Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session L3: Quantum Information Theory and New Implementations |
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Sponsoring Units: DAMOP GQI Chair: Marianna Safronova, University of Delaware Room: Gilmer Hall 190 |
Thursday, May 21, 2009 2:00PM - 2:12PM |
L3.00001: Quantum computing with an electron spin ensemble Klaus Moelmer, Arzhang Ardavan, Andrew Briggs, John Morton, Janus Wesenberg, Robert Schoelkopf, Dave Schuster We propose to encode a register of quantum bits in different collective electron spin wave excitations in a solid medium.~ Communication between spins is enabled by locating them in the vicinity of a stripline cavity, and making use of their strong~collective coupling to the quantized radiation field. The transformation between different spin waves is achieved by applying gradient magnetic fields across the sample, while a Cooper Pair Box,~ resonant with the cavity field, may be used to carry out one- and~ two-qubit gate operations. In the presentation we will address the achievable number of bits and number of gates with current measured and estimated physical parameters. [Preview Abstract] |
Thursday, May 21, 2009 2:12PM - 2:24PM |
L3.00002: Constructing General Unitary Maps from State Preparations Seth Merkel, Gavin Brennen, Poul Jessen, Ivan Deutsch We present an open-loop protocol for constructing arbitrary unitary maps on $d$-dimensional quantum systems. This protocol utilizes both geometric and stochastic construction techniques, and as such is reasonably efficient and places only minor restrictions on the form of the dynamics necessary to drive the system. We extend this construction to maps that are defined only on a subspace of the Hilbert space, and show that the fundamental scaling depends on the subspace dimension. Additionally, we show how these techniques can be used in atomic spin systems to create general qudit operators as well to perform a simple form of error correction on an embedded qubit. [Preview Abstract] |
Thursday, May 21, 2009 2:24PM - 2:36PM |
L3.00003: Superradiant cascade emission Hsiang-Hua Jen, Stewart Jenkins, Brian Kennedy We develop a quantum theory of counter-propagating light fields produced in a two-photon cascade emission in a dense atomic vapor. We include the effects of radiative atom-atom coupling in the medium, leading to Heisenberg-Maxwell equations with superradiant characteristics. Our model relates to experiments in ultra-cold rubidium vapor which aim to generate entangled light fields at telecom wavelength and matter spin waves. These in turn provide building blocks for a long-distance quantum repeater. [Preview Abstract] |
Thursday, May 21, 2009 2:36PM - 2:48PM |
L3.00004: Quantum memory for light via a Raman process in an optically dense atomic system Dmitriy Kupriyanov, Oksana Mishina, Aleksandra Sheremet, Nickolay Larionov, Igor Sokolov Quantum memories for light pose an extremely important problem for various protocols of quantum computing and secure communication, which might be solvable with currently existing technology. In the present paper we consider the coherent stimulated Raman process developing in an optically dense disordered atomic medium in application to a quantum memory scheme and we point out the difference in its physical nature from a similar but not identical protocol based on the effect of electromagnetically induced transparency (EIT). We show that the Raman and EIT memory schemes do not compete but complement one another and each of them has a certain advantages in the area of its applicability. We include in our consideration analysis of the transient processes associated with switching off/on the control pulse and follow how they modify the probe pulse dynamics on the retrieval stage of the memory protocol. The importance of the hyperfine interaction for the atomic systems consisting of alkali atoms is also discussed. [Preview Abstract] |
Thursday, May 21, 2009 2:48PM - 3:00PM |
