: We present a scheme for the dissipative preparation of an entangled steady state of two superconducting qubits in a circuit quantum electrodynamics (QED) setup. Combining resonator photon loss-a dissipative process already present in the setup-with an effective two-photon microwave drive, we engineer an effective decay mechanism which prepares a maximally entangled state of the two qubits. This state is then maintained as the steady state of the driven, dissipative evolution. The performance of the dissipative state preparation protocol is studied analytically and verified numerically. In view of the experimental implementation of the presented scheme we investigate the effects of potential experimental imperfections and show that our scheme is robust to small deviations in the parameters. We find that high fidelities with the target state can be achieved both with state-of-the-art three-dimensional, as well as with the more commonly used two-dimensional transmons. The promising results of our study thus open a route for the demonstration of a highly entangled steady state in circuit QED.
BibTeX:
@article{Reiter:2013,
  author = {Reiter, Florentin and Tornberg, L. and Johansson, Goran and Sørensen, Anders S.},
  title = {Steady-state entanglement of two superconducting qubits engineered by dissipation},
  journal = {Phys. Rev. A},
  year = {2013},
  volume = {88},
  number = {3},
  doi = {10.1103/PhysRevA.88.032317}
}
Borregaard J and Sørensen AS (2013), "Near-Heisenberg-Limited Atomic Clocks in the Presence of Decoherence", Phys. Rev. Let.., AUG 27, 2013. Vol. 111(9)
Abstract: The ultimate stability of atomic clocks is limited by the quantum noise of the atoms. To reduce this noise it has been suggested to use entangled atomic ensembles with reduced atomic noise. Potentially this can push the stability all the way to the limit allowed by the Heisenberg uncertainty relation, which is denoted the Heisenberg limit. In practice, however, entangled states are often more prone to decoherence, which may prevent reaching this performance. Here we present an adaptive measurement protocol that in the presence of a realistic source of decoherence enables us to get near-Heisenberg-limited stability of atomic clocks using entangled atoms. The protocol may thus realize the full potential of entanglement for quantum metrology despite the detrimental influence of decoherence.
BibTeX:
@article{Borregaard:2013,
  author = {Borregaard, J. and Sørensen, A. S.},
  title = {Near-Heisenberg-Limited Atomic Clocks in the Presence of Decoherence},
  journal = {Phys. Rev. Let.},
  year = {2013},
  volume = {111},
  number = {9},
  doi = {10.1103/PhysRevLett.111.090801}
}
Borregaard J and Sørensen AS (2013), "Efficient Atomic Clocks Operated with Several Atomic Ensembles", Phys. Rev. Let.., AUG 27, 2013. Vol. 111(9)
Abstract: Atomic clocks are typically operated by locking a local oscillator (LO) to a single atomic ensemble. In this Letter, we propose a scheme where the LO is locked to several atomic ensembles instead of one. This results in an exponential improvement compared to the conventional method and provides a stability of the clock scaling as (alpha N)^-m/2 with N being the number of atoms in each of the m ensembles and alpha a constant depending on the protocol being used to lock the LO.
BibTeX:
@article{Borregaard:2013a,
  author = {Borregaard, J. and Sørensen, A. S.},
  title = {Efficient Atomic Clocks Operated with Several Atomic Ensembles},
  journal = {Phys. Rev. Let.},
  year = {2013},
  volume = {111},
  number = {9},
  doi = {10.1103/PhysRevLett.111.090802}
}
Iakoupov I and Sørensen AS (2013), "An efficient quantum memory based on two-level atoms", New Journal of Physics., AUG 13, 2013. Vol. 15
Abstract: We propose a method to implement a quantum memory for light based on ensembles of two-level atoms. Our protocol is based on controlled reversible inhomogeneous broadening (CRIB), where an external field first dephases the atomic polarization and thereby stores an incoming light pulse into collective states of the atomic ensemble, and later a reversal of the applied field leads to a rephasing of the atomic polarization and a reemission of the light. As opposed to previous proposals for CRIB-based quantum memories, we propose only applying the broadening for a short period after most of the pulse has already been absorbed by the ensemble. We show that with this procedure there exist certain modes of the incoming light field that can be stored with an efficiency approaching 100% in the limit of high optical depth and long coherence time of the atoms. These results demonstrate that it is possible to operate an efficient quantum memory without any optical control fields.
BibTeX:
@article{Iakoupov:2013,
  author = {Iakoupov, Ivan and Sørensen, Anders S.},
  title = {An efficient quantum memory based on two-level atoms},
  journal = {New Journal of Physics},
  year = {2013},
  volume = {15},
  doi = {10.1088/1367-2630/15/8/085012}
}
Gawarecki K, Lueker S, Reiter DE, Kuhn T, Glaessl M, Axt VM, Grodecka-Grad A and Machnikowski P (2012), "Dephasing in the adiabatic rapid passage in quantum dots: Role of phonon-assisted biexciton generation", Phys. Rev. B., DEC 3, 2012. Vol. 86(23)
Abstract: We study the evolution of an exciton confined in a quantum dot adiabatically controlled by a frequency-swept (chirped) laser pulse in the presence of carrier-phonon coupling. We focus on the dynamics induced by a linearly polarized beam and analyze the decoherence due to phonon-assisted biexciton generation. We show that if the biexciton state is shifted down by a few meV, as is typically the case, then the resulting decoherence is strong even at low temperatures. As a result, efficient state preparation is restricted to a small parameter area corresponding to low temperatures, positive chirps, and moderate pulse areas.
BibTeX:
@article{Gawarecki:2012,
  author = {Gawarecki, Krzysztof and Lueker, Sebastian and Reiter, Doris E. and Kuhn, Tilmann and Glaessl, Martin and Axt, Vollrath Martin and Grodecka-Grad, Anna and Machnikowski, Pawel},
  title = {Dephasing in the adiabatic rapid passage in quantum dots: Role of phonon-assisted biexciton generation},
  journal = {Phys. Rev. B},
  year = {2012},
  volume = {86},
  number = {23},
  doi = {10.1103/PhysRevB.86.235301}
}
Reiter DE, Lueker S, Gawarecki K, Grodecka-Grad A, Machnikowski P, Axt VM and Kuhn T (2012), "Phonon Effects on Population Inversion in Quantum Dots: Resonant, Detuned and Frequency-Swept Excitations", Acta Physica Polonica A., DEC, 2012. Vol. 122(6), pp. 1065-1068.
Abstract: The effect of acoustic phonons on different light-induced excitations of a semiconductor quantum dot is investigated. Resonant excitation of the quantum dot leads to the Rabi oscillations, which are damped due to the phonon interaction. When the excitation frequency is detuned, an occupation can only occur due to phonon absorption or emission processes. For frequency-swept excitations a population inversion is achieved through adiabatic rapid passage, but the inversion is also damped by phonons. For all three scenarios the influence of the phonons depends non-monotonically on the pulse area.
BibTeX:
@article{Reiter:2012,
  author = {Reiter, D. E. and Lueker, S. and Gawarecki, K. and Grodecka-Grad, A. and Machnikowski, P. and Axt, V. M. and Kuhn, T.},
  title = {Phonon Effects on Population Inversion in Quantum Dots: Resonant, Detuned and Frequency-Swept Excitations},
  journal = {Acta Physica Polonica A},
  year = {2012},
  volume = {122},
  number = {6},
  pages = {1065-1068},
  note = {41st International School and Conference on the Physics of Semiconductors (Jaszowiec), Krynica Zdroj, POLAND, JUN 08-15, 2012}
}
Pekker D, Wunsch B, Kitagawa T, Manousakis E, Sørensen AS and Demler E (2012), "Signatures of the superfluid to Mott insulator transition in equilibrium and in dynamical ramps", Phys. Rev. B., OCT 22, 2012. Vol. 86(14)
Abstract: We investigate the equilibrium and dynamical properties of the Bose-Hubbard model and the related particle-hole symmetric spin-1 model in the vicinity of the superfluid to Mott insulator quantum phase transition. We employ the following methods: exact-diagonalization, mean-field (Gutzwiller), clustermean-field, andmean-field plus Gaussian fluctuations. In the first part of the paper we benchmark the four methods by analyzing the equilibrium problem and give numerical estimates for observables such as the density of double occupancies and their correlation function. In the second part, we study parametric ramps from the superfluid to the Mott insulator and map out the crossover from the regime of fast ramps, which is dominated by local physics, to the regime of slow ramps with a characteristic universal power law scaling, which is dominated by long wavelength excitations. We calculate values of several relevant physical observables, characteristic time scales, and an optimal protocol needed for observing universal scaling.
BibTeX:
@article{Pekker:2012,
  author = {Pekker, D. and Wunsch, B. and Kitagawa, T. and Manousakis, E. and Sørensen, A. S. and Demler, E.},
  title = {Signatures of the superfluid to Mott insulator transition in equilibrium and in dynamical ramps},
  journal = {Phys. Rev. B},
  year = {2012},
  volume = {86},
  number = {14},
  doi = {10.1103/PhysRevB.86.144527}
}
Grodecka-Grad A, Zeuthen E and Sørensen AS (2012), "High-Capacity Spatial Multimode Quantum Memories Based on Atomic Ensembles", Phys. Rev. Let.., SEP 27, 2012. Vol. 109(13)
Abstract: We study spatial multimode quantum memories based on light storage in extended ensembles of Lambda-type atoms. We show that such quantum light-matter interfaces allow for highly efficient storage of many spatial modes. In particular, forward operating memories possess excellent scaling with the important physical parameters: quadratic scaling with the Fresnel number and even cubic with the optical depth of the atomic ensemble. Thus, the simultaneous use of both the longitudinal and transverse shape of the stored spin wave modes constitutes a valuable and so far overlooked resource for multimode quantum memories.
BibTeX:
@article{Grodecka-Grad:2012,
  author = {Grodecka-Grad, Anna and Zeuthen, Emil and Sørensen, Anders S.},
  title = {High-Capacity Spatial Multimode Quantum Memories Based on Atomic Ensembles},
  journal = {Phys. Rev. Let.},
  year = {2012},
  volume = {109},
  number = {13},
  doi = {10.1103/PhysRevLett.109.133601}
}
Borregaard J, Brask JB and Sørensen AS (2012), "Hybrid quantum repeater protocol with fast local processing", Phys. Rev. A., JUL 26, 2012. Vol. 86(1)
Abstract: We propose a hybrid quantum repeater protocol combining the advantages of continuous and discrete variables. The repeater is based on the previous work of Brask et al. [Phys. Rev. Lett. 105, 160501 (2010)] but we present two ways of improving this protocol. In the previous protocol entangled single-photon states are produced and grown into superpositions of coherent states, known as two-mode cat states. The entanglement is then distributed using homodyne detection. To improve the protocol, we replace the time-consuming nonlocal growth of cat states with local growth of single-mode cat states, eliminating the need for classical communication during growth. Entanglement is generated in subsequent connection processes. Furthermore the growth procedure is optimized. We review the main elements of the original protocol and present the two modifications. Finally the two protocols are compared and the modified protocol is shown to perform significantly better than the original protocol.
BibTeX:
@article{Borregaard:2012,
  author = {Borregaard, J. and Brask, J. B. and Sørensen, A. S.},
  title = {Hybrid quantum repeater protocol with fast local processing},
  journal = {Phys. Rev. A},
  year = {2012},
  volume = {86},
  number = {1},
  doi = {10.1103/PhysRevA.86.012330}
}
Kot E, Grønbech-Jensen N, Nielsen BM, Neergaard-Nielsen JS, Polzik ES and Sørensen AS (2012), "Breakdown of the Classical Description of a Local System", Phys. Rev. Let.., JUN 6, 2012. Vol. 108(23)
Abstract: We provide a straightforward demonstration of a fundamental difference between classical and quantum mechanics for a single local system: namely, the absence of a joint probability distribution of the position x and momentum p. Elaborating on a recently reported criterion by Bednorz and Belzig [Phys. Rev. A 83, 052113 (2011)] we derive a simple criterion that must be fulfilled for any joint probability distribution in classical physics. We demonstrate the violation of this criterion using the homodyne measurement of a single photon state, thus proving a straightforward sigNature of the breakdown of a classical description of the underlying state. Most importantly, the criterion used does not rely on quantum mechanics and can thus be used to demonstrate nonclassicality of systems not immediately apparent to exhibit quantum behavior. The criterion is directly applicable to any system described by the continuous canonical variables x and p, such as a mechanical or an electrical oscillator and a collective spin of a large ensemble.
