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| = ! Volume 92 |
| = !! Issue 21 |
| = * __''Exact Decoherence to Pointer States in Free Open Quantum Systems is Universal''__\n |
| = ;;:Jens Eisert\n |
| = Institut fur Physik, Universitat Potsdam, Am Neuen Palais 10, D-14469 Potsdam, Germany |
| = QOLS, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2BW, United Kingdom \n |
| = (Received 14 October 2003; published 26 May 2004)\n\n |
| = In this Letter it is shown that exact decoherence to minimal uncertainty Gaussian pointer states is generic for free quantum particles coupled to a heat bath. More specifically, the Letter is concerned with damped free particles linearly coupled under product initial conditions to a heat bath at arbitrary temperature, with arbitrary coupling strength and spectral densities covering the Ohmic, sub-Ohmic, and supra-Ohmic regime. Then it is true that there exists a time tc such that for times t>tc the state can always be exactly represented as a mixture (convex combination) of particular minimal uncertainty Gaussian states, regardless of and independent from the initial state. This exact "localization" is hence not a feature specific to high temperature and weak damping limit, but is a generic property of damped free particles. 使2004 The American Physical Society |
| = **URL: http://link.aps.org/abstract/PRL/v92/e210401 |
| = **doi:10.1103/PhysRevLett.92.210401 |
| = **PACS: 03.65.Yz, 03.67.-a, 05.40.Jc |
| = * __''Realization of the Einstein-Podolsky-Rosen Paradox Using Momentum- and Position-Entangled Photons from Spontaneous Parametric Down Conversion''__ |
| = ;;:John C. Howell,1 Ryan S. Bennink,2 Sean J. Bentley,<sup>2</sup> and R. W. Boyd<sup>2</sup> \n |
| = <sup>1</sup>Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA\n |
| = <sup>2</sup>The Institute of Optics, University of Rochester, Rochester, New York 14627, USA \n |
| = (Received 15 September 2003; published 28 May 2004)\n\n |
| = We report on a momentum-position realization of the EPR paradox using direct detection in the near and far fields of the photons emitted by collinear type-II phase-matched parametric down conversion. Using this approach we achieved a measured two-photon momentum-position variance product of 0.012, which dramatically violates the bounds for the EPR and separability criteria. 使2004 The American Physical Society |
| = **URL: http://link.aps.org/abstract/PRL/v92/e210403 |
| = **doi:10.1103/PhysRevLett.92.210403 |
| = |
| = !! Issue 18 |
| = __''Experimental Demonstration of Single Photon Nonlocality''__ |
| = ;;:Bjorn Hessmo,1 Pavel Usachev,<sup>2</sup> Hoshang Heydari,<sup>1</sup> and Gunnar Bjork<sup>1</sup>\n |
| = <sup>1</sup>Department of Microelectronics and Information Technology, Royal Institute of Technology (KTH), S-16440 Kista, Sweden\n |
| = <sup>2</sup>Ioffe Physico-technical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia\n |
| = (Received 26 November 2003; published 4 May 2004)\n\n |
| = In this Letter we experimentally implement a single photon Bell test based on the ideas of S. Tan et al. |
| = \[Phys. Rev. Lett. 66, 252 (1991)\] and L. Hardy \[Phys. Rev. Lett. 73, 2279 (1994)\]. |
| = A double homodyne measurement is used to measure correlations in the Fock space spanned by zero and one photons. |
| = Local oscillators used in the correlation measurement are distributed to two observers by copropagating it |
| = in an orthogonal polarization mode. This method eliminates the need for interferometrical stability in the setup, |
| = consequently making it a robust and scalable method. 使2004 The American Physical Society |
| = **URL: http://link.aps.org/abstract/PRL/v92/e180401 |
| = **doi:10.1103/PhysRevLett.92.180401 |
| = **PACS: 03.65.Ud, 42.50.Xa |
| = **PACS: 03.65.Ud, 03.65.Ta, 42.50.Xa, 42.65.Lm |
| = * __''How to Observe High-Dimensional Two-Photon Entanglement with Only Two Detectors''__\n |
