Bulletin of the American Physical Society
2005 36th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 17–21, 2005; Lincoln, Nebraska
Session E4: Quantum Information I |
Hide Abstracts |
Chair: Christopher Monroe, University of Michigan Room: Burnham Yates Conference Center Hawthorne |
Thursday, May 19, 2005 8:00AM - 8:12AM |
E4.00001: Quantum state reconstruction via continuous mesaurement Andrew Silberfarb, Greg Smith, Poul Jessen, Ivan Deutsch We present a new procedure for quantum state reconstruction based on weak continuous measurement of an ensemble average. By applying controlled evolution to the initial state new information is continually mapped onto the measured observable. A Bayesian filter is then used to update the state-estimate in accordance with the measurement record. We discuss how this technique is employed to reconstruct the spin of a cesium atom, and present some relevant experimental data. [Preview Abstract] |
Thursday, May 19, 2005 8:12AM - 8:24AM |
E4.00002: Entanglement Evolution in the Presence of Decoherence Jin Wang, Herman Batelaan, Jeremy R. Podany, Anthony F. Starace The pairwise entanglement of a Heisenberg XY interacting spin chain in the presence of a uniform magnetic field and decoherence due to population relaxation is studied. The time dependent concurrence is analyzied both analytically and numerically for some typical initial states, including a separable (unentangled) initial state. An analytical formula for non-zero steady state concurrence is found for any initial state, and optimal parameter values for maximizing the steady state concurrence are given. We also distinguish the effects of global versus local coherence for this system. Since our model interaction Hamiltonian describes also mesoscopic objects that interact via their spins, it may be that a certain level of entanglement is robust against decohering interactions with an environment even for mesoscopic objects. [Preview Abstract] |
Thursday, May 19, 2005 8:24AM - 8:36AM |
E4.00003: Remote control of the visibility of an interference pattern Enrique Galvez, Matthew Pysher, Kartik Misra We prepared polarization-entangled photon pairs in a superposition of Bell states by sending one of the photons of each pair through a Mach-Zehnder interferometer. As a consequence, the visibility of the interference pattern was controlled by projecting the state of the pair with a polarizer on the path of the photon that does not go through the interferometer. Theory as well as experimental results will be presented. This work is funded by NSF DUE-9952626 and PHY-9988004. [Preview Abstract] |
Thursday, May 19, 2005 8:36AM - 8:48AM |
E4.00004: Geometric Phase of Unitary Evolution Operator Dmitry Uskov, Ravi Rau The common geometric phase is understood in terms of curvature of the space of normalized quantum states (Berry-Simon and Aharonov-Anandan geometric phase). We demonstrate how the geometric phase can be self-consistently defined in the space of unitary operators if the dynamic group of a quantum system has physically meaningful partitioning into a subgroup and complimentary coset space. To motivate the introduction of a geometric phase of a unitary operator, we demonstrate that this phase does not vanish even if the quantum system is in a mixed state, described by a density matrix. Illustrative examples of calculating the geometric phase of a unitary operator are provided for SU(2) and SU(4) groups i.e. one- and two-qubit transformations. [Preview Abstract] |
Thursday, May 19, 2005 8:48AM - 9:00AM |
E4.00005: Robust Manipulation of Neutral Atom Qubits Brian Mischuck, Worawarong Rakreungdet, Poul Jessen Quantum information can be encoded in the hyperfine ground states of Cesium atoms trapped in optical lattices. Arbitrary manipulations of single atomic qubits on the Bloch sphere may be performed by driving the transition between those ground states with a resonant microwave field. We explore the use of composite pulses similar to those used in NMR in our atom/lattice system, and show experimentally that they are robust against errors in pulse timing, microwave power and detuning from resonance. [Preview Abstract] |
Thursday, May 19, 2005 9:00AM - 9:12AM |