BibTeX:
@article{Kot:2012,
  author = {Kot, Eran and Grønbech-Jensen, Niels and Nielsen, Bo M. and Neergaard-Nielsen, Jonas S. and Polzik, Eugene S. and Sørensen, Anders S.},
  title = {Breakdown of the Classical Description of a Local System},
  journal = {Phys. Rev. Let.},
  year = {2012},
  volume = {108},
  number = {23},
  doi = {10.1103/PhysRevLett.108.233601}
}
Vasilyev DV, Hammerer K, Korolev N and Sørensen AS (2012), "Quantum noise for Faraday light-matter interfaces", Journal of Physics B --- Atomic Molecular and Optical Physics., JUN 28, 2012. Vol. 45(12, SI)
Abstract: In light-matter interfaces based on the Faraday effect, quite a number of quantum information protocols have been successfully demonstrated. In order to further increase the performance and fidelities achieved in these protocols, a deeper understanding of the relevant noise and decoherence processes needs to be gained. In this paper, we provide for the first time a complete description of the decoherence from spontaneous emission. We derive from first principles the effects of photons being spontaneously emitted into unobserved modes. Our results relate the resulting decay and noise terms in effective equations of motion for collective atomic spins and the forward-propagating light modes to the full atomic level structure. We illustrate and apply our results to the case of a quantum memory protocol. Our results can be applied to any alkali atoms, and the general approach taken in this paper can be applied to light-matter interfaces and quantum memories based on different mechanisms.
BibTeX:
@article{Vasilyev:2012,
  author = {Vasilyev, D. V. and Hammerer, K. and Korolev, N. and Sørensen, A. S.},
  title = {Quantum noise for Faraday light-matter interfaces},
  journal = {Journal of Physics B --- Atomic Molecular and Optical Physics},
  year = {2012},
  volume = {45},
  number = {12, SI},
  doi = {10.1088/0953-4075/45/12/124007}
}
Pedersen KGL, Andersen BM, Syljuåsen OF, Bruun GM and Sørensen AS (2012), "Inducing spin-dependent tunneling to probe magnetic correlations in optical lattices", Phys. Rev. A., MAY 29, 2012. Vol. 85(5)
Abstract: We suggest a simple experimental method for probing antiferromagnetic spin correlations of two-component Fermi gases in optical lattices. The method relies on a spin selective Raman transition to excite atoms of one spin species to their first excited vibrational mode where the tunneling is large. The resulting difference in the tunneling dynamics of the two spin species can then be exploited, to reveal the spin correlations by measuring the number of doubly occupied lattice sites at a later time. We perform quantum Monte Carlo simulations of the spin system and solve the optical lattice dynamics numerically to show how the timed probe can be used to identify antiferromagnetic spin correlations in optical lattices.
BibTeX:
@article{Pedersen:2012,
  author = {Pedersen, K. G. L. and Andersen, B. M. and Syljuåsen, O. F. and Bruun, G. M. and Sørensen, A. S.},
  title = {Inducing spin-dependent tunneling to probe magnetic correlations in optical lattices},
  journal = {Phys. Rev. A},
  year = {2012},
  volume = {85},
  number = {5},
  doi = {10.1103/PhysRevA.85.053642}
}
Reiter F, Kastoryano MJ and Sørensen AS (2012), "Driving two atoms in an optical cavity into an entangled steady state using engineered decay", New Journal of Physics., MAY 18, 2012. Vol. 14
Abstract: We propose various schemes for the dissipative preparation of a maximally entangled steady state of two atoms in an optical cavity. Harnessing the natural decay processes of cavity photon loss and spontaneous emission, we use an effective operator formalism to identify and engineer effective decay processes, which reach an entangled steady state of two atoms as the unique fixed point of the dissipative time evolution. We investigate various aspects that are crucial for the experimental implementation of our schemes in present-day and future cavity quantum electrodynamics systems and analytically derive the optimal parameters, the error scaling and the speed of convergence of our protocols. Our study shows promising performance of our schemes for existing cavity experiments and favorable scaling of fidelity and speed with respect to the cavity parameters.
BibTeX:
@article{Reiter:2012a,
  author = {Reiter, Florentin and Kastoryano, Michael J. and Sørensen, Anders S.},
  title = {Driving two atoms in an optical cavity into an entangled steady state using engineered decay},
  journal = {New Journal of Physics},
  year = {2012},
  volume = {14},
  doi = {10.1088/1367-2630/14/5/053022}
}
Cappellaro P, Goldstein G, Hodges JS, Jiang L, Maze JR, Sørensen AS and Lukin MD (2012), "Environment-assisted metrology with spin qubits", Phys. Rev. A., MAR 30, 2012. Vol. 85(3)
Abstract: We investigate the sensitivity of a recently proposed method for precision measurement [Phys. Rev. Lett. 106, 140502 (2011)], focusing on an implementation based on solid-state spin systems. The scheme amplifies a quantum sensor response to weak external fields by exploiting its coupling to spin impurities in the environment. We analyze the limits to the sensitivity due to decoherence and propose dynamical decoupling schemes to increase the spin coherence time. The sensitivity is also limited by the environment spin polarization; therefore, we discuss strategies to polarize the environment spins and present a method to extend the scheme to the case of zero polarization. The coherence time and polarization determine a figure of merit for the environment's ability to enhance the sensitivity compared to echo-based sensing schemes. This figure of merit can be used to engineer optimized samples for high-sensitivity nanoscalemagnetic sensing, such as diamond nanocrystals with controlled impurity density.
BibTeX:
@article{Cappellaro:2012,
  author = {Cappellaro, P. and Goldstein, G. and Hodges, J. S. and Jiang, L. and Maze, J. R. and Sørensen, A. S. and Lukin, M. D.},
  title = {Environment-assisted metrology with spin qubits},
  journal = {Phys. Rev. A},
  year = {2012},
  volume = {85},
  number = {3},
  doi = {10.1103/PhysRevA.85.032336}
}
Lueker S, Gawarecki K, Reiter DE, Grodecka-Grad A, Axt VM, Machnikowski P and Kuhn T (2012), "Influence of acoustic phonons on the optical control of quantum dots driven by adiabatic rapid passage", Phys. Rev. B., MAR 15, 2012. Vol. 85(12)
Abstract: The role of phonons for adiabatic rapid passage in semiconductor quantum dots is studied theoretically. While in an ideal system adiabatic rapid passage results in a full inversion of the quantum dot occupation, phonons hamper this behavior drastically. We show that the transitions between the adiabatic states lead to a temperature-dependent decrease of the final exciton occupation. In contrast to the ideal evolution, the phonon-related perturbation induces dependencies on the pulse power and on the sign of the chirp.
BibTeX:
@article{Lueker:2012,
  author = {Lueker, S. and Gawarecki, K. and Reiter, D. E. and Grodecka-Grad, A. and Axt, V. M. and Machnikowski, P. and Kuhn, T.},
  title = {Influence of acoustic phonons on the optical control of quantum dots driven by adiabatic rapid passage},
  journal = {Phys. Rev. B},
  year = {2012},
  volume = {85},
  number = {12},
  doi = {10.1103/PhysRevB.85.121302}
}
Reiter F and Sørensen AS (2012), "Effective operator formalism for open quantum systems", Phys. Rev. A., MAR 9, 2012. Vol. 85(3)
Abstract: We present an effective operator formalism for open quantum systems. Employing perturbation theory and adiabatic elimination of excited states for a weakly driven system, we derive an effective master equation which reduces the evolution to the ground-state dynamics. The effective evolution involves a single effective Hamiltonian and one effective Lindblad operator for each naturally occurring decay process. Simple expressions are derived for the effective operators which can be directly applied to reach effective equations of motion for the ground states. We compare our method with the hitherto existing concepts for effective interactions and present physical examples for the application of our formalism, including dissipative state preparation by engineered decay processes.
BibTeX:
@article{Reiter:2012b,
  author = {Reiter, Florentin and Sørensen, Anders S.},
  title = {Effective operator formalism for open quantum systems},
  journal = {Phys. Rev. A},
  year = {2012},
  volume = {85},
  number = {3},
  doi = {10.1103/PhysRevA.85.032111}
}
Witthaut D, Lukin MD and Sørensen AS (2012), "Photon sorters and QND detectors using single photon emitters", Europhysics Letters., MAR, 2012. Vol. 97(5)
Abstract: We discuss a new method for realizing number-resolving and non-demolition photo detectors by strong coupling of light to individual single photon emitters, which act as strong optical non-linearities. As a specific application we show how these elements can be integrated into an error-proof Bell state analyzer, whose efficiency exceeds the best possible performance with linear optics even for a modest atom-field coupling. The methods are error-proof in the sense that every detection event unambiguously projects the photon state onto a Fock or Bell state and imperfections only result in reduced success probability, not in wrong results.
BibTeX:
@article{Witthaut:2012,
  author = {Witthaut, D. and Lukin, M. D. and Sørensen, A. S.},
  title = {Photon sorters and QND detectors using single photon emitters},
  journal = {Europhysics Letters},
  year = {2012},
  volume = {97},
  number = {5},
  doi = {10.1209/0295-5075/97/50007}
}
Taylor JM, Sørensen AS, Marcus CM and Polzik ES (2011), "Laser Cooling and Optical Detection of Excitations in a LC Electrical Circuit", Phys. Rev. Let.., DEC 27, 2011. Vol. 107(27)
Abstract: We explore a method for laser cooling and optical detection of excitations in a room temperature LC electrical circuit. Our approach uses a nanomechanical oscillator as a transducer between optical and electronic excitations. An experimentally feasible system with the oscillator capacitively coupled to the LC and at the same time interacting with light via an optomechanical force is shown to provide strong electromechanical coupling. Conditions for improved sensitivity and quantum limited readout of electrical signals with such an ``optical loud speaker'' are outlined.
BibTeX:
@article{Taylor:2011,
  author = {Taylor, J. M. and Sørensen, A. S. and Marcus, C. M. and Polzik, E. S.},
  title = {Laser Cooling and Optical Detection of Excitations in a LC Electrical Circuit},
  journal = {Phys. Rev. Let.},
  year = {2011},
  volume = {107},
  number = {27},
  doi = {10.1103/PhysRevLett.107.273601}
}
Pedersen KGL, Andersen BM, Bruun GM, Syljuåsen OF and Sørensen AS (2011), "Measuring spin correlations in optical lattices using superlattice potentials", Phys. Rev. A., OCT 11, 2011. Vol. 84(4)
Abstract: We suggest two experimental methods for probing both short-and long-range spin correlations of atoms in optical lattices using superlattice potentials. The first method involves an adiabatic doubling of the periodicity of the underlying lattice to probe neighboring singlet (triplet) correlations for fermions (bosons) by the occupation of the resulting vibrational ground state. The second method utilizes a time-dependent superlattice potential to generate spin-dependent transport by any number of prescribed lattice sites, and probes correlations by the resulting number of doubly occupied sites. For experimentally relevant parameters, we demonstrate how both methods yield large sigNatures of antiferromagnetic correlations of strongly repulsive fermionic atoms in a single shot of the experiment. Lastly, we show how this method may also be applied to probe d-wave pairing, a possible ground-state candidate for the doped repulsive Hubbard model.
BibTeX:
@article{Pedersen:2011,
  author = {Pedersen, K. G. L. and Andersen, B. M. and Bruun, G. M. and Syljuåsen, O. F. and Sørensen, A. S.},
  title = {Measuring spin correlations in optical lattices using superlattice potentials},
  journal = {Phys. Rev. A},
  year = {2011},
  volume = {84},
  number = {4},
  doi = {10.1103/PhysRevA.84.041603}
}
Stannigel K, Rabl P, Sørensen AS, Lukin MD and Zoller P (2011), "Optomechanical transducers for quantum-information processing", Phys. Rev. A., OCT 31, 2011. Vol. 84(4)
Abstract: We discuss the implementation of optical quantum networks where the interface between stationary and photonic qubits is realized by optomechanical transducers [K. Stannigel et al., Phys. Rev. Lett. 105, 220501 (2010)]. This approach does not rely on the optical properties of the qubit and thereby enables optical quantum communication applications for a wide range of solid-state spin-and charge-based systems. We present an effective description of such networks for many qubits and give a derivation of a state transfer protocol for long-distance quantum communication. We also describe how to mediate local on-chip interactions by means of the optomechanical transducers that can be used for entangling gates. We finally discuss experimental systems for the realization of our proposal.