| = ;;:S. S. R. Oemrawsingh, A. Aiello, E. R. Eliel, G. Nienhuis, and J. P. Woerdman \n |
| = Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands |
| = (Received 2 February 2004; published 24 May 2004)\n\n |
| = We propose a novel setup to investigate the entanglement of orbital angular momentum states living |
| = in a high-dimensional Hilbert space. We incorporate noninteger spiral phase plates in spatial analyzers, |
| = enabling us to use only two detectors. The two-photon states that are produced are not confined to a 2▼2-dimensional |
| = Hilbert space, and the setup allows the probing of correlations in a high-dimensional space. |
| = For the special case of half-integer spiral phase plates, we predict that |
| = the Clauser-Horne-Shimony-Holt-Bell parameter S is larger than achievable for two qubits (S = 2), namely, S = 3. |
| = 使2004 The American Physical Society |
| = **URL: http://link.aps.org/abstract/PRL/v92/e217901 |
| = **doi:10.1103/PhysRevLett.92.217901 |
| = **PACS: 03.67.Mn, 42.50.Dv |
| = * __''Qubits versus Bits for Measuring an Integral of a Classical Field''__ |
| = ;;:Lev Vaidman and Zion Mitrani |
| = School of Physics and Astronomy, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel-Aviv University, Tel-Aviv 69978, Israel\n |
| = (Received 31 December 2002; published 25 May 2004)\n\n |
| = Methods for measuring an integral of a classical field via local interaction of classical bits or local interaction |
| = of qubits passing through the field one at a time are analyzed. A quantum method, which has an exponentially |
| = better precision than any classical method we could see, is described. 使2004 The American Physical Society |
| = **URL: http://link.aps.org/abstract/PRL/v92/e217902 |
| = **doi:10.1103/PhysRevLett.92.217902 |
| = **PACS: 03.67.-a, 03.65.Ud, 03.65.Ta |
| = * __''Entanglement as a Precondition for Secure Quantum Key Distribution''__ |
| = ;;:Marcos Curty,<sup>1</sup> Maciej Lewenstein,<sup>2</sup> and Norbert Lutkenhaus<sup>1</sup>\n |
| = <sup>1</sup>Quantum Information Theory Group, Institut fur Theoretische Physik, Universitat Erlangen-Nurnberg, 91058 Erlangen, Germany |
| = <sup>2</sup>Institut fur Theoretische Physik, Universitat Hannover, 30167 Hannover, Germany \n |
| = (Received 21 July 2003; published 27 May 2004)\n\n |
| = We demonstrate that a necessary precondition for an unconditionally secure quantum key distribution |
| = is that both sender and receiver can use the available measurement results to prove the presence of |
| = entanglement in a quantum state that is effectively distributed between them. |
| = One can thus systematically search for entanglement using the class of entanglement witness operators |
| = that can be constructed from the observed data. We apply such analysis to two well-known quantum key distribution |
| = protocols, namely, the 4-state protocol and the 6-state protocol. As a special case, we show that, |
| = for some asymmetric error patterns, the presence of entanglement can be proven even for error rates above 25% |
| = (4-state protocol) and 33% (6-state protocol). 使2004 The American Physical Society |
| = **URL: http://link.aps.org/abstract/PRL/v92/e217903 |
| = **doi:10.1103/PhysRevLett.92.217903 |
| = **PACS: 03.67.Dd, 03.65.Ud, 03.67.Mn |
| = |
| = !! Issue 19 |
| = * __''Density Matrix Perturbation Theory''__ |
| = ;;:Anders M. N. Niklasson and Matt Challacombe\n |
| = Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA\n |
| = (Received 19 September 2003; published 14 May 2004)\n\n |
| = An orbital-free quantum perturbation theory is proposed. |
| = It gives the response of the density matrix upon variation of the Hamiltonian by quadratically convergent recursions |
| = based on perturbed projections. The technique allows treatment of embedded quantum subsystems with a computational |