E4.00006: Density Matrix Reconstruction of a Large Angular Momentum by Continuous Weak Measurement on Cold Cs Atoms Greg A. Smith, Andrew Silberfarb, Poul S. Jessen We experimentally demonstrate how a continuous measurement on an ensemble of laser-cooled Cs atoms can be used to reconstruct the density matrix for their ground state spin angular momentum. By employing a carefully crafted time-varying magnetic field and a probe-induced nonlinear light-shift, the angular momentum state space is fully explored and complete information about the initial state is gradually mapped onto the measured observable. Based on the time-dependent measurement record one can then obtain an estimate of the entire initial density matrix. The reconstruction can in principle be done in real time and with minimal disturbance, and therefore provides a starting point for feedback control based on knowledge of the entire quantum state. [Preview Abstract] |
Thursday, May 19, 2005 9:12AM - 9:24AM |
E4.00007: Remote Atom Entanglement David Moehring, Martin Madsen, Boris Blinov, Rudolph Kohn, Christopher Monroe We report on progress toward the quantum entanglement between two remotely located atoms via the joint detection of emitted photons from each atom. In the experiment, single $^{111}$Cd$^{+}$ ions are trapped in two different ion traps spaced by 1 meter. Each atom is excited to a state with multiple decay channels and the atomic spin becomes probabilistically entangled with the spontaneously emitted photon's polarization [1]. When the two photons are detected in coincidence, the two atoms are expected to be left in a known Bell state. Even though this is established at random times, the atom-atom entanglement is ``heralded'' by the joint detection of the two photons, and becomes a resource for further quantum information processing. This work is supported by the U.S. National Security Agency and the Advanced Research and Development Activity under Army Research Office contract, and the National Science Foundation ITR program. 1. \textit{``Observation of Entanglement between a Single Trapped Atom and a Single Photon''}, B.B. Blinov, D.L. Moehring, L.-M. Duan, C. Monroe, Nature (London) \textbf{428}, 153 (2004). [Preview Abstract] |
Thursday, May 19, 2005 9:24AM - 9:36AM |
E4.00008: Entangling Trapped Ions through Spin-Dependent Bichromatic Forces Kathy-Anne Brickman, Mark Acton, Louis Deslauriers, Paul Haljan, Patricia Lee, Christopher Monroe We experimentally demonstrate a two-ion entangling gate utilizing the Molmer-Sorensen gate scheme. Pairs of hyperfine ground states of $^{111}$Cd$^{+}$ ions, insensitive to magnetic fields to first order, are used as qubits that are entangled through bichromatic stimulated Raman transitions. The spectral arrangement of the Raman beams is tailored to suppress phase noise accumulation between gates. This suppression may be critical for reliably performing consecutive gates of this type within quantum algorithms. This work is supported by the U.S. National Security Agency and the Advanced Research and Development Activity under Army Research Office contract, and the National Science Foundation ITR Program. [Preview Abstract] |
Thursday, May 19, 2005 9:36AM - 9:48AM |
E4.00009: Four-level and two-qubit systems, sub-algebras, and unitary integration A.R.P. Rau, G. Selvaraj, D. Uskov Four-level systems in quantum optics, and for representing two qubits in quantum computing, are difficult to solve for general time-dependent Hamiltonians. A systematic procedure is presented which combines analytical handling of the algebraic operator aspects with simple solutions of classical, first-order differential equations. In particular, by exploiting su(2) X su(2) and su(2) X su(2) X u(1) sub-algebras of the full SU(4) dynamical group of the system, the non-trivial part of the final calculation is reduced to a single Riccati (first order, quadratically nonlinear) equation, itself simply solved. Examples are provided of two-qubit problems from the recent literature, including implementation of two-qubit gates with Josephson junctions. [Preview Abstract] |
Thursday, May 19, 2005 9:48AM - 10:00AM |
E4.00010: Scalable quantum computing with highly magnetic atoms Andrei Derevianko, Caleb Cannon Considering recent success in cooling and trapping of open shell atoms with large magnetic moments, we propose a quantum computer based on magnetic interactions of such atoms. This architecture is based on cold atoms confined to sites of a tight optical lattice. The lattice is placed in a non-uniform magnetic field and the resulting Zeeman sublevels define qubit states. Microwave pulses tuned to space-dependent resonant frequencies are used for individual addressing. The atoms interact via magnetic-dipole interactions allowing implementation of a universal controlled- NOT gate. The resulting gate operation times are as fast as 100 microseconds, much faster then the anticipated decoherence times. Single qubit operations take about 10 microseconds.\\ Details can be found in A. Derevianko and C. Cannon, Phys. Rev. A {\bf 70}, 062319 (2004). [Preview Abstract] |