BibTeX:
@article{Stannigel:2011,
  author = {Stannigel, K. and Rabl, P. and Sørensen, A. S. and Lukin, M. D. and Zoller, P.},
  title = {Optomechanical transducers for quantum-information processing},
  journal = {Phys. Rev. A},
  year = {2011},
  volume = {84},
  number = {4},
  doi = {10.1103/PhysRevA.84.042341}
}
Zeuthen E, Grodecka-Grad A and Sørensen AS (2011), "Three-dimensional theory of quantum memories based on Lambda-type atomic ensembles", Phys. Rev. A., OCT 24, 2011. Vol. 84(4)
Abstract: We develop a three-dimensional theory for quantum memories based on light storage in ensembles of Lambda-type atoms, where two long-lived atomic ground states are employed. We consider light storage in an ensemble of finite spatial extent and we show that within the paraxial approximation the Fresnel number of the atomic ensemble and the optical depth are the only important physical parameters determining the quality of the quantum memory. We analyze the influence of these parameters on the storage of light followed by either forward or backward read-out from the quantum memory. We show that for small Fresnel numbers the forward memory provides higher efficiencies, whereas for large Fresnel numbers the backward memory is advantageous. The optimal light modes to store in the memory are presented together with the corresponding spin waves and outcoming light modes. We show that for high optical depths such Lambda-type atomic ensembles allow for highly efficient backward and forward memories even for small Fresnel numbers F greater than or similar to 0.1.
BibTeX:
@article{Zeuthen:2011,
  author = {Zeuthen, Emil and Grodecka-Grad, Anna and Sørensen, Anders S.},
  title = {Three-dimensional theory of quantum memories based on Lambda-type atomic ensembles},
  journal = {Phys. Rev. A},
  year = {2011},
  volume = {84},
  number = {4},
  doi = {10.1103/PhysRevA.84.043838}
}
Goldstein G, Cappellaro P, Maze JR, Hodges JS, Jiang L, Sørensen AS and Lukin MD (2011), "Environment-Assisted Precision Measurement", Phys. Rev. Let.., APR 8, 2011. Vol. 106(14)
Abstract: We describe a method to enhance the sensitivity of precision measurements that takes advantage of the environment of a quantum sensor to amplify the response of the sensor to weak external perturbations. An individual qubit is used to sense the dynamics of surrounding ancillary qubits, which are in turn affected by the external field to be measured. The resulting sensitivity enhancement is determined by the number of ancillas that are coupled strongly to the sensor qubit; it does not depend on the exact values of the coupling strengths and is resilient to many forms of decoherence. The method achieves nearly Heisenberg-limited precision measurement, using a novel class of entangled states. We discuss specific applications to improve clock sensitivity using trapped ions and magnetic sensing based on electronic spins in diamond.
BibTeX:
@article{Goldstein:2011,
  author = {Goldstein, G. and Cappellaro, P. and Maze, J. R. and Hodges, J. S. and Jiang, L. and Sørensen, A. S. and Lukin, M. D.},
  title = {Environment-Assisted Precision Measurement},
  journal = {Phys. Rev. Let.},
  year = {2011},
  volume = {106},
  number = {14},
  doi = {10.1103/PhysRevLett.106.140502}
}
Andersen ML, Stobbe S, Sørensen AS and Lodahl P (2011), "Strongly modified plasmon-matter interaction with mesoscopic quantum emitters", Nature Physics., MAR, 2011. Vol. 7(3), pp. 215-218.
Abstract: Semiconductor quantum dots (QDs) provide useful means to couple light and matter in applications such as light-harvesting(1,2) and all-solid-state quantum information processing(3,4). This coupling can be increased by placing QDs in nanostructured optical environments such as photonic crystals or metallic nanostructures that enable strong confinement of light and thereby enhance the light-matter interaction. It has thus far been assumed that QDs can be described in the same way as atomic photon emitters-as point sources with wavefunctions whose spatial extent can be disregarded. Here we demonstrate that this description breaks down for QDs near plasmonic nanostructures. We observe an eightfold enhancement of the plasmon excitation rate, depending on QD orientation as a result of their mesoscopic character. Moreover, we show that the interaction can be enhanced or suppressed, determined by the geometry of the plasmonic nanostructure, consistent with a newly developed theory that takes mesoscopic effects into account. This behaviour has no equivalence in atomic systems and offers new opportunities to exploit the unique mesoscopic characteristics of QDs in the development of nanophotonic devices that use the increased light-matter interaction.
BibTeX:
@article{Andersen:2011,
  author = {Andersen, Mads Lykke and Stobbe, Soren and Sørensen, Anders S. and Lodahl, Peter},
  title = {Strongly modified plasmon-matter interaction with mesoscopic quantum emitters},
  journal = {Nature Physics},
  year = {2011},
  volume = {7},
  number = {3},
  pages = {215-218},
  doi = {10.1038/NPHYS1870}
}
Kastoryano MJ, Reiter F and Sørensen AS (2011), "Dissipative Preparation of Entanglement in Optical Cavities", Phys. Rev. Let.., FEB 28, 2011. Vol. 106(9)
Abstract: We propose a novel scheme for the preparation of a maximally entangled state of two atoms in an optical cavity. Starting from an arbitrary initial state, a singlet state is prepared as the unique fixed point of a dissipative quantum dynamical process. In our scheme, cavity decay is no longer undesirable, but plays an integral part in the dynamics. As a result, we get a qualitative improvement in the scaling of the fidelity with the cavity parameters. Our analysis indicates that dissipative state preparation is more than just a new conceptual approach, but can allow for significant improvement as compared to preparation protocols based on coherent unitary dynamics.
BibTeX:
@article{Kastoryano:2011,
  author = {Kastoryano, M. J. and Reiter, F. and Sørensen, A. S.},
  title = {Dissipative Preparation of Entanglement in Optical Cavities},
  journal = {Phys. Rev. Let.},
  year = {2011},
  volume = {106},
  number = {9},
  doi = {10.1103/PhysRevLett.106.090502}
}
Marcos D, Wubs M, Taylor JM, Aguado R, Lukin MD and Sørensen AS (2010), "Coupling Nitrogen-Vacancy Centers in Diamond to Superconducting Flux Qubits", Phys. Rev. Let.., NOV 17, 2010. Vol. 105(21)
Abstract: We propose a method to achieve coherent coupling between nitrogen-vacancy (NV) centers in diamond and superconducting (SC) flux qubits. The resulting coupling can be used to create a coherent interaction between the spin states of distant NV centers mediated by the flux qubit. Furthermore, the magnetic coupling can be used to achieve a coherent transfer of quantum information between the flux qubit and an ensemble of NV centers. This enables a long-term memory for a SC quantum processor and possibly an interface between SC qubits and light.
BibTeX:
@article{Marcos:2010,
  author = {Marcos, D. and Wubs, M. and Taylor, J. M. and Aguado, R. and Lukin, M. D. and Sørensen, A. S.},
  title = {Coupling Nitrogen-Vacancy Centers in Diamond to Superconducting Flux Qubits},
  journal = {Phys. Rev. Let.},
  year = {2010},
  volume = {105},
  number = {21},
  doi = {10.1103/PhysRevLett.105.210501}
}
Stannigel K, Rabl P, Sørensen AS, Zoller P and Lukin MD (2010), "Optomechanical Transducers for Long-Distance Quantum Communication", Phys. Rev. Let.., NOV 23, 2010. Vol. 105(22)
Abstract: We describe a new scheme to interconvert stationary and photonic qubits which is based on indirect qubit-light interactions mediated by a mechanical resonator. This approach does not rely on the specific optical response of the qubit and thereby enables optical quantum interfaces for a wide range of solid state spin and charge based systems. We discuss the implementation of state transfer protocols between distant nodes of a quantum network and show that high transfer fidelities can be achieved under realistic experimental conditions.
BibTeX:
@article{Stannigel:2010,
  author = {Stannigel, K. and Rabl, P. and Sørensen, A. S. and Zoller, P. and Lukin, M. D.},
  title = {Optomechanical Transducers for Long-Distance Quantum Communication},
  journal = {Phys. Rev. Let.},
  year = {2010},
  volume = {105},
  number = {22},
  doi = {10.1103/PhysRevLett.105.220501}
}
Brask JB, Rigas I, Polzik ES, Andersen UL and Sørensen AS (2010), "Hybrid Long-Distance Entanglement Distribution Protocol", Phys. Rev. Let.., OCT 11, 2010. Vol. 105(16)
Abstract: We propose a hybrid (continuous-discrete variable) quantum repeater protocol for long-distance entanglement distribution. Starting from states created by single-photon detection, we show how entangled coherent state superpositions can be generated by means of homodyne detection. We show that near-deterministic entanglement swapping with such states is possible using only linear optics and homodyne detectors, and we evaluate the performance of our protocol combining these elements.
BibTeX:
@article{Brask:2010a,
  author = {Brask, J. B. and Rigas, I. and Polzik, E. S. and Andersen, U. L. and Sørensen, A. S.},
  title = {Hybrid Long-Distance Entanglement Distribution Protocol},
  journal = {Phys. Rev. Let.},
  year = {2010},
  volume = {105},
  number = {16},
  doi = {10.1103/PhysRevLett.105.160501}
}
Dzsotjan D, Sørensen AS and Fleischhauer M (2010), "Quantum emitters coupled to surface plasmons of a nanowire: A Green's function approach", Phys. Rev. B., AUG 27, 2010. Vol. 82(7)
Abstract: We investigate a system consisting of a single, as well as two emitters strongly coupled to surface plasmon modes of a nanowire using a Green's function approach. Explicit expressions are derived for the spontaneous decay rate into the plasmon modes and for the atom-plasmon coupling as well as a plasmon-mediated atomatom coupling. Phenomena due to the presence of losses in the metal are discussed. In case of two atoms, we observe Dicke subradiance and superradiance resulting from their plasmon-mediated interaction. Based on this phenomenon, we propose a scheme for a deterministic two-qubit quantum gate. We also discuss a possible realization of interesting many-body Hamiltonians, such as the spin-boson model, using strong emitterplasmon coupling.
BibTeX:
@article{Dzsotjan:2010,
  author = {Dzsotjan, David and Sørensen, Anders S. and Fleischhauer, Michael},
  title = {Quantum emitters coupled to surface plasmons of a nanowire: A Green's function approach},
  journal = {Phys. Rev. B},
  year = {2010},
  volume = {82},
  number = {7},
  doi = {10.1103/PhysRevB.82.075427}
}
Togan E, Chu Y, Trifonov AS, Jiang L, Maze J, Childress L, Dutt MVG, Sørensen AS, Hemmer PR, Zibrov AS and Lukin MD (2010), "Quantum entanglement between an optical photon and a solid-state spin qubit", Nature., AUG 5, 2010. Vol. 466(7307), pp. 730-U4.
Abstract: Quantum entanglement is among the most fascinating aspects of quantum theory(1). Entangled optical photons are now widely used for fundamental tests of quantum mechanics(2) and applications such as quantum cryptography(1). Several recent experiments demonstrated entanglement of optical photons with trapped ions(3), atoms(4,5) and atomic ensembles(6-8), which are then used to connect remote long-term memory nodes in distributed quantum networks(9-11). Here we realize quantum entanglement between the polarization of a single optical photon and a solid-state qubit associated with the single electronic spin of a nitrogen vacancy centre in diamond. Our experimental entanglement verification uses the quantum eraser technique(5,12), and demonstrates that a high degree of control over interactions between a solid-state qubit and the quantum light field can be achieved. The reported entanglement source can be used in studies of fundamental quantum phenomena and provides a key building block for the solid-state realization of quantum optical networks(13,14).
BibTeX:
@article{Togan:2010,
  author = {Togan, E. and Chu, Y. and Trifonov, A. S. and Jiang, L. and Maze, J. and Childress, L. and Dutt, M. V. G. and Sørensen, A. S. and Hemmer, P. R. and Zibrov, A. S. and Lukin, M. D.},
  title = {Quantum entanglement between an optical photon and a solid-state spin qubit},
  journal = {Nature},
  year = {2010},
  volume = {466},
  number = {7307},
  pages = {730-U4},
  doi = {10.1038/Nature09256}
}
Gawarecki K, Pochwala M, Grodecka-Grad A and Machnikowski P (2010), "Phonon-assisted relaxation and tunneling in self-assembled quantum dot molecules", Phys. Rev. B., JUN 14, 2010. Vol. 81(24)
Abstract: We study theoretically phonon-assisted relaxation processes in a system consisting of one or two electrons confined in two vertically stacked self-assembled quantum dots. The calculation is based on a k.p approximation for single-particle wave functions in a strained self-assembled structure. From these, two-particle states are calculated by including the Coulomb interaction and the transition rates between the lowest-energy eigenstates are derived. We take into account phonon couplings via deformation potential and piezoelectric interaction and show that they both can play a dominant role in different parameter regimes. Within the Fermi golden rule approximation, we calculate the relaxation rates between the lowest-energy eigenstates which lead to thermalization on a picosecond time scale in a narrow range of dot sizes.