| = cost scaling linearly with the size of the perturbed region, O(Npert.), and as O(1) with the total system size. |
| = The method allows efficient high order perturbation expansions, as demonstrated with an example involving |
| = a 10th order expansion. Density matrix analogs of Wigner's 2n + 1 rule are also presented. |
| = 使2004 The American Physical Society |
| = **URL: http://link.aps.org/abstract/PRL/v92/e193001 |
| = **doi:10.1103/PhysRevLett.92.193001 |
| = **PACS: 31.15.-p, 02.70.-c, 71.15.-m |
| = * __''Quantum Control of Electron-Phonon Scatterings in Artificial Atoms''__ |
| = ;;:Ulrich Hohenester<sup>1</sup> and Georg Stadler<sup>2</sup>\n |
| = <sup>1</sup>Institut fur Theoretische Physik, Karl-Franzens-Universitat Graz, Universitatsplatz 5, 8010 Graz, Austria\n |
| = <sup>2</sup>Institut fur Mathematik, Karl-Franzens-Universitat Graz, Heinrichstra帕e 36, 8010 Graz, Austria |
| = (Received 25 November 2003; published 10 May 2004)\n\n |
| = The phonon-induced dephasing dynamics in optically excited semiconductor quantum dots is studied |
| = within the frameworks of the independent boson model and optimal control. We show that appropriate tailoring |
| = of laser pulses allow complete control of the optical excitation despite the phonon dephasing, |
| = a finding in marked contrast to other environment couplings. 使2004 The American Physical Society |
| = **URL: http://link.aps.org/abstract/PRL/v92/e196801 |
| = **doi:10.1103/PhysRevLett.92.196801 |
| = **PACS: 73.21.La, 02.60.Pn, 42.50.Ct, 71.38.-k |
| = * __''Complete Conditions for Entanglement Transfer''__ |
| = ;;:M. Paternostro, W. Son, and M. S. Kim\n |
| = School of Mathematics and Physics, The Queen's University, Belfast BT7 1NN, United Kingdom\n |
| = (Received 13 November 2003; published 14 May 2004)\n\n |
| = We investigate the conditions to entangle two qubits interacting with local environments driven by |
| = a continuous-variable correlated field. We find the conditions to transfer the entanglement from |
| = the driving field to the qubits both in dynamical and steady-state cases. |
| = We see how the quantum correlations initially present in the driving field play a critical role |
| = in the entanglement-transfer process. The system we treat is general enough to be adapted to different physical setups. |
| = 使2004 The American Physical Society |
| = **URL: http://link.aps.org/abstract/PRL/v92/e197901 |
| = **doi:10.1103/PhysRevLett.92.197901 |
| = **PACS: 03.67.Mn, 42.50.Dv, 42.50.Pq |
| = * __''Stability of Macroscopic Entanglement under Decoherence''__ |
| = ;;:W. Dur1,<sup>2</sup> and H.-J. Briegel<sup>1</sup>,<sup>2</sup>,<sup>3</sup>\n |
| = <sup>1</sup>Sektion Physik, Ludwig-Maximilians-Universitat Munchen, Theresienstrasse 37, D-80333 Munchen, Germany\n |
| = <sup>2</sup>Institut fur Theoretische Physik, Universitat Innsbruck, A-6020 Innsbruck, Austria\n |
| = <sup>3</sup>Institut fur Quantenoptik and Quanteninformation der Osterreichischen Akademie der Wissenschaften, Innsbruck, Austria\n |
| = (Received 25 July 2003; published 6 May 2004)\n\n |
| = We investigate the lifetime of macroscopic entanglement under the influence of decoherence. |
| = For Greenberger-Horne-Zeilinger type superposition states, we find that the lifetime decreases with the size of the system |
| = (i.e., the number of independent degrees of freedom), and the effective number of subsystems that remain |
| = entangled decreases with time. For a class of other states (e.g., cluster states), however, we show that the lifetime |
| = of entanglement is independent of the size of the system. 使2004 The American Physical Society |
| = **URL: http://link.aps.org/abstract/PRL/v92/e180403 |
| = **doi:10.1103/PhysRevLett.92.180403 |
| = **PACS: 03.65.Yz, 03.65.Ud, 03.67.Mn |
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