Bulletin of the American Physical Society
38th Annual Meeting of the Division of Atomic, Molecular, and Optical Physics
Volume 52, Number 7
Tuesday–Saturday, June 5–9, 2007; Calgary, Alberta, Canada
Session H5: Decoherence, Entanglement, and Chaos |
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Chair: P. Turner, University of Calgary Room: TELUS Convention Centre Glen 205 |
Thursday, June 7, 2007 10:30AM - 10:42AM |
H5.00001: Coherent control and entanglement in a decoherence-free subspace of two multi-level atoms Martin Kiffner, J\"{o}rg Evers, Christoph H. Keitel Decoherence-free subspaces (DFS) in a system of two dipole-dipole interacting multi-level atoms are investigated theoretically. The ground state of each atom is a $S_0$ singlet state, and the excited state multiplet is a $P_1$ triplet. Since we consider arbitrary geometrical alignments of the atoms, all Zeeman sublevels of the atomic multiplets have to be taken into account~[1]. It is shown that the collective state space of the two dipole-dipole interacting four-level atoms contains a four-dimensional DFS~[2]. We describe a method that allows to populate the antisymmetric states of the DFS by means of a laser field. These antisymmetric states are identified as long-lived entangled states. Further, we show that any single-qubit operation between two states of the DFS can be induced by means of a microwave field. Typical operation times of these qubit rotations can be significantly shorter than for a nuclear spin system. \newline [1] M. Kiffner, J. Evers, and C. H. Keitel, arXiv:quant-ph/0611071. \newline [2] M. Kiffner, J. Evers, and C. H. Keitel, Phys. Rev. A in print (arXiv:quant-ph/0611084). [Preview Abstract] |
Thursday, June 7, 2007 10:42AM - 10:54AM |
H5.00002: Neutral atoms with cavity-assisted interaction for robust long distant quantum communication Peng Xue We show how to realize long distance quantum communication with long-lived quantum memories, acting on decoherence-free subspace (DFS), with neutral atoms whose interactions are catalyzed by single photons, or weak coherent light. In this matter, generation, purification and swapping of logical entangled states are obtained through cavity-assisted photon scattering that is robust to random variation in the atom-photon coupling rate, thereby avoiding the requirement for location in the Lamb-Dicke regime. The logical qubits are immunized to the dominant source of decoherence; while, additional errors as photon losses in our scheme are automatically detected, leading to signaled errors which do not affect the fidelity of the logical entanglement. Our scheme is also robust to the changes in the path lengths during long distance communication since no interferometer is required here. We show it can be implemented in the context of prominent experimental setups for quantum information processing. [Preview Abstract] |
Thursday, June 7, 2007 10:54AM - 11:06AM |
H5.00003: Decoherence of the two-level atom in the Jaynes-Cummings model Hoofar Daneshvar, Gordon W.F. Drake The evolution of coherences as well as $Tr(\rho^{2})$ which is a measure of the purity of the system is investigated over long times for the Jaynes-Cummings model of a two-level atom, interacting with a quantized single-mode field and we present the specific initial conditions for the atom as well as the quantized field for which the decoherence of the atom may be delayed. We also present a specific initial condition for which not only we observe a delayed decoherence of the state of the two-level atom, but also the amplitude of the sub-oscillations becomes very small and we observe a smooth decay of both coherences and $Tr(\rho^{2})$. As we will present, the calculation of the real entropy of the two-level atom in these regimes, verifies these results. The effect of the phase of the initial atomic and field states is also studied and in particular it is explicitly shown that only the relative phase between the initial states of the atom and the field is important for the subsequent evolution of the atomic coherences. [Preview Abstract] |
Thursday, June 7, 2007 11:06AM - 11:18AM |
H5.00004: Criteria for dynamically stable decoherence-free subspaces R. Karasik, K.-P. Marzlin, B.C. Sanders, K.B. Whaley A decoherence-free subspace (DFS) is a collection of states for a system that is impervious to dominant noise effects created by the environment. The DFS approach provides an important strategy for quantum information processing because it would allow quantum circuit simplification by reducing the need for quantum error correction and providing stable quantum memory. Experimental demonstrations of DFSs show the efficacy of this approach. We analyze similarities and differences between various approaches to DFSs present in the literature and show that an excessively restrictive assumption on immunity from decoherence for an arbitrary initial environment state can be relaxed for practical DFS cases. In the important class of systems whose dynamics is described by Markovian master equations, we provide necessary and sufficient conditions for the existence of a dynamically stable DFS. We also present examples that show why previous work in this direction was not sufficient. [Preview Abstract] |
Thursday, June 7, 2007 11:18AM - 11:30AM |
H5.00005: Robust transmission of non-Gaussian entanglement over optical fibers Asoka Biswas, Daniel A. Lidar We show how the entanglement in a wide range of continuous variable non-Gaussian states can be preserved against decoherence for long-range quantum communication through an optical fiber. We apply protection via decoherence-free subspaces and quantum dynamical decoupling to this end. The latter is implemented by inserting phase shifters at regular intervals $\Delta $ inside the fiber, where $\Delta $ is roughly the ratio of the speed of light in the fiber to the bath high-frequency cutoff. Detailed estimates of relevant parameters are provided using the boson-boson model of system-bath interaction for silica fibers, and $\Delta $ is found to be on the order of a millimeter. [Preview Abstract] |
Thursday, June 7, 2007 11:30AM - 11:42AM |