BibTeX:
@article{Gawarecki:2010,
  author = {Gawarecki, Krzysztof and Pochwala, Michal and Grodecka-Grad, Anna and Machnikowski, Pawel},
  title = {Phonon-assisted relaxation and tunneling in self-assembled quantum dot molecules},
  journal = {Phys. Rev. B},
  year = {2010},
  volume = {81},
  number = {24},
  doi = {10.1103/PhysRevB.81.245312}
}
Sørensen AS, Altman E, Gullans M, Porto JV, Lukin MD and Demler E (2010), "Adiabatic preparation of many-body states in optical lattices", Phys. Rev. A., JUN 22, 2010. Vol. 81(6)
Abstract: We analyze a technique for the preparation of low-entropy many-body states of atoms in optical lattices based on adiabatic passage. In particular, we show that this method allows preparation of strongly correlated states as stable highest energy states of Hamiltonians that have trivial ground states. As an example, we analyze the generation of antiferromagnetically ordered states by adiabatic change of a staggered field acting on the spins of bosonic atoms with ferromagnetic interactions.
BibTeX:
@article{Sorensen:2010,
  author = {Sørensen, Anders S. and Altman, Ehud and Gullans, Michael and Porto, J. V. and Lukin, Mikhail D. and Demler, Eugene},
  title = {Adiabatic preparation of many-body states in optical lattices},
  journal = {Phys. Rev. A},
  year = {2010},
  volume = {81},
  number = {6},
  doi = {10.1103/PhysRevA.81.061603}
}
Witthaut D and Sørensen AS (2010), "Photon scattering by a three-level emitter in a one-dimensional waveguide", New Journal of Physics., APR 30, 2010. Vol. 12
Abstract: We discuss the scattering of photons from a three-level emitter in a one-dimensional waveguide, where the transport is governed by the interference of spontaneously emitted and directly transmitted waves. The scattering problem is solved in closed form for different level structures. Several possible applications are discussed: the state of the emitter can be switched deterministically by Raman scattering, thus enabling applications in quantum computing such as a single-photon transistor. An array of emitters gives rise to a photonic band gap structure, which can be tuned by a classical driving laser. A disordered array leads to Anderson localization of photons, where the localization length can again be controlled by an external driving.
BibTeX:
@article{Witthaut:2010,
  author = {Witthaut, D. and Sørensen, A. S.},
  title = {Photon scattering by a three-level emitter in a one-dimensional waveguide},
  journal = {New Journal of Physics},
  year = {2010},
  volume = {12},
  doi = {10.1088/1367-2630/12/4/043052}
}
Hammerer K, Sørensen AS and Polzik ES (2010), "Quantum interface between light and atomic ensembles", Rev. Mod. Phys.., APR 5, 2010. Vol. 82(2), pp. 1041-1093.
Abstract: During the past decade the interaction of light with multiatom ensembles has attracted much attention as a basic building block for quantum information processing and quantum state engineering. The field started with the realization that optically thick free space ensembles can be efficiently interfaced with quantum optical fields. By now the atomic ensemble-light interfaces have become a powerful alternative to the cavity-enhanced interaction of light with single atoms. Various mechanisms used for the quantum interface are discussed, including quantum nondemolition or Faraday interaction, quantum measurement and feedback, Raman interaction, photon echo, and electromagnetically induced transparency. This review provides a common theoretical frame for these processes, describes basic experimental techniques and media used for quantum interfaces, and reviews several key experiments on quantum memory for light, quantum entanglement between atomic ensembles and light, and quantum teleportation with atomic ensembles. The two types of quantum measurements which are most important for the interface are discussed: homodyne detection and photon counting. This review concludes with an outlook on the future of atomic ensembles as an enabling technology in quantum information processing.
BibTeX:
@article{Hammerer:2010,
  author = {Hammerer, Klemens and Sørensen, Anders S. and Polzik, Eugene S.},
  title = {Quantum interface between light and atomic ensembles},
  journal = {Rev. Mod. Phys.},
  year = {2010},
  volume = {82},
  number = {2},
  pages = {1041-1093},
  doi = {10.1103/RevModPhys.82.1041}
}
Brask JB, Jiang L, Gorshkov AV, Vuletic V, Sørensen AS and Lukin MD (2010), "Fast entanglement distribution with atomic ensembles and fluorescent detection", Phys. Rev. A., FEB, 2010. Vol. 81(2)
Abstract: Quantum repeaters based on atomic ensemble quantum memories are promising candidates for achieving scalable distribution of entanglement over long distances. Recently, important experimental progress has been made toward their implementation. However, the entanglement rates and scalability of current approaches are limited by relatively low retrieval and single-photon detector efficiencies. We propose a scheme which makes use of fluorescent detection of stored excitations to significantly increase the efficiency of connection and hence the rate. Practical performance and possible experimental realizations of the new protocol are discussed.
BibTeX:
@article{Brask:2010,
  author = {Brask, J. B. and Jiang, L. and Gorshkov, A. V. and Vuletic, V. and Sørensen, A. S. and Lukin, M. D.},
  title = {Fast entanglement distribution with atomic ensembles and fluorescent detection},
  journal = {Phys. Rev. A},
  year = {2010},
  volume = {81},
  number = {2},
  doi = {10.1103/PhysRevA.81.020303}
}
Jiang L, Taylor JM, Sørensen AS and Lukin MD (2010), "Scalable quantum networks based on few-qubit registers", Int. J. of Quantum Information., FEB-MAR, 2010. Vol. 8(1-2), pp. 93-104.
Abstract: We describe and analyze a hybrid approach to scalable quantum computation based on an optically connected network of few-qubit quantum registers. We show that probabilistically connected five-qubit quantum registers suffice for deterministic, fault-tolerant quantum computation even when state preparation, measurement, and entanglement generation all have substantial errors. We discuss requirements for achieving fault-tolerant operation for two specific implementations of our approach.
BibTeX:
@article{Jiang:2010,
  author = {Jiang, Liang and Taylor, Jacob M. and Sørensen, Anders S. and Lukin, Mikhail D.},
  title = {Scalable quantum networks based on few-qubit registers},
  journal = {Int. J. of Quantum Information},
  year = {2010},
  volume = {8},
  number = {1-2},
  pages = {93-104},
  doi = {10.1142/S0219749910006058}
}
Bruun GM, Syljuåsen OF, Pedersen KGL, Andersen BM, Demler E and Sørensen AS (2009), "Antiferromagnetic noise correlations in optical lattices", Phys. Rev. A., SEP, 2009. Vol. 80(3)
Abstract: We analyze how noise correlations probed by time-of-flight experiments reveal antiferromagnetic (AF) correlations of fermionic atoms in two-dimensional and three-dimensional optical lattices. Combining analytical and quantum Monte Carlo calculations using experimentally realistic parameters, we show that AF correlations can be detected for temperatures above and below the critical temperature for AF ordering. It is demonstrated that spin-resolved noise correlations yield important information about the spin ordering. Finally, we show how to extract the spin correlation length and the related critical exponent of the AF transition from the noise.
BibTeX:
@article{Bruun:2009a,
  author = {Bruun, G. M. and Syljuåsen, O. F. and Pedersen, K. G. L. and Andersen, B. M. and Demler, E. and Sørensen, A. S.},
  title = {Antiferromagnetic noise correlations in optical lattices},
  journal = {Phys. Rev. A},
  year = {2009},
  volume = {80},
  number = {3},
  doi = {10.1103/PhysRevA.80.033622}
}
Sørensen MW and Sørensen AS (2009), "Three-dimensional theory of stimulated Raman scattering", Phys. Rev. A., SEP, 2009. Vol. 80(3)
Abstract: We present a three-dimensional theory of stimulated Raman scattering (SRS) or super-radiance. In particular we address how the spatial and temporal properties of the generated SRS beam or Stokes beam of radiation depends on the spatial properties of the gain medium. Maxwell equations for the Stokes field operators and of the atomic operators are solved analytically and a correlation function for the Stokes field is derived. In the analysis we identify a super-radiating part of the Stokes radiation that exhibit beam characteristics. We show how the intensity in this beam builds up in time and at some point largely dominates the total Stokes radiation of the gain medium. We show how the SRS depends on the Fresnel number and the optical depth and that in fact these two factors are the only factors describing the coherent radiation.
BibTeX:
@article{Sorensen:2009,
  author = {Sørensen, Martin W. and Sørensen, Anders S.},
  title = {Three-dimensional theory of stimulated Raman scattering},
  journal = {Phys. Rev. A},
  year = {2009},
  volume = {80},
  number = {3},
  doi = {10.1103/PhysRevA.80.033804}
}
Tualle-Brouri R, Ourjoumtsev A, Dantan A, Grangier P, Wubs M and Sørensen AS (2009), "Multimode model for projective photon-counting measurements", Phys. Rev. A., JUL, 2009. Vol. 80(1)
Abstract: We present a general model to account for the multimode Nature of the quantum electromagnetic field in projective photon-counting measurements. We focus on photon-subtraction experiments, where non-Gaussian states are produced conditionally. These are useful states for continuous-variable quantum-information processing. We present a general method called mode reduction that reduces the multimode model to an effective two-mode problem. We apply this method to a multimode model describing broadband parametric down-conversion, thereby improving the analysis of existing experimental results. The main improvement is that spatial and frequency filters before the photon detector are taken into account explicitly. We find excellent agreement with previously published experimental results, using fewer free parameters than before, and discuss the implications of our analysis for the optimized production of states with negative Wigner functions.
BibTeX:
@article{Tualle-Brouri:2009,
  author = {Tualle-Brouri, Rosa and Ourjoumtsev, Alexei and Dantan, Aurelien and Grangier, Philippe and Wubs, Martijn and Sørensen, Anders S.},
  title = {Multimode model for projective photon-counting measurements},
  journal = {Phys. Rev. A},
  year = {2009},
  volume = {80},
  number = {1},
  doi = {10.1103/PhysRevA.80.013806}
}
Huck A, Smolka S, Lodahl P, Sørensen AS, Boltasseva A, Janousek J and Andersen UL (2009), "Demonstration of Quadrature-Squeezed Surface Plasmons in a Gold Waveguide", Phys. Rev. Let.., JUN 19, 2009. Vol. 102(24)
Abstract: We report on the efficient generation, propagation, and reemission of squeezed long-range surface-plasmon polaritons in a gold waveguide. Squeezed light is used to excite the nonclassical surface-plasmon polaritons, and the reemitted quantum state is fully characterized by complete quantum tomographic reconstruction of the density matrix. We find that the plasmon-assisted transmission of nonclassical light in metallic waveguides can be described by a beam splitter relation. This result is explained theoretically.
BibTeX:
@article{Huck:2009,
  author = {Huck, Alexander and Smolka, Stephan and Lodahl, Peter and Sørensen, Anders S. and Boltasseva, Alexandra and Janousek, Jiri and Andersen, Ulrik L.},
  title = {Demonstration of Quadrature-Squeezed Surface Plasmons in a Gold Waveguide},
  journal = {Phys. Rev. Let.},
  year = {2009},
  volume = {102},
  number = {24},
  doi = {10.1103/PhysRevLett.102.246802}
}
Doll R, Haenggi P, Kohler S and Wubs M (2009), "Fast initial qubit dephasing and the influence of substrate dimensions on error correction rates", European physical journal B., APR, 2009. Vol. 68(4), pp. 523-527.
Abstract: Keeping single-qubit quantum coherence above some threshold value not far below unity is a prerequisite for fault-tolerant quantum error correction (QEC). We study the initial dephasing of solid-state qubits in the independent-boson model, which describes well recent experiments on quantum dot (QD) excitons both in bulk and in substrates of reduced geometry such as nanotubes. Using explicit expressions for the exact coherence dynamics, a minimal QEC rate is identified in terms of error threshold, temperature, and qubit-environment coupling strength. This allows us to systematically study the benefit of a current trend towards substrates with reduced dimensions.
BibTeX:
@article{Doll:2009,
  author = {Doll, R. and Haenggi, P. and Kohler, S. and Wubs, M.},
  title = {Fast initial qubit dephasing and the influence of substrate dimensions on error correction rates},
  journal = {European physical journal B},
  year = {2009},
  volume = {68},
  number = {4},
  pages = {523-527},
  doi = {10.1140/epjb/e2009-00115-6}
}
Bruun GM, Andersen BM, Demler E and Sørensen AS (2009), "Probing Spatial Spin Correlations of Ultracold Gases by Quantum Noise Spectroscopy", Phys. Rev. Let.., JAN 23, 2009. Vol. 102(3)
Abstract: Spin noise spectroscopy with a single laser beam is demonstrated theoretically to provide a direct probe of the spatial correlations of cold fermionic gases. We show how the generic many-body phenomena of antibunching, pairing, antiferromagnetic, and algebraic spin liquid correlations can be revealed by measuring the spin noise as a function of laser width, temperature, and frequency.
