Volume 92
Issue 21
- Exact Decoherence to Pointer States in Free Open Quantum Systems is Universal
- Jens Eisert
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
(Received 14 October 2003; published 26 May 2004)
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
- 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,2 and R. W. Boyd2
1Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA
2The Institute of Optics, University of Rochester, Rochester, New York 14627, USA
(Received 15 September 2003; published 28 May 2004)
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
Issue 18
Experimental Demonstration of Single Photon Nonlocality
- Bjorn Hessmo,1 Pavel Usachev,2 Hoshang Heydari,1 and Gunnar Bjork1
1Department of Microelectronics and Information Technology, Royal Institute of Technology (KTH), S-16440 Kista, Sweden
2Ioffe Physico-technical Institute, Russian Academy of Sciences, Politekhnicheskaya ul. 26, St. Petersburg, 194021 Russia
(Received 26 November 2003; published 4 May 2004)
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
- How to Observe High-Dimensional Two-Photon Entanglement with Only Two Detectors
- S. S. R. Oemrawsingh, A. Aiello, E. R. Eliel, G. Nienhuis, and J. P. Woerdman
Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands
(Received 2 February 2004; published 24 May 2004)
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
- 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
(Received 31 December 2002; published 25 May 2004)
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
- Entanglement as a Precondition for Secure Quantum Key Distribution
- Marcos Curty,1 Maciej Lewenstein,2 and Norbert Lutkenhaus1
1Quantum Information Theory Group, Institut fur Theoretische Physik, Universitat Erlangen-Nurnberg, 91058 Erlangen, Germany
2Institut fur Theoretische Physik, Universitat Hannover, 30167 Hannover, Germany
(Received 21 July 2003; published 27 May 2004)
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
Issue 19
- Density Matrix Perturbation Theory
- Anders M. N. Niklasson and Matt Challacombe
Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
(Received 19 September 2003; published 14 May 2004)
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
- Quantum Control of Electron-Phonon Scatterings in Artificial Atoms
- Ulrich Hohenester1 and Georg Stadler2
1Institut fur Theoretische Physik, Karl-Franzens-Universitat Graz, Universitatsplatz 5, 8010 Graz, Austria
2Institut fur Mathematik, Karl-Franzens-Universitat Graz, Heinrichstra帕e 36, 8010 Graz, Austria
(Received 25 November 2003; published 10 May 2004)
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
- Complete Conditions for Entanglement Transfer
- M. Paternostro, W. Son, and M. S. Kim
School of Mathematics and Physics, The Queen's University, Belfast BT7 1NN, United Kingdom
(Received 13 November 2003; published 14 May 2004)
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
- Stability of Macroscopic Entanglement under Decoherence
- W. Dur1,2 and H.-J. Briegel1,2,3
1Sektion Physik, Ludwig-Maximilians-Universitat Munchen, Theresienstrasse 37, D-80333 Munchen, Germany
2Institut fur Theoretische Physik, Universitat Innsbruck, A-6020 Innsbruck, Austria
3Institut fur Quantenoptik and Quanteninformation der Osterreichischen Akademie der Wissenschaften, Innsbruck, Austria
(Received 25 July 2003; published 6 May 2004)
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