Thursday, May 19, 2005 10:00AM - 10:12AM |
E4.00011: Robust quantum memory using magnetic-field-independent atomic qubits C. Langer, R. Ozeri, J. D. Jost, B. DeMarco, A. Ben-Kish, B. Blakestad, J. Britton, J. Chiaverini, D. Hume, W. M. Itano, D. Leibfried, R. Reichle, T. Rosenband, P. Schmidt, D. J. Wineland Scalable quantum information processing (QIP) requires physical systems capable of reliably storing coherent superpositions for times over which quantum error correction can be implemented. Here, we experimentally demonstrate a robust quantum memory using a magnetic-field-independent hyperfine transition in $^{9} $Be$^{+}$ atomic ion qubits at a field B = 0.01194 T using Raman spectroscopy implemented with two-photon stimulated-Raman transitions. We observe the single physical qubit memory coherence time to be greater than 10 seconds, an improvement of approximately five orders of magnitude from previous experiments effectively eliminating memory errors in this system. Future QIP experiments employing this qubit will be discussed. [Preview Abstract] |
Thursday, May 19, 2005 10:12AM - 10:24AM |
E4.00012: Coherence time of a cold atomic ensemble as a quantum memory C. W. Chou, D. Felinto, H. de Riedmatten, S. Polyakov, H. J. Kimble In the quantum communication scheme proposed by Duan, Lukin, Cirac, and Zoller (DLCZ)[1], two distant ensembles of atoms can be entangled through the concepts of quantum repeaters. Since the scheme is probabilistic, long coherence time is essential for a scalable quantum network. However, in all experiments reported so far that employ cold atomic ensembles for the DLCZ protocol [2], the coherence times were short (of the order of 100 ns). The major cause of decoherence is identified as the inhomogeneous broadening of the atomic ground states due to the quadrupole magnetic field of the magneto-optical trap (MOT). We have developed a theory to describe this decoherence and made comparisions to the observed rate of decay of field correlations. We have also developed a technique to probe the effect of the magnetic field on the coherence time by way of in situ Raman spectroscopy between hyperfine ground states. By fast switching of the quadrupole magnetic field and nulling the residual magnetic field, we improved the coherence time to a few microseconds. Other solutions that we are investigating include utilizing a field-insensitive set of states for the DLCZ protocol. [1] Duan, L.-M., et. al, Nature 414, 413 (2001) [2] A. Kuzmich, et, al, Nature 423 726-729 (2003), C. W. Chou et. al, Phys. Rev. Lett. 92, 213601 (2004), S. Polyakov et al, Phys. Rev. Lett. 93, 263601 (2004), D. N. Matsukevich and A. Kuzmich, Science 306, 663(2004) [Preview Abstract] |
Thursday, May 19, 2005 10:24AM - 10:36AM |
E4.00013: A two reservoir model of quantum error correction James Clemens, Julio Gea-Banacloche We consider a two reservoir model of quantum error correction with a hot bath causing errors in the qubits and a cold bath cooling the ancilla qubits to a fiducial state. The error correction acts as a kind of refrigeration process to maintain the data qubits in a low entropy state by periodically moving the entropy to the ancilla qubits and then to the cold reservoir. We quantify the performance of the error correction as a function of the reservoir temperatures and cooling rate by means of the fidelity and the residual entropy of the data qubits. We also make a comparison with the continuous quantum error correction model of Sarovar and Milburn (quant-ph/0501038). [Preview Abstract] |
Thursday, May 19, 2005 10:36AM - 10:48AM |
E4.00014: Collapse and revival of entanglement in the system of trapped ions Vladimir Malinovsky Dynamics of the entangled states of two trapped ions is considered. We demonstrate the strong dependence of the population dynamics on the relative phase of the pulses used to prepare the entangled states. When the initial distribution of phonons is a coherent state, the population and entanglement exhibits collapses and full revivals. For an initial thermal distribution the revivals are not completed and the system loses entanglement and coherence. Unique discreteness of the effective Rabi frequency, proportional to $\sqrt{1+4 n ( n + 1 )\sin^2\phi}$, where $n$ is the number of phonon state, and $\phi$ is the relative phase, is found. As a result, periodic collapse and perfect revival take place in the population dynamics even for thermal distribution of motional states. Effect of the relative phase on the entanglement dynamics in the system of trapped ions is also discussed. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700