BibTeX:
@article{Bruun:2009,
  author = {Bruun, G. M. and Andersen, Brian M. and Demler, Eugene and Sørensen, Anders S.},
  title = {Probing Spatial Spin Correlations of Ultracold Gases by Quantum Noise Spectroscopy},
  journal = {Phys. Rev. Let.},
  year = {2009},
  volume = {102},
  number = {3},
  doi = {10.1103/PhysRevLett.102.030401}
}
Trimborn F, Witthaut D and Korsch HJ (2009), "Beyond mean-field dynamics of small Bose-Hubbard systems based on the number-conserving phase-space approach", Phys. Rev. A., JAN, 2009. Vol. 79(1)
Abstract: The number-conserving quantum phase space description of the Bose-Hubbard model is discussed for the illustrative case of two and three modes, as well as the generalization of the two-mode case to an open quantum system. The phase-space description based on generalized SU(M) coherent states yields a Liouvillian flow in the macroscopic limit, which can be efficiently simulated using Monte Carlo methods even for large systems. We show that this description clearly goes beyond the common mean-field limit. In particular it resolves well-known problems where the common mean-field approach fails, such as the description of dynamical instabilities and chaotic dynamics. Moreover, it provides a valuable tool for a semiclassical approximation of many interesting quantities, which depend on higher moments of the quantum state and are therefore not accessible within the common approach. As a prominent example, we analyze the depletion and heating of the condensate. A comparison to methods ignoring the fixed particle number shows that in this case artificial number fluctuations lead to ambiguities and large deviations even for quite simple examples.
BibTeX:
@article{Trimborn:2009,
  author = {Trimborn, F. and Witthaut, D. and Korsch, H. J.},
  title = {Beyond mean-field dynamics of small Bose-Hubbard systems based on the number-conserving phase-space approach},
  journal = {Phys. Rev. A},
  year = {2009},
  volume = {79},
  number = {1},
  doi = {10.1103/PhysRevA.79.013608}
}
Hafezi M, Sørensen AS, Lukin MD and Demler E (2008), "Characterization of topological states on a lattice with Chern number", Europhysics Letters. Vol. 81(1)
Abstract: We study Chern numbers to characterize the ground state of strongly interacting systems on a lattice. This method allows us to perform a numerical characterization of bosonic fractional quantum Hall (FQH) states on a lattice where the conventional overlap calculation with the known continuum case such as the Laughlin state, breaks down due to the lattice structure or dipole-dipole interaction. The non-vanishing Chern number indicates the existence of a topological order in the degenerate ground-state manifold.
BibTeX:
@article{Hafezi:2008,
  author = {Hafezi, M. and Sørensen, A. S. and Lukin, M. D. and Demler, E.},
  title = {Characterization of topological states on a lattice with Chern number},
  journal = {Europhysics Letters},
  year = {2008},
  volume = {81},
  number = {1},
  doi = {10.1209/0295-5075/81/10005}
}
Witthaut D, Trimborn F and Wimberger S (2008), "Dissipation Induced Coherence of a Two-Mode Bose-Einstein Condensate", Phys. Rev. Let.., NOV 14, 2008. Vol. 101(20)
Abstract: We discuss the dynamics of a Bose-Einstein condensate in a double-well trap subject to phase noise and particle loss. The phase coherence of a weakly interacting condensate as well as the response to an external driving show a pronounced stochastic resonance effect: Both quantities become maximal for a finite value of the dissipation rate matching the intrinsic time scales of the system. Even stronger effects are observed when dissipation acts in concurrence with strong interparticle interactions, restoring the purity of the condensate almost completely and increasing the phase coherence significantly.
BibTeX:
@article{Witthaut:2008,
  author = {Witthaut, D. and Trimborn, F. and Wimberger, S.},
  title = {Dissipation Induced Coherence of a Two-Mode Bose-Einstein Condensate},
  journal = {Phys. Rev. Let.},
  year = {2008},
  volume = {101},
  number = {20},
  doi = {10.1103/PhysRevLett.101.200402}
}
Trimborn F, Witthaut D and Wimberger S (2008), "Mean-field dynamics of a two-mode Bose-Einstein condensate subject to noise and dissipation", J. Phys. B - Atomic molecular and optical physicseu., SEP 14, 2008. Vol. 41(17)
Abstract: We discuss the dynamics of an open two-mode Bose-Hubbard system subject to phase noise and particle dissipation. Starting from the full many-body dynamics described by a master equation the mean-field limit is derived resulting in an effective non-Hermitian (discrete) Gross-Pitaevskii equation which has been introduced only phenomenologically up to now. The familiar mean-field phase-space structure is substantially altered by the dissipation. In particular, the character of the fixed points shows an abrupt transition from elliptic or hyperbolic to attractive or repulsive, respectively. This reflects the metastable behaviour of the corresponding many-body system which surprisingly also leads to a significant increase of the purity of the condensate. A comparison of the mean-field approximation to simulations of the full master equation using the Monte Carlo wavefunction method shows an excellent agreement for wide parameter ranges.
BibTeX:
@article{Trimborn:2008,
  author = {Trimborn, F. and Witthaut, D. and Wimberger, S.},
  title = {Mean-field dynamics of a two-mode Bose-Einstein condensate subject to noise and dissipation},
  journal = {J. Phys. B - Atomic molecular and optical physicseu},
  year = {2008},
  volume = {41},
  number = {17},
  doi = {10.1088/0953-4075/41/17/171001}
}
Fernholz T, Krauter H, Jensen K, Sherson JF, Sørensen AS and Polzik ES (2008), "Spin squeezing of atomic ensembles via nuclear-electronic spin entanglement", Phys. Rev. Let.., AUG 15, 2008. Vol. 101(7)
Abstract: We demonstrate spin squeezing in a room temperature ensemble of approximate to 10^12 cesium atoms using their internal structure, where the necessary entanglement is created between nuclear and electronic spins of each individual atom. This state provides improvement in measurement sensitivity beyond the standard quantum limit for quantum memory experiments and applications in quantum metrology and is thus a complementary alternative to spin squeezing obtained via interatom entanglement. Squeezing of the collective spin is verified by quantum state tomography.
BibTeX:
@article{Fernholz:2008,
  author = {Fernholz, T. and Krauter, H. and Jensen, K. and Sherson, J. F. and Sørensen, A. S. and Polzik, E. S.},
  title = {Spin squeezing of atomic ensembles via nuclear-electronic spin entanglement},
  journal = {Phys. Rev. Let.},
  year = {2008},
  volume = {101},
  number = {7},
  doi = {10.1103/PhysRevLett.101.073601}
}
Brask JB and Sørensen AS (2008), "Memory imperfections in atomic-ensemble-based quantum repeaters", Phys. Rev. A., JUL, 2008. Vol. 78(1)
Abstract: Quantum repeaters promise to deliver long-distance entanglement overcoming loss in realistic quantum channels. A promising class of repeaters, based on atomic ensemble quantum memories and linear optics, follows the proposal by L.-M. Duan , Nature (London) 414, 413 (2001). Here we analyze this protocol in terms of a very general model for the quantum memories employed. We derive analytical expressions for scaling of entanglement with memory imperfections, dark counts, loss, and distance, and we apply our results to two specific quantum memory protocols. Our methods apply to any quantum memory with an interaction Hamiltonian at most quadratic in the mode operators and are in principle extendible to more recent modifications of the original proposal of Duan, Lukin, Cirac, and Zoller.
BibTeX:
@article{Brask:2008,
  author = {Brask, Jonatan Bohr and Sørensen, Anders Søndberg},
  title = {Memory imperfections in atomic-ensemble-based quantum repeaters},
  journal = {Phys. Rev. A},
  year = {2008},
  volume = {78},
  number = {1},
  doi = {10.1103/PhysRevA.78.012350}
}
Doll R, Wubs M, Kohler S and Haenggi P (2008), "Fidelity and entanglement of a spatially extended linear three-qubit register", Int. J. Quantum Inf.., JUL, 2008. Vol. 6(1), pp. 681-687.
Abstract: We study decoherence of a three-qubit array coupled to substrate phonons. Assuming an initial three-qubit entangled state that would be decoherence-free for identical qubit positions, allows us to focus on non-Markovian effects of the inevitable spatial qubit separation. It turns out that the coherence is most affected when the qubits are regularly spaced. Moreover, we find that up to a constant scaling factor, two-qubit entanglement is not influenced by the presence of the third qubit, even though all qubits interact via the phonon field.
BibTeX:
@article{Doll:2008,
  author = {Doll, Roland and Wubs, Martijn and Kohler, Sigmund and Haenggi, Peter},
  title = {Fidelity and entanglement of a spatially extended linear three-qubit register},
  journal = {Int. J. Quantum Inf.},
  year = {2008},
  volume = {6},
  number = {1},
  pages = {681-687},
  note = {International Meeting on Noise, Information and Complexity AT Quantum Scale, Erice, ITALY, NOV 02-10, 2007},
  doi = {10.1142/S0219749908003955}
}
Gorshkov AV, Calarco T, Lukin MD and Sørensen AS (2008), "Photon storage in Lambda-type optically dense atomic media. IV. Optimal control using gradient ascent", Phys. Rev. A., APR, 2008. Vol. 77(4)
Abstract: We use the numerical gradient ascent method from optimal control theory to extend efficient photon storage in Lambda-type media to previously inaccessible regimes and to provide simple intuitive explanations for our optimization techniques. In particular, by using gradient ascent to shape classical control pulses used to mediate photon storage, we open up the possibility of high efficiency photon storage in the nonadiabatic limit, in which analytical solutions to the equations of motion do not exist. This control shaping technique enables an order-of-magnitude increase in the bandwidth of the memory. We also demonstrate that the often discussed connection between time reversal and optimality in photon storage follows naturally from gradient ascent. Finally, we discuss the optimization of controlled reversible inhomogeneous broadening.
BibTeX:
@article{Gorshkov:2008,
  author = {Gorshkov, Alexey V. and Calarco, Tommaso and Lukin, Mikhail D. and Sørensen, Anders S.},
  title = {Photon storage in Lambda-type optically dense atomic media. IV. Optimal control using gradient ascent},
  journal = {Phys. Rev. A},
  year = {2008},
  volume = {77},
  number = {4},
  doi = {10.1103/PhysRevA.77.043806}
}
Sørensen MW and Sørensen AS (2008), "Three-dimensional theory for light-matter interaction", Phys. Rev. A., JAN, 2008. Vol. 77(1)
Abstract: We present a full quantum mechanical three-dimensional theory describing an electromagnetic field interacting with an ensemble of identical atoms. The theory is constructed such that it describes recent experiments on light-matter quantum interfaces, where the quantum fluctuations of light are mapped onto the atoms and back onto light. We show that the interaction of the light with the atoms may be separated into a mean effect of the ensemble and a deviation from the mean. The mean effect of the interaction effectively gives rise to an index of refraction of the gas. We formally change to a dressed state picture, where the light modes are solutions to the diffraction problem, and develop a perturbative expansion in the fluctuations. The fluctuations are due to quantum fluctuations as well as the random positions of the atoms. In this perturbative expansion we show how the quantum fluctuations are mapped between atoms and light while the random positioning of the atoms give rise to decay due to spontaneous emission. Furthermore we identify limits, where the full three-dimensional theory reduces to the one-dimensional theory typically used to describe the interaction.
BibTeX:
@article{Sorensen:2008,
  author = {Sørensen, Martin W. and Sørensen, Anders S.},
  title = {Three-dimensional theory for light-matter interaction},
  journal = {Phys. Rev. A},
  year = {2008},
  volume = {77},
  number = {1},
  doi = {10.1103/PhysRevA.77.013826}
}
Jiang L, Taylor JM, Sørensen AS and Lukin MD (2007), "Distributed quantum computation based on small quantum registers", Phys. Rev. A., DEC, 2007. Vol. 76(6)
Abstract: We describe and analyze an efficient register-based hybrid quantum computation scheme. Our scheme is based on a probabilistic, heralded optical connection among local five-qubit quantum registers. We assume high-fidelity local unitary operations within each register, but the error probability for initialization, measurement, and entanglement generation can be very high (similar to 5. We demonstrate that with a reasonable time overhead our scheme can achieve deterministic nonlocal coupling gates between arbitrary two registers with very high fidelity, limited only by the imperfections from the local unitary operation. We estimate the clock cycle and the effective error probability for implementation of quantum registers with ion traps or nitrogen-vacancy centers. Our scheme capitalizes on an efficient two-level pumping scheme that in principle can create Bell pairs with arbitrarily high fidelity. We introduce a Markov chain model to study the stochastic process of entanglement pumping and map it onto a deterministic process. Finally we discuss requirements for achieving fault-tolerant operation with our register-based hybrid scheme and also present an alternative approach to fault-tolerant preparation of Greenberger-Horne-Zeilinger states.