H5.00006: Decoherence-induced geometric phase in an open multilevel atomic system Shubhrangshu Dasgupta, Daniel A. Lidar We consider the process of stimulated Raman adiabatic passage (STIRAP) in a three-level atom. Viewed as a closed system, no geometric phase is acquired. But in the presence of spontaneous emission and/or collisional relaxation we show numerically that a non-vanishing, purely real, geometric phase is acquired during STIRAP, whose magnitude grows with the decay rates. Rather than viewing this decoherence-induced geometric phase as a nuisance, it can be considered an example of beneficial decoherence : the environment provides a mechanism for the generation of geometric phases which would otherwise require an extra experimental control knob. [Preview Abstract] |
Thursday, June 7, 2007 11:42AM - 11:54AM |
H5.00007: Studying chaos, entanglement and decoherence in the quantum kicked top with cold atoms Shohini Ghose, Rene Stock, Roshan Lal, Andrew Silberfarb We propose and analyze an experiment to study the dynamics of the quantum kicked top using cold Cesium atoms interacting with laser and magnetic fields. This would be the first experimental realization of this well known chaotic system in a deeply quantum regime, and would allow detailed studies of the effects of chaos on entanglement and decoherence. These studies are of importance for understanding quantum-classical correspondence as well as for designing quantum information processing applications. We describe the process of state preparation in the system, and show how the nonlinear AC Stark shift together with a pulsed magnetic field can be used to realize the kicked top Hamiltonian. Signatures of chaos are evident in the entanglement between the electronic and nuclear spin, which can be monitored via Faraday rotation spectroscopy. We analyze and explain the predicted dynamics by decomposing the initial states into regular and chaotic Floquet eigenstates. Our accurate simulations show that dynamical signatures of chaos persist in the presence of decoherence due to photon scattering. Furthermore, chaos affects the decoherence rate itself due to the rapid mixing in phase space caused by chaotic dynamics, even in a deeply quantum regime. [Preview Abstract] |
Thursday, June 7, 2007 11:54AM - 12:06PM |
H5.00008: Chaos and Entanglement with Two Coupled Spins Leigh Norris, Parin Sripakdeevong, Arjendu Pattanayak, Collin Trail, Ivan Deutsch, Shohini Ghose We study the correlation between chaos and entanglement in a system consisting of two spins that evolve via hyperfine and Zeeman interactions in the presence of a time-varying external magnetic field. Here, chaos arises due to the coupling between subsystems, in contrast with previously studied cases where two coupled subsystems are independently chaotic (e.g. coupled kicked tops). Using a common Hamiltonian to generate quantum and classical dynamics, we study how the entanglement generated by initially uncoupled spin-coherent states correlates with the mixed nature of the underlying the classical phase space consisting of regular islands and a chaotic sea. We report on the relationship between the mean entanglement of the eigenstates of the Floquet operator and the presence of chaos in the classical phase space. We also analyze the performance of an entanglement measure dependent on the eigenvalues of the Floquet operator. [Preview Abstract] |
Thursday, June 7, 2007 12:06PM - 12:18PM |
H5.00009: Chaotic Escape of Particles from a Vase-Shaped Billiard Jaison Novick, John Delos, Kevin Mitchell We study the escape of particles from a two dimensional, open billiard with the shape of a vase. At the narrowest point of the vase's neck lies an unstable periodic orbit that defines a dividing surface between orbits that escape and those that are turned back into the cavity. We imagine a burst of particles emanating from a point source with all possible launch angles. We show that the particles arrive at the detector in pulses. We record the time for escaping particles to reach the dividing surface. The escape time, as a function of the launch angle, displays a fractal structure that is understood upon transformation to a suitable phase space. Here, we find two infinitely long, invariant curves, called stable and unstable manifolds, emanating from the unstable fixed point. These curves intersect in a complicated way forming a structure known as a homoclinic tangle. The intersection of the initial conditions with the tangle produces the escape time fractal. We present a topological method that, given a finite development of the tangle, allows us to predict a subset of the fractal seen in numerical experiments. [Preview Abstract] |
Thursday, June 7, 2007 12:18PM - 12:30PM |
H5.00010: ABSTRACT HAS BEEN MOVED TO R1.00096 |
Thursday, June 7, 2007 12:30PM - 12:42PM |
H5.00011: Dynamically Invariant Entanglement in Particle Collisions Nathan Harshman When most people think of entanglement, they think of interparticle entanglement. For example, how is Alice's particle correlated to Bob's particle? However, other kinds of entanglement can be defined. Any partition of a complete set of commuting observables implies a tensor product structure, and therewith, a kind of entanglement. These other kinds of entanglement can be useful for understanding particles and the generation of entanglement in collisions. For example, only certain kinds of entanglement are invariant under a change of reference frame. In particular, the entanglement between internal and external degrees of freedom is invariant and it is conserved in collisions. [Preview Abstract] |
Thursday, June 7, 2007 12:42PM - 12:54PM |
H5.00012: Entanglement of mixed states in systems of n qubits Renan Cabrera, William Baylis With the help of geometric algebra, we define a sequence of unitary operators that fulfills a controllability condition and whose operation on a reference or pass state spans the complete Hilbert space. The unitary operators can be classified according to their ability to induce entanglement, and the operators that can entangle a system provide a natural parametrization of the process. The entanglement of pure states is measured using reduced traces, and this can be extended to define a measure of the entanglement of mixed states in a system of n qubits. Practical expressions for average state fidelity of n-qubit systems have also been obtained. [Preview Abstract] |
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