Bulletin of the American Physical Society
39th Annual Meeting of the APS Division of Atomic, Molecular, and Optical Physics
Volume 53, Number 7
Tuesday–Saturday, May 27–31, 2008; State College, Pennsylvania
Session K5: Entanglement and Decoherence |
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Chair: Chris Monroe, University of Maryland Room: Nittany Lion Inn Boardroom I |
Thursday, May 29, 2008 2:00PM - 2:12PM |
K5.00001: Generation of Entanglement Outside of the Light Cone James Franson Although it may seem counterintuitive, correlations, entanglement, and mutual information can all be generated at two distant locations in less time than it would take for light to travel between them. These effects are due to the fact that the Feynman propagator, which is proportional to the probability amplitude to emit a photon at one location and then annihilate it at another location, has nonzero values outside of the forward light cone. It should be emphasized that this does not allow messages to be transmitted faster than the speed of light, which would violate causality. These effects are illustrated by considering two distant atoms, one of which is initially in its excited state and the other in its ground state. The probability amplitude for the two atoms to exchange a photon and make a transition to the other state is calculated using perturbation theory and commutator techniques, which gives a result proportional to the Feynman propagator. These effects can be interpreted as being due to the propagation of virtual photons outside of the light cone or as a transfer of pre-existing entanglement from the quantum vacuum. [Preview Abstract] |
Thursday, May 29, 2008 2:12PM - 2:24PM |
K5.00002: The memory loophole Daniel Shanahan The memory loophole supposes that the measurement of an entangled pair is influenced by the measurements of earlier pairs in the same run of measurements. To assert the memory loophole is thus to deny that measurement is intrinsically random. It is argued that measurement might instead involve a process of recovery and equilibrium in the measuring apparatus akin to that described in thermodynamics by Le Chatelier's principle. The predictions of quantum mechanics would then arise from conservation of the measured property in the combined system of apparatus and measured ensemble. Measurement would be consistent with classical laws of conservation, not simply in the classical limit of large numbers, but whatever the size of the ensemble. However variances from quantum mechanical predictions would be self-correcting and centripetal, rather than Markovian and increasing as under the standard theory. Entanglement correlations would persist, not because the entangled particles act in concert (which would entail nonlocality), but because the measurements of the particles were influenced by the one fluctuating state of imbalance in the process of measurement. [Preview Abstract] |
Thursday, May 29, 2008 2:24PM - 2:36PM |
K5.00003: Entanglement versus nonlocality in 3-qubit states Shohini Ghose, Neil Sinclair Quantum correlations or entanglement in coupled systems leads to violations of Bell's inequalities that cannot be explained by a local hidden variable theory. For 2-qubits states, bipartite entanglement (concurrence) is simply related to the Bell-CHSH nonlocality parameter. Our studies focus on a class of 3-qubit states. We consider pure states that cannot be uniquely reconstructed from the reduced two-qubit states, implying that some information is encoded in tripartite correlations. We quantify tripartite entanglement by the 3-tangle [1] and investigate its relationship to the Svetlichny inequality [2] for testing tripartite nonlocality. Although the 3-tangle is always non-zero, we identify some tripartite entangled states for which the Svetlichny inequality is not violated. We analytically confirm previous numerical results [3], and extend past results by using the Svetlichny inequality, which has some advantages over the Mermin inequality used in previous work. We also show that the states considered here are those pure states that have the minimum value of the Svetlichny parameter for a given value of 3-tangle, thus identifying their unique nature in the class of all pure states. Our results highlight the complex nature of entanglement and nonlocality in multipartite states. [1] V. Coffman, et al., Phys. Rev. A 61, 052306 (2000) [2] G. Svetlichny, Phys. Rev. D 35, 3066 (1987) [3] V. Scarani and N. Gisin, J. Phys. A 34, 6043 (2001) [Preview Abstract] |
Thursday, May 29, 2008 2:36PM - 2:48PM |
K5.00004: Entanglement Mechanisms in One-Dimensional Potential Scattering Nathan Harshman, Pavneet Singh When two non-relativistic particles scatter in one dimension, they can become entangled. This entanglement process is constrained by the symmetries of the scattering system and the boundary conditions on the incoming state. Applying these constraints, three different mechanisms of entanglement can be identified: the superposition of reflected and transmitted modes, momentum correlations of the reflected mode due to inversion of the relative momentum, and momentum correlations in the transmitted and reflected modes due to dependence of the scattering amplitude on the relative momentum. We consider three standard potentials, the hard core, Dirac delta, and double Dirac delta, and show that the relative importance of these mechanisms depends on the interaction and on the properties of the incoming wave function. We find that even when the momenta distributions of the incoming articles are sharply peaked, entanglement due to the momentum correlations generated by reflection can be quite large for particles with unequal mass. [Preview Abstract] |
Thursday, May 29, 2008 2:48PM - 3:00PM |
K5.00005: Bell inequality violation with two remote atomic qubits Dzmitry Matsukevich, Peter Maunz, David Moehring, Steven Olmschenk, Chris Monroe We report the violation of a Bell inequality between the quantum states of two remote Yb ions separated by about one meter. First, we prepare the two spatially separated ions, each entangled with the polarization state of a photon it has emitted. Next, the heralded entanglement of two ions is established via interference and joint detection of these photons. The near unit detection efficiency of the quantum state of the remote trapped ions allows us to close the detection loophole in a Bell inequality measurement. This experiment also offers an approach to a loophole-free test of a Bell inequality. [Preview Abstract] |