BibTeX:
@article{Jiang:2007,
  author = {Jiang, Liang and Taylor, Jacob M. and Sørensen, Anders S. and Lukin, Mikhail D.},
  title = {Distributed quantum computation based on small quantum registers},
  journal = {Phys. Rev. A},
  year = {2007},
  volume = {76},
  number = {6},
  doi = {10.1103/PhysRevA.76.062323}
}
Chang DE, Sørensen AS, Demler EA and Lukin MD (2007), "A single-photon transistor using nanoscale surface plasmons", Nature Physics., NOV, 2007. Vol. 3(11), pp. 807-812.
Abstract: Photons rarely interact-which makes it challenging to build all-optical devices in which one light signal controls another. Even in nonlinear optical media, in which two beams can interact because of their influence on the medium's refractive index, this interaction is weak at low light levels. Here, we propose a novel approach to realizing strong nonlinear interactions at the single-photon level, by exploiting the strong coupling between individual optical emitters and propagating surface plasmons confined to a conducting nanowire. We show that this system can act as a nonlinear two-photon switch for incident photons propagating along the nanowire, which can be coherently controlled using conventional quantum-optical techniques. Furthermore, we discuss how the interaction can be tailored to create a single-photon transistor, where the presence (or absence) of a single incident photon in a `gate' field is sufficient to allow (or prevent) the propagation of subsequent `signal' photons along the wire.
BibTeX:
@article{Chang:2007,
  author = {Chang, Darrick E. and Sørensen, Anders S. and Demler, Eugene A. and Lukin, Mikhail D.},
  title = {A single-photon transistor using nanoscale surface plasmons},
  journal = {Nature Physics},
  year = {2007},
  volume = {3},
  number = {11},
  pages = {807-812},
  doi = {10.1038/nphys708}
}
Gorshkov AV, Andre A, Lukin MD and Sørensen AS (2007), "Photon storage in Lambda-type optically dense atomic media. I. Cavity model", Phys. Rev. A., SEP, 2007. Vol. 76(3)
Abstract: In a recent paper [Gorshkov , Phys. Rev. Lett. 98, 123601 (2007)], we used a universal physical picture to optimize and demonstrate equivalence between a wide range of techniques for storage and retrieval of photon wave packets in Lambda-type atomic media in free space, including the adiabatic reduction of the photon group velocity, pulse-propagation control via off-resonant Raman techniques, and photon-echo-based techniques. In the present paper, we perform the same analysis for the cavity model. In particular, we show that the retrieval efficiency is equal to C/(1+C) independent of the retrieval technique, where C is the cooperativity parameter. We also derive the optimal strategy for storage and, in particular, demonstrate that at any detuning one can store, with the optimal efficiency of C/(1+C), any smooth input mode satisfying TC γ >> 1 and a certain class of resonant input modes satisfying TC γ similar to 1, where T is the duration of the input mode and 2 γ is the transition linewidth. In the two subsequent papers of the series, we present the full analysis of the free-space model and discuss the effects of inhomogeneous broadening on photon storage.
BibTeX:
@article{Gorshkov:2007a,
  author = {Gorshkov, Alexey V. and Andre, Axel and Lukin, Mikhail D. and Sørensen, Anders S.},
  title = {Photon storage in Lambda-type optically dense atomic media. I. Cavity model},
  journal = {Phys. Rev. A},
  year = {2007},
  volume = {76},
  number = {3},
  doi = {10.1103/PhysRevA.76.033804}
}
Gorshkov AV, Andre A, Lukin MD and Sørensen AS (2007), "Photon storage in Lambda-type optically dense atomic media. II. Free-space model", Phys. Rev. A., SEP, 2007. Vol. 76(3)
Abstract: In a recent paper [Gorshkov , Phys. Rev. Lett. 98, 123601 (2007)], we presented a universal physical picture for describing a wide range of techniques for storage and retrieval of photon wave packets in Lambda-type atomic media in free space, including the adiabatic reduction of the photon group velocity, pulse-propagation control via off-resonant Raman techniques, and photon-echo based techniques. This universal picture produced an optimal control strategy for photon storage and retrieval applicable to all approaches and yielded identical maximum efficiencies for all of them. In the present paper, we present the full details of this analysis as well some of its extensions, including the discussion of the effects of non-degeneracy of the two lower levels of the Lambda system. The analysis in the present paper is based on the intuition obtained from the study of photon storage in the cavity model in the preceding paper [Gorshkov , previous paper, Phys. Rev. A. 76, 033804 (2007)].
BibTeX:
@article{Gorshkov:2007b,
  author = {Gorshkov, Alexey V. and Andre, Axel and Lukin, Mikhail D. and Sørensen, Anders S.},
  title = {Photon storage in Lambda-type optically dense atomic media. II. Free-space model},
  journal = {Phys. Rev. A},
  year = {2007},
  volume = {76},
  number = {3},
  doi = {10.1103/PhysRevA.76.033805}
}
Gorshkov AV, Andre A, Lukin MD and Sørensen AS (2007), "Photon storage in Lambda-type optically dense atomic media. III. Effects of inhomogeneous broadening", Phys. Rev. A., SEP, 2007. Vol. 76(3)
Abstract: In a recent paper [Gorshkov , Phys. Rev. Lett. 98, 123601 (2007)] and in the two preceding papers [Gorshkov , this issue, Phys. Rev. A 76, 033804 (2006); 76, 033805 (2006)], we used a universal physical picture to optimize and demonstrate equivalence between a wide range of techniques for storage and retrieval of photon wave packets in homogeneously broadened Lambda-type atomic media, including the adiabatic reduction of the photon group velocity, pulse-propagation control via off-resonant Raman techniques, and photon-echo-based techniques. In the present paper, we generalize this treatment to include inhomogeneous broadening. In particular, we consider the case of Doppler-broadened atoms and assume that there is a negligible difference between the Doppler shifts of the two optical transitions. In this situation, we show that, at high enough optical depth, all atoms contribute coherently to the storage process as if the medium were homogeneously broadened. We also discuss the effects of inhomogeneous broadening in solid state samples. In this context, we discuss the advantages and limitations of reversing the inhomogeneous broadening during the storage time, as well as suggest a way for achieving high efficiencies with a nonreversible inhomogeneous profile.
BibTeX:
@article{Gorshkov:2007c,
  author = {Gorshkov, Alexey V. and Andre, Axel and Lukin, Mikhail D. and Sørensen, Anders S.},
  title = {Photon storage in Lambda-type optically dense atomic media. III. Effects of inhomogeneous broadening},
  journal = {Phys. Rev. A},
  year = {2007},
  volume = {76},
  number = {3},
  doi = {10.1103/PhysRevA.76.033806}
}
Hafezi M, Sørensen AS, Demler E and Lukin MD (2007), "Fractional quantum Hall effect in optical lattices", Phys. Rev. A., AUG, 2007. Vol. 76(2)
Abstract: We analyze a recently proposed method to create fractional quantum Hall (FQH) states of atoms confined in optical lattices [A. Sørensen , Phys. Rev. Lett. 94, 086803 (2005)]. Extending the previous work, we investigate conditions under which the FQH effect can be achieved for bosons on a lattice with an effective magnetic field and finite on-site interaction. Furthermore, we characterize the ground state in such systems by calculating Chern numbers which can provide direct sigNatures of topological order and explore regimes where the characterization in terms of wave-function overlap fails. We also discuss various issues which are relevant for the practical realization of such FQH states with ultracold atoms in an optical lattice, including the presence of a long-range dipole interaction which can improve the energy gap and stabilize the ground state. We also investigate a detection technique based on Bragg spectroscopy to probe these systems in an experimental realization.
BibTeX:
@article{Hafezi:2007,
  author = {Hafezi, M. and Sørensen, A. S. and Demler, E. and Lukin, M. D.},
  title = {Fractional quantum Hall effect in optical lattices},
  journal = {Phys. Rev. A},
  year = {2007},
  volume = {76},
  number = {2},
  doi = {10.1103/PhysRevA.76.023613}
}
Chang DE, Sørensen AS, Hemmer PR and Lukin MD (2007), "Strong coupling of single emitters to surface plasmons", Phys. Rev. B., JUL, 2007. Vol. 76(3)
Abstract: We propose a method that enables strong, coherent coupling between individual optical emitters and electromagnetic excitations in conducting nanostructures. The excitations are optical plasmons that can be localized to subwavelength dimensions. Under realistic conditions, the tight confinement causes optical emission to be almost entirely directed into the propagating plasmon modes via a mechanism analogous to cavity quantum electrodynamics. We first illustrate this result for the case of a nanowire, before considering the optimized geometry of a nanotip. We describe an application of this technique involving efficient single-photon generation on demand, in which the plasmons are efficiently outcoupled to a dielectric waveguide. Finally, we analyze the effects of increased scattering due to surface roughness on these nanostructures.
BibTeX:
@article{Chang:2007a,
  author = {Chang, D. E. and Sørensen, A. S. and Hemmer, P. R. and Lukin, M. D.},
  title = {Strong coupling of single emitters to surface plasmons},
  journal = {Phys. Rev. B},
  year = {2007},
  volume = {76},
  number = {3},
  doi = {10.1103/PhysRevB.76.035420}
}
Flindt C, Sørensen AS, Lukin MD and Taylor JM (2007), "Spin-photon entangling diode", Phys. Rev. Let.., JUN 15, 2007. Vol. 98(24)
Abstract: We propose a semiconductor device that can electrically generate entangled electron spin-photon states, providing a building block for entanglement of distant spins. The device consists of a p-i-n diode structure that incorporates a coupled double quantum dot. We show that electronic control of the diode bias and local gating allow for the generation of single photons that are entangled with a robust quantum memory based on the electron spins. Practical performance of this approach to controlled spin-photon entanglement is analyzed.
BibTeX:
@article{Flindt:2007,
  author = {Flindt, Christian and Sørensen, Anders S. and Lukin, Mikhail D. and Taylor, Jacob M.},
  title = {Spin-photon entangling diode},
  journal = {Phys. Rev. Let.},
  year = {2007},
  volume = {98},
  number = {24},
  doi = {10.1103/PhysRevLett.98.240501}
}
Novikova I, Gorshkov AV, Phillips DF, Sørensen AS, Lukin MD and Walsworth RL (2007), "Optimal control of light pulse storage and retrieval", Phys. Rev. Let.., JUN 15, 2007. Vol. 98(24)
Abstract: We demonstrate experimentally a procedure to obtain the maximum efficiency for the storage and retrieval of light pulses in atomic media. The procedure uses time-reversal to obtain optimal input signal pulse shapes. Experimental results in warm Rb vapor are in good agreement with theoretical predictions and demonstrate a substantial improvement of efficiency. This optimization procedure is applicable to a wide range of systems.
BibTeX:
@article{Novikova:2007,
  author = {Novikova, Irina and Gorshkov, Alexey V. and Phillips, David F. and Sørensen, Anders S. and Lukin, Mikhail D. and Walsworth, Ronald L.},
  title = {Optimal control of light pulse storage and retrieval},
  journal = {Phys. Rev. Let.},
  year = {2007},
  volume = {98},
  number = {24},
  doi = {10.1103/PhysRevLett.98.243602}
}
Gorshkov AV, Andre A, Fleischhauer M, Sørensen AS and Lukin MD (2007), "Universal approach to optimal photon storage in atomic media", Phys. Rev. Let.., MAR 23, 2007. Vol. 98(12)
Abstract: We present a universal physical picture for describing storage and retrieval of photon wave packets in a Lambda-type atomic medium. This physical picture encompasses a variety of different approaches to pulse storage ranging from adiabatic reduction of the photon group velocity and pulse-propagation control via off-resonant Raman fields to photon-echo-based techniques. Furthermore, we derive an optimal control strategy for storage and retrieval of a photon wave packet of any given shape. All these approaches, when optimized, yield identical maximum efficiencies, which only depend on the optical depth of the medium.
BibTeX:
@article{Gorshkov:2007,
  author = {Gorshkov, Alexey V. and Andre, Axel and Fleischhauer, Michael and Sørensen, Anders S. and Lukin, Mikhail D.},
  title = {Universal approach to optimal photon storage in atomic media},
  journal = {Phys. Rev. Let.},
  year = {2007},
  volume = {98},
  number = {12},
  doi = {10.1103/PhysRevLett.98.123601}
}
Flindt C, Sørensen AS and Flensberg K (2006), "Spin-orbit mediated control of spin qubits", Phys. Rev. Let.., DEC 15, 2006. Vol. 97(24)
Abstract: We propose to use the spin-orbit interaction as a means to control electron spins in quantum dots, enabling both single-qubit and two-qubit operations. Very fast single-qubit operations may be achieved by temporarily displacing the electrons. For two-qubit operations the coupling mechanism is based on a combination of the spin-orbit coupling and the mutual long-ranged Coulomb interaction. Compared to existing schemes using the exchange coupling, the spin-orbit induced coupling is less sensitive to random electrical fluctuations in the electrodes defining the quantum dots.