L3.00005: Localizable Entanglement and Partial K-way Negativities of Four Qubit States Santosh Shelly Sharma, Naresh Kumar Sharma We use selective partial tansposition to construct partial K- way negativities (K is 2 to 4) that measure the bi-partite, tripartite, and genuine 4-partite entanglement of single copy four qubit states in normal form. For a state in normal form, the partial K-way negativities are polynomial functions of local invariants characterizing the state, as such proper entanglement measures. Nine families of four qubit states, obtained by Versraete et al. [F. Versraete, J. Dehaene, B. De Moor, and H. Verschelde, Phys. Rev. A65, 052112 (2002)], are grouped in two distinct classes that is, (i) states with zero three-way partial negativity and, (ii) states with finite three-way partial negativity. We derive relations between the contribution of a K-way partial transpose to negativity of global partial transpose and the optimum localizable entanglement that may be filtered out from the state through qubit state measurement and classical communication. [Preview Abstract] |
Thursday, May 21, 2009 3:00PM - 3:12PM |
L3.00006: Negativity of the Coarse Grained Wigner Function as a Measure of Quantal Behavior Tyler Keating, Adam Steege, Arjendu Pattanayak The negativity of a given state's Wigner function has been proposed as a measure of quantumness of that state in a unipartite system. This otherwise physically intuitive and useful phase-space measure however does not yield the right correspondence principle limit, and also turns out to yield infinite values of the infinite square well. We show that both these issues can be sensibly resolved using coarse-graining of the Wigner function. [Preview Abstract] |
Thursday, May 21, 2009 3:12PM - 3:24PM |
L3.00007: Quantum-walk analogues of optical phenomena Daniel D. Powell, Joshua M. Grossman Quantum walks (QWs), the counterpart of classical random walks (CRWs), offer the prospect of efficient quantum algorithms as they spread more quickly and more thoroughly through state space than CRWs. Projecting each time step of the discrete infinite-line QW along a second dimension produces probability amplitudes that evolve as the walker traverses this array of coordinates, in analogy to a photon traversing a two-dimensional array of beamsplitters. When we start the walker in a superposition of locations rather than a single point (as if it has passed through a beamsplitter before entering the QW), we observe analogues of optical phenomena in free-space propagation. Starting the particle in a range of adjacent positions produces, in the far field, a probability distribution that fits the diffraction pattern of light from a single finite-width slit, despite the fact that the particle traverses an array of scatterers. Starting the particle in separated positions produces probability distributions that fit multi-slit interference patterns. When the particle starts from a single position and we remove certain beamsplitters after the first step, we observe an even more uniform probability distribution that the standard QW. Removing larger numbers of beamsplitters causes localization of the particle. [Preview Abstract] |
Thursday, May 21, 2009 3:24PM - 3:36PM |
L3.00008: Qubit-based Model for Simulation of Discrete Quantum Systems Steven Peil We present an approach to simulating quantum computation based on a classical model developed to directly imitate discrete quantum systems. Qubits are represented as harmonic functions in a 2D vector space. Multiplication of qubit representations of different frequencies results in exponential growth of the state space similar to the tensor-product composition of qubit spaces in quantum mechanics. Individual qubits remain accessible, though entanglement imposes a demand on resources that scales exponentially with the number of entangled qubits. We demonstrate a simulation of Shor's factoring algorithm for the number 21 and discuss a simpler implementation of factoring in a classical model. [Preview Abstract] |
Thursday, May 21, 2009 3:36PM - 3:48PM |
L3.00009: Only $n$-qubit Greenberger-Horne-Zeilinger states contain $n$-partite information Scott Walck, David Lyons The generalized $n$-qubit Greenberger-Horne-Zeilinger (GHZ) states and their local unitary equivalents are the only pure states of $n$ qubits that are not uniquely determined (among arbitrary states, pure or mixed) by their reduced density matrices of $n-1$ qubits. Thus, the generalized GHZ states are the only ones containing information at the $n$-party level. [Preview Abstract] |
Thursday, May 21, 2009 3:48PM - 4:00PM |
L3.00010: Low-loss nonlinear polaritonics Barry Sanders, Sergey Moiseev, Ali Kamli We show how to achieve low-loss giant nonlinear phase shifts between weak surface polaritons by nano-confined propagation through an electromagnetically-induced transparent medium in the vicinity of a negative-index metamaterial. [Preview Abstract] |
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