Thursday, May 29, 2008 3:00PM - 3:12PM |
K5.00006: Robust creation of arbitrary-sized Dicke states of trapped ions by global addressing Ian Linington, Nikolay Vitanov We propose a novel technique for the creation of entangled Dicke states in a chain of trapped ions. Our approach is robust against experimental imperfections, largely insensitive to motional heating and applicable to arbitrary numbers of ions and excitations. Individual addressing is not required, since the method uses only a single laser, interacting simultaneously with all ions. By factorizing the overall Hilbert space, we show that the dynamics is confined to a ladder of symmetric states which are invariant under permutation of the ions. Sweeping the laser detuning through resonance and enforcing adiabatic evolution then induces a ``bow-tie'' level-crossing wherein the two ends of the ladder are smoothly connected. An initial product state may be transformed into an entangled Dicke state using only two laser pulses. The technique is naturally robust against fluctuations in the laser intensity and the chirp rate. Furthermore, because the method is significantly faster than existing approaches, heating effects can be very small, even when the centre-of-mass mode is used. We quantify the adiabaticity requirements and the effects of motional heating, and estimate an overall fidelity exceeding $98\%$ for the generation of a ten-ion Dicke state. The method may readily be adapted in order to create non-classical superposition states of the ions' collective motion and Greenberger-Horne-Zeilinger states of their internal states. [Preview Abstract] |
Thursday, May 29, 2008 3:12PM - 3:24PM |
K5.00007: Compact Source of Entangled Images and Squeezed Light Using Four-Wave Mixing in Rubidium Vapor Raphael Pooser, Alberto Marino, Vincent Boyer, Paul Lett Recently great interest has been generated in the quantum correlations and entanglement present in multi-mode squeezed fields, which have applications in quantum information, quantum imaging, and quantum computing. We use a nondegenerate four-wave mixing (4WM) process in Rubidium vapor at 795 nm to demonstrate generation of entangled images and multi-mode output beams. We have verified the presence of entanglement between the multi-mode beams by analyzing the amplitude difference and the phase sum noise using a dual homodyne detection scheme, measuring more than 4 dB of squeezing in both cases. This amount of squeezing also implies EPR correlations between the output beams. The multi-spatial-mode character of the entanglement is shown by using an arbitrarily shaped local oscillator during these measurements. Transferring the quantum correlations from the light to atoms in order to generate correlated atom beams is another interesting prospect. We have built a compact 4WM source of squeezed light, using semiconductor lasers only, for use in such experiments. We show that tapered amplifier lasers can be made to emit low enough input pump noise to the four wave mixing process to preserve the squeezing properties of the output beams. [Preview Abstract] |
Thursday, May 29, 2008 3:24PM - 3:36PM |
K5.00008: Incoherently generated coherence and immunity to decoherence Barry Sanders, Raisa Karasik, Karl-Peter Marzlin, K. Birgitta Whaley The decoherence free subspace is important as states in this subspace are immune to the decohering effects of open system dynamics. We introduce a new kind of state, which, for certain open system dynamics, can be made immune to decoherence by driving the system with an appropriate driving field: we refer to these states as incoherently generated coherent states, and they are pure states that evolve unitarily despite coupling to the open system. The seemingly non-unitary open system driving term becomes essential as a partner with the driving field to generate coherences that stabilize these special states. We prove that such states cannot exist for most open system models with finite-dimensional systems but are readily found for infinite-dimensional systems, and we present examples of such states for suitable open systems. [Preview Abstract] |
Thursday, May 29, 2008 3:36PM - 3:48PM |
K5.00009: Multi-particle decoherence free subspaces in extended systems Raisa Karasik, Karl-Peter Marzlin, Barry C. Sanders, K. Birgitta Whaley A decoherence-free subspace (DFS) is a collection of states that are immune to the noise derived from interactions with the environment. DFS is especially of interest for states involving two or more particles and is considered a prominent candidate for quantum memory and quantum information processing. We develop a method for finding DFS in real quantum systems. For systems with a homogeneous environment and energy-conserving coupling to the particles in 3D space, our methods show that perfect DFS exists for co-located particles only . This restriction does not exist for confined systems, such as atoms embedded in an optical fiber. [Preview Abstract] |
Thursday, May 29, 2008 3:48PM - 4:00PM |
K5.00010: Light-Atoms-Mirror Entanglement in an Optical Cavity Claudiu Genes, David Vitali, Paolo Tombesi We propose a scheme for the realization of a hybrid, strongly quantum-correlated system formed of an atomic ensemble surrounded by a high-finesse optical cavity with a vibrating mirror. We show that the steady state of the system shows tripartite and bipartite continuous variable entanglement in experimentally accessible parameter regimes, which is robust against temperature. This work builds on previous theoretical results that have proven the possibility of ground state cooling of mechanical oscillators and generation of intra-cavity optomechanical entanglement between light and a vibrational degree of freedom of a micromirror. [Preview Abstract] |
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