BibTeX:
@article{Flindt:2006,
  author = {Flindt, Christian and Sørensen, Anders S. and Flensberg, Karsten},
  title = {Spin-orbit mediated control of spin qubits},
  journal = {Phys. Rev. Let.},
  year = {2006},
  volume = {97},
  number = {24},
  doi = {10.1103/PhysRevLett.97.240501}
}
Chang DE, Sørensen AS, Hemmer PR and Lukin MD (2006), "Quantum optics with surface plasmons", Phys. Rev. Let.., AUG 4, 2006. Vol. 97(5)
Abstract: We describe a technique that enables strong, coherent coupling between individual optical emitters and guided plasmon excitations in conducting nanostructures at optical frequencies. We show that under realistic conditions optical emission can be almost entirely directed into the plasmon modes. As an example, we describe an application of this technique involving efficient generation of single photons on demand, in which the plasmon is efficiently outcoupled to a dielectric waveguide.
BibTeX:
@article{Chang:2006,
  author = {Chang, D. E. and Sørensen, A. S. and Hemmer, P. R. and Lukin, M. D.},
  title = {Quantum optics with surface plasmons},
  journal = {Phys. Rev. Let.},
  year = {2006},
  volume = {97},
  number = {5},
  doi = {10.1103/PhysRevLett.97.053002}
}
Sherson J, Sørensen AS, Fiurasek J, Mølmer K and Polzik ES (2006), "Light qubit storage and retrieval using macroscopic atomic ensembles", Phys. Rev. A., JUL, 2006. Vol. 74(1)
Abstract: We present an experimentally feasible protocol for the complete storage and retrieval of arbitrary light states in an atomic quantum memory using the Faraday interaction between light and matter. Our protocol relies on multiple passages of a single light pulse through the atomic ensemble without the impractical requirement of kilometer-long delay lines between the passages. A time-dependent interaction strength enables the storage and retrieval of states with arbitrary pulse shapes. The fidelity approaches unity exponentially without squeezed or entangled initial states, as illustrated by calculations for a photonic qubit. (c) 2006 American Institute of Physics.
BibTeX:
@article{Sherson:2006,
  author = {Sherson, J. and Sørensen, A. S. and Fiurasek, J. and Mølmer, K. and Polzik, E. S.},
  title = {Light qubit storage and retrieval using macroscopic atomic ensembles},
  journal = {Phys. Rev. A},
  year = {2006},
  volume = {74},
  number = {1},
  doi = {10.1103/PhysRevA.74.011802}
}
Childress L, Taylor J, Sørensen A and Lukin M (2006), "Fault-tolerant quantum communication based on solid-state photon emitters", Phys. Rev. Let.., FEB 24, 2006. Vol. 96(7)
Abstract: We describe a novel protocol for a quantum repeater that enables long-distance quantum communication through realistic, lossy photonic channels. Contrary to previous proposals, our protocol incorporates active purification of arbitrary errors at each step of the protocol using only two qubits at each repeater station. Because of these minimal physical requirements, the present protocol can be realized in simple physical systems such as solid-state single photon emitters. As an example, we show how nitrogen-vacancy color centers in diamond can be used to implement the protocol, using the nuclear and electronic spin to form the two qubits.
BibTeX:
@article{Childress:2006,
  author = {Childress, L and Taylor, JM and Sørensen, AS and Lukin, MD},
  title = {Fault-tolerant quantum communication based on solid-state photon emitters},
  journal = {Phys. Rev. Let.},
  year = {2006},
  volume = {96},
  number = {7},
  doi = {10.1103/PhysRevLett.96.070504}
}
Childress L, Taylor J, Sørensen A and Lukin M (2005), "Fault-tolerant quantum repeaters with minimal physical resources and implementations based on single-photon emitters", Phys. Rev. A., NOV, 2005. Vol. 72(5)
Abstract: We analyze a method that uses fixed, minimal physical resources to achieve generation and nested purification of quantum entanglement for quantum communication over arbitrarily long distances and discuss its implementation using realistic photon emitters and photonic channels. In this method, we use single-photon emitters with two internal degrees of freedom formed by an electron spin and a nuclear spin to build intermediate nodes in a quantum channel. State-selective fluorescence is used for probablistic entanglement generation between electron spins in adjacent nodes. We analyze in detail several approaches which are applicable to realistic, homogeneously broadened single-photon emitters. Furthermore, the coupled electron and nuclear spins can be used to efficiently implement entanglement swapping and purification. We show that these techniques can be combined to generate high-fidelity entanglement over arbitrarily long distances. We present a specific protocol that functions in polynomial time and tolerates percent-level errors in entanglement fidelity and local operations. The scheme has the lowest requirements on physical resources of any current scheme for fully fault-tolerant quantum repeaters.
BibTeX:
@article{Childress:2005,
  author = {Childress, L and Taylor, JM and Sørensen, AS and Lukin, MD},
  title = {Fault-tolerant quantum repeaters with minimal physical resources and implementations based on single-photon emitters},
  journal = {Phys. Rev. A},
  year = {2005},
  volume = {72},
  number = {5},
  doi = {10.1103/PhysRevA.72.052330}
}
Sørensen A, Demler E and Lukin M (2005), "Fractional quantum hall states of atoms in optical lattices", Phys. Rev. Let.., MAR 4, 2005. Vol. 94(8)
Abstract: We describe a method to create fractional quantum Hall states of atoms confined in optical lattices. We show that the dynamics of the atoms in the lattice is analogous to the motion of a charged particle in a magnetic field if an oscillating quadrupole potential is applied together with a periodic modulation of the tunneling between lattice sites. In a suitable parameter regime the ground state in the lattice is of the fractional quantum Hall type, and we show how these states can be reached by melting a Mott-insulator state in a superlattice potential. Finally, we discuss techniques to observe these strongly correlated states.
BibTeX:
@article{Sorensen:2005,
  author = {Sørensen, AS and Demler, E and Lukin, MD},
  title = {Fractional quantum hall states of atoms in optical lattices},
  journal = {Phys. Rev. Let.},
  year = {2005},
  volume = {94},
  number = {8},
  doi = {10.1103/PhysRevLett.94.086803}
}
Andre A, Sørensen A and Lukin M (2004), "Stability of atomic clocks based on entangled atoms", Phys. Rev. Let.., JUN 11, 2004. Vol. 92(23)
Abstract: We analyze the effect of realistic noise sources for an atomic clock consisting of a local oscillator that is actively locked to a spin-squeezed (entangled) ensemble of N atoms. We show that the use of entangled states can lead to an improvement of the long-term stability of the clock when the measurement is limited by decoherence associated with instability of the local oscillator combined with fluctuations in the atomic ensemble's Bloch vector. Atomic states with a moderate degree of entanglement yield the maximal clock stability, resulting in an improvement that scales as N-1/6 compared to the atomic shot noise level.
BibTeX:
@article{Andre:2004,
  author = {Andre, A and Sørensen, AS and Lukin, MD},
  title = {Stability of atomic clocks based on entangled atoms},
  journal = {Phys. Rev. Let.},
  year = {2004},
  volume = {92},
  number = {23},
  doi = {10.1103/PhysRevLett.92.230801}
}
Childress L, Sørensen A and Lukin M (2004), "Mesoscopic cavity quantum electrodynamics with quantum dots", Phys. Rev. A., APR, 2004. Vol. 69(4)
Abstract: We describe an electrodynamic mechanism for coherent, quantum-mechanical coupling between spatially separated quantum dots on a microchip. The technique is based on capacitive interactions between the electron charge and a superconducting transmission line resonator, and is closely related to atomic cavity quantum electrodynamics. We investigate several potential applications of this technique which have varying degrees of complexity. In particular, we demonstrate that this mechanism allows design and investigation of an on-chip double-dot microscopic maser. Moreover, the interaction may be extended to couple spatially separated electron-spin states while only virtually populating fast-decaying superpositions of charge states. This represents an effective, controllable long-range interaction, which may facilitate implementation of quantum information processing with electron-spin qubits and potentially allow coupling to other quantum systems such as atomic or superconducting qubits.
BibTeX:
@article{Childress:2004,
  author = {Childress, L and Sørensen, AS and Lukin, MD},
  title = {Mesoscopic cavity quantum electrodynamics with quantum dots},
  journal = {Phys. Rev. A},
  year = {2004},
  volume = {69},
  number = {4},
  doi = {10.1103/PhysRevA.69.042302}
}
Sørensen A, van der Wal C, Childress L and Lukin M (2004), "Capacitive coupling of atomic systems to mesoscopic conductors", Phys. Rev. Let.., FEB 13, 2004. Vol. 92(6)
Abstract: We describe a technique that enables a strong, coherent coupling between isolated neutral atoms and mesoscopic conductors. The coupling is achieved by exciting atoms trapped above the surface of a superconducting transmission line into Rydberg states with large electric dipole moments that induce voltage fluctuations in the transmission line. Using a mechanism analogous to cavity quantum electrodynamics, an atomic state can be transferred to a long-lived mode of the fluctuating voltage, atoms separated by millimeters can be entangled, or the quantum state of a solid-state device can be mapped onto atomic or photonic states.
BibTeX:
@article{Sorensen:2004,
  author = {Sørensen, AS and van der Wal, CH and Childress, LI and Lukin, MD},
  title = {Capacitive coupling of atomic systems to mesoscopic conductors},
  journal = {Phys. Rev. Let.},
  year = {2004},
  volume = {92},
  number = {6},
  doi = {10.1103/PhysRevLett.92.063601}
}
Sørensen A and Mølmer K (2003), "Measurement induced entanglement and quantum computation with atoms in optical cavities", Phys. Rev. Let.., AUG 29, 2003. Vol. 91(9)
Abstract: We propose a method to prepare entangled states and implement quantum computation with atoms in optical cavities. The internal states of the atoms are entangled by a measurement of the phase of light transmitted through the cavity. By repeated measurements an entangled state is created with certainty, and this entanglement can be used to implement gates on qubits which are stored in different internal degrees of freedom of the atoms. This method, based on measurement induced dynamics, has a higher fidelity than schemes making use of controlled unitary dynamics.
BibTeX:
@article{Sorensen:2003a,
  author = {Sørensen, AS and Mølmer, K},
  title = {Measurement induced entanglement and quantum computation with atoms in optical cavities},
  journal = {Phys. Rev. Let.},
  year = {2003},
  volume = {91},
  number = {9},
  doi = {10.1103/PhysRevLett.91.097905}
}
Sørensen A and Mølmer K (2003), "Probabilistic generation of entanglement in optical cavities", Phys. Rev. Let.., MAR 28, 2003. Vol. 90(12)
Abstract: We propose to produce entanglement by measuring the reflection from an optical cavity. Conditioned on the detection of a reflected photon, pairs of atoms in the cavity are prepared in maximally entangled states. The success probability depends on the cavity parameters, but high quality entangled states may be produced with a high probability even for cavities of moderate quality.
BibTeX:
@article{Sorensen:2003,
  author = {Sørensen, AS and Mølmer, K},
  title = {Probabilistic generation of entanglement in optical cavities},
  journal = {Phys. Rev. Let.},
  year = {2003},
  volume = {90},
  number = {12},
  doi = {10.1103/PhysRevLett.90.127903}
}
Sørensen A and Mølmer K (2002), "Entangling atoms in bad cavities", Phys. Rev. A., AUG, 2002. Vol. 66(2)
Abstract: We propose a method to produce entangled spin squeezed states of a large number of atoms inside an optical cavity. By illuminating the atoms with bichromatic light, the coupling to the cavity induces pairwise exchange of excitations which entangles the atoms. Unlike most proposals for entangling atoms by cavity QED, our proposal does not require the strong coupling regime g^2/κ>1, where g is the atom cavity coupling strength, kappa is the cavity decay rate, and Γ is the decay rate of the atoms. In this work the important parameter is Ng^2/κΓ, where N is the number of atoms, and our proposal permits the production of entanglement in bad cavities as long as they contain a large number of atoms.
BibTeX:
@article{Sorensen:2002a,
  author = {Sørensen, AS and Mølmer, K},
  title = {Entangling atoms in bad cavities},
  journal = {Phys. Rev. A},
  year = {2002},
  volume = {66},
  number = {2},
  doi = {10.1103/PhysRevA.66.022314}
}
Sørensen A (2002), "Bogoliubov theory of entanglement in a Bose-Einstein condensate", Phys. Rev. A., APR, 2002. Vol. 65(4, B)
Abstract: We consider a Bose-Einstein condensate that is illuminated by a short resonant light pulse that coherently couples two internal states of the atoms. We show that the subsequent time evolution prepares the atoms in an interesting entangled state called a spin-squeezed state. This evolution is analyzed in detail by developing a Bogoliubov theory that describes the entanglement of the atoms. Our calculation is a consistent expansion in 1/N, where N is the number of particles in the condensate, and our theory predicts that it is possible to produce spin-squeezing by at least a factor of 1/N. Within the Bogoliubov approximation this result is independent of temperature.
BibTeX:
@article{Sorensen:2002,
  author = {Sørensen, AS},
  title = {Bogoliubov theory of entanglement in a Bose-Einstein condensate},
  journal = {Phys. Rev. A},
  year = {2002},
  volume = {65},
  number = {4, B},
  doi = {10.1103/PhysRevA.65.043610}
}
Wang X, Sørensen A and Mølmer K (2001), "Spin squeezing in the Ising model", Phys. Rev. A., NOV, 2001. Vol. 64(5)
Abstract: We analyze the collective spin noise in interacting spin systems. General expressions are derived for the short-time behavior of spin systems with general spin-spin interactions, and we suggest optimum experimental conditions for the detection of spin squeezing. For Ising models with site-dependent nearest-neighbor interactions, general expressions are presented for the spin squeezing parameter for all times. The reduction of collective spin noise can be used to verify the entangling powers of quantum computer architectures based on interacting spins.
BibTeX:
@article{Wang:2001a,
  author = {Wang, XG and Sørensen, AS and Mølmer, K},
  title = {Spin squeezing in the Ising model},
  journal = {Phys. Rev. A},
  year = {2001},
  volume = {64},
  number = {5}
}
Sørensen A and Mølmer K (2001), "Entanglement and extreme spin squeezing", Phys. Rev. Let.., MAY 14, 2001. Vol. 86(20), pp. 4431-4434.
Abstract: For any mean value of a Cartesian component of a spin vector we identify the smallest possible uncertainty in any of the orthogonal components. The corresponding states are optimal for spectroscopy and atomic clocks. We show that the results for different spin J can be used to identify entanglement and to quantify the depth of entanglement in systems with many particles. With the procedure developed in this Letter, collective spin measurements on an ensemble of particles can be used as an experimental proof of multiparticle entanglement.
BibTeX:
@article{Sorensen:2001a,
  author = {Sørensen, AS and Mølmer, K},
  title = {Entanglement and extreme spin squeezing},
  journal = {Phys. Rev. Let.},
  year = {2001},
  volume = {86},
  number = {20},
  pages = {4431-4434},
  doi = {10.1103/PhysRevLett.86.4431}
}
Wang X, Sørensen A and Mølmer K (2001), "Multibit gates for quantum computing", Phys. Rev. Let.., APR 23, 2001. Vol. 86(17), pp. 3907-3910.
Abstract: We present a general technique to implement products of many qubit operators communicating via a joint harmonic oscillator degree of freedom in a quantum computer. By conditional displacements and rotations we can implement Hamiltonians which are trigonometric functions of qubit operators. With such operators we can effectively implement higher order gates such as Toffoli gates and C-n-NOT gates, and we show that the entire Grover search algorithm can be implemented in a direct way.
BibTeX:
@article{Wang:2001,
  author = {Wang, XG and Sørensen, A and Mølmer, K},
  title = {Multibit gates for quantum computing},
  journal = {Phys. Rev. Let.},
  year = {2001},
  volume = {86},
  number = {17},
  pages = {3907-3910},
  doi = {10.1103/PhysRevLett.86.3907}
}
Sørensen A, Duan L, Cirac J and Zoller P (2001), "Many-particle entanglement with Bose-Einstein condensates", Nature., JAN 4, 2001. Vol. 409(6816), pp. 63-66.
Abstract: The possibility of creating and manipulating entangled states of systems of many particles is of significant interest for quantum information processing; such a capability could lead to new applications that rely on the basic principles of quantum mechanics(1). So far, up to four atoms have been entangled in a controlled way(2,3). A crucial requirement for the production of entangled states is that they can be considered pure at the single-particle level. Bose-Einstein condensates(4-6) fulfil this requirement; hence it is natural to investigate whether they can also be used in some applications of quantum information. Here we propose a method to achieve substantial entanglement of a large number of atoms in a Bose-Einstein condensate. A single resonant laser pulse is applied to all the atoms in the condensate, which is then allowed to evolve freely; in this latter stage, collisional interactions produce entanglement between the atoms. The technique should be realizable with present technology.
BibTeX:
@article{Sorensen:2001,
  author = {Sørensen, A and Duan, LM and Cirac, JI and Zoller, P},
  title = {Many-particle entanglement with Bose-Einstein condensates},
  journal = {Nature},
  year = {2001},
  volume = {409},
  number = {6816},
  pages = {63-66},
  doi = {10.1038/35051038}
}
Sørensen A and Mølmer K (2000), "Ion trap quantum computer with bichromatic light", Fortschritte der Physik---Progress of Physics. Vol. 48(9-11), pp. 811-821.
Abstract: In the ion trap quantum computer, quantum bits are represented by the internal state of the trapped ions, and the collective vibrations of the ions are exploited for the coherent manipulation of the state of several bits. We have developed a method to perform gates on different ions by illuminating the: ions with bichromatic light. Our gates are independent of the motional state of the ions and they may be applied to ions in any state or, e.g., thermal mixture of vibrational states. Moreover, our proposal allows the production of multiparticle entangled states even in situations without experimental access to the individual ions in the trap. Maximally entangled states of any number of ions are produced by simply illuminating all ions with the same beams of bichromatic light.
BibTeX:
@article{Sorensen:2000a,
  author = {Sørensen, A and Mølmer, K},
  title = {Ion trap quantum computer with bichromatic light},
  journal = {Fortschritte der Physik---Progress of Physics},
  year = {2000},
  volume = {48},
  number = {9-11},
  pages = {811-821},
  doi = {10.1002/1521-3978(200009)48:9/113.0.CO;2-Y}
}
Duan L, Sørensen A, Cirac J and Zoller P (2000), "Squeezing and entanglement of atomic beams", Phys. Rev. Let.., NOV 6, 2000. Vol. 85(19), pp. 3991-3994.
Abstract: We propose and analyze a scheme for generating entangled atomic beams out of a Bose-Einstein condensate using spin-exchanging collisions. In particular, we show how to create both atomic squeezed states and entangled states of pairs of atoms.
BibTeX:
@article{Duan:2000,
  author = {Duan, LM and Sørensen, A and Cirac, JI and Zoller, P},
  title = {Squeezing and entanglement of atomic beams},
  journal = {Phys. Rev. Let.},
  year = {2000},
  volume = {85},
  number = {19},
  pages = {3991-3994},
  doi = {10.1103/PhysRevLett.85.3991}
}
Mølmer K and Sørensen A (2000), "RISQ - reduced instruction set quantum computers", Journal of Modern Optics Optics., NOV, 2000. Vol. 47(14-15), pp. 2515-2527.
Abstract: Candidates for quantum computing which offer only restricted control, e.g. due to lack of access to individual qubits, are not useful for general purpose quantum computing. We present concrete proposals for the use of systems with such limitations as RISQ-reduced instruction set quantum computers and devices-for simulation of quantum dynamics, for multi-particle entanglement and squeezing of collective spin variables. These tasks are useful in their own right, and they also provide experimental probes for the functioning of quantum gates in premature prototypes of quantum computers.
BibTeX:
@article{Molmer:2000,
  author = {Mølmer, K and Sørensen, A},
  title = {RISQ - reduced instruction set quantum computers},
  journal = {Journal of Modern Optics Optics},
  year = {2000},
  volume = {47},
  number = {14-15},
  pages = {2515-2527},
  note = {7th Meeting on Laser Phenomena, TYROL, AUSTRIA, JAN 16-21, 2000}
}
Sørensen A and Mølmer K (2000), "Entanglement and quantum computation with ions in thermal motion", Phys. Rev. A., AUG, 2000. Vol. 62(2)
Abstract: With bichromatic fields, it is possible to deterministically produce entangled states of trapped ions. In this paper we present a unified analysis of this process for both weak and strong fields, for slow and fast gates. Simple expressions for the fidelity of creating maximally entangled states of two or an arbitrary number of ions under nonideal conditions are derived and discussed.
BibTeX:
@article{Sorensen:2000,
  author = {Sørensen, A and Mølmer, K},
  title = {Entanglement and quantum computation with ions in thermal motion},
  journal = {Phys. Rev. A},
  year = {2000},
  volume = {62},
  number = {2},
  doi = {10.1103/PhysRevA.62.022311}
}
Sørensen A and Mølmer K (1999), "Spin-spin interaction and spin squeezing in an optical lattice", Phys. Rev. Let.., SEP 13, 1999. Vol. 83(11), pp. 2274-2277.
Abstract: We show that by displacing two optical lattices with respect to each other, we may produce interactions similar to the ones describing ferromagnetism and antiferromagnetism in condensed matter physics. We also show that particularly simple choices of the interaction lead to spin squeezing, which may be used to improve the sensitivity of atomic clocks. Spin squeezing is generated even with partially, and randomly, filled lattices, and our proposal may be implemented with current technology.
BibTeX:
@article{Sorensen:1999a,
  author = {Sørensen, A and Mølmer, K},
  title = {Spin-spin interaction and spin squeezing in an optical lattice},
  journal = {Phys. Rev. Let.},
  year = {1999},
  volume = {83},
  number = {11},
  pages = {2274-2277},
  doi = {10.1103/PhysRevLett.83.2274}
}
Sørensen A and Mølmer K (1999), "Quantum computation with ions in thermal motion", Phys. Rev. Let.., MAR 1, 1999. Vol. 82(9), pp. 1971-1974.
Abstract: We propose an implementation of quantum logic gates via virtual vibrational excitations in an ion-trap quantum computer. Transition paths involving unpopulated vibrational states interfere destructively to eliminate the dependence of rates and revolution frequencies on vibrational quantum numbers. As a consequence, quantum computation becomes feasible with ions whose vibrations are strongly coupled to a thermal reservoir.
BibTeX:
@article{Sorensen:1999,
  author = {Sørensen, A and Mølmer, K},
  title = {Quantum computation with ions in thermal motion},
  journal = {Phys. Rev. Let.},
  year = {1999},
  volume = {82},
  number = {9},
  pages = {1971-1974},
  doi = {10.1103/PhysRevLett.82.1971}
}
Mølmer K and Sørensen A (1999), "Multiparticle entanglement of hot trapped ions", Phys. Rev. Let.., MAR 1, 1999. Vol. 82(9), pp. 1835-1838.
Abstract: We propose an efficient method to produce multiparticle entangled states of ions in an ion trap for which a wide range of interesting effects and applications have been suggested. Our preparation scheme exploits the collective vibrational motion of the ions, but it works in such a way that this motion need not be fully controlled in the experiment. The ions may, e.g., be in thermal motion and exchange mechanical energy with a surrounding heat bath without detrimental effects on the internal state preparation. Our scheme does not require access to the individual ions in the trap. [S0031-9007(99)08580-4].
BibTeX:
@article{Molmer:1999,
  author = {Mølmer, K and Sørensen, A},
  title = {Multiparticle entanglement of hot trapped ions},
  journal = {Phys. Rev. Let.},
  year = {1999},
  volume = {82},
  number = {9},
  pages = {1835-1838},
  doi = {10.1103/PhysRevLett.82.1835}
}
Sørensen A and Mølmer K (1998), "Error-free quantum communication through noisy channels", Phys. Rev. A., OCT, 1998. Vol. 58(4), pp. 2745-2749.
Abstract: We suggest a method to perform a quantum logic gate between distant quantum bits (qubits) by off-resonant field-atom dispersive interactions. The scheme we present is shown to work ideally even in the presence of errors in the photon channels used for communication. The stability against errors arises from the paradoxical situation that the transmitted photons carry no information about the state of the qubits. In contrast to a previous proposal for ideal communication [Phys. Rev. Lett. 78, 4293 (1997)] our proposal only involves single atoms in the sending and receiving devices. [S1050-2947(98)11809-7].
BibTeX:
@article{Sorensen:1998,
  author = {Sørensen, A and Mølmer, K},
  title = {Error-free quantum communication through noisy channels},
  journal = {Phys. Rev. A},
  year = {1998},
  volume = {58},
  number = {4},
  pages = {2745-2749},
  doi = {10.1103/PhysRevA.58.2745}
}

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