Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session R40: Focus Session: Pathways to Practical Quantum Computing II |
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Sponsoring Units: DCOMP TGQI Chair: Juan Pablo Paz, Los Alamos National Laboratory Room: Baltimore Convention Center 343 |
Wednesday, March 15, 2006 2:30PM - 3:06PM |
R40.00001: Liquid NMR quantum computations Invited Speaker: Raymond Laflamme |
Wednesday, March 15, 2006 3:06PM - 3:18PM |
R40.00002: Generating Spin Echoes in Dipolar Solids with $\pi$-Pulses: More is Different Dale Li, Yanqun Dong, Rona Ramos, Anatoly Dementyev, Sean Barrett NMR spin echo measurements of $^{13}$C in C$_{60}$, $^{89}$Y in Y$_{2}$O$_{3}$, and $^{29}$Si in Silicon are shown to defy conventional expectations when more than one $\pi$-pulse is used. Multiple $\pi$-pulse echo trains may either freeze-out or accelerate the dipolar decay of the signal, depending upon $\pi$-pulse phase, which suggests a connection to quantum dynamical decoupling and the formation of quantum Zeno subspaces. Exact quantum calculations (without a spin bath) reveal an intrinsic cause for these coherent phenomena: the dipolar coupling has a many-body effect during any real, finite pulse. [Preview Abstract] |
Wednesday, March 15, 2006 3:18PM - 3:30PM |
R40.00003: From Large to Small Spin Systems: Exploring Many-body Effects in NMR Yanqun Dong, Dale Li, Rona Ramos, Sean Barrett NMR multi-pulse spin echo measurements on different nuclei and different samples show a dramatic Pulse Sequence Sensitivity (PSS): the multiple pi-pulse echo trains may either freeze out or accelerate the dipolar decay of the signal depending upon pi pulse phase. Finite-pulse spin simulations on small spin systems (7 spins) also show a tiny PSS. We propose that the dramatic PSS in experiments may be due to many-body effects during the finite pi pulses. If this is the case, the PSS should be less in small spin systems. This idea can be tested by performing spin echo experiments on small spin systems. The liquid crystal 5CB has a nematic phase, which enables the molecules to act like isolated spins systems in NMR. Since each molecule has 19 $^{1}$H spins, this is an attractive system for our test. I will present experimental results on 5CB, and discuss their implication on our model. [Preview Abstract] |
Wednesday, March 15, 2006 3:30PM - 3:42PM |
R40.00004: Observation of Anomolously Long-Lived Spin Echoes in a Dense Dipolar Spin System Rona Ramos, Yanqun Dong, Dale Li, Sean Barrett Continuing the investigation of anomolously long-lived spin echoes found in multipulse $^{29}$Si NMR experiments, similar proton NMR experiments were performed on adamantane (C$_{10}$H$_{16}$, a molecular solid that tumbles about its fcc lattice sites). In contrast to the dilute dipolar silicon samples from previous experiments [A.E. Dementyev, D. Li, K. MacLean, S.E. Barrett, Phys. Rev. B 68, 153302 (2003).], adamantane presents a densely populated, strongly coupled proton spin system in which to probe the basis of this puzzle. Despite these changes, this phenomenon, which defies conventional NMR theory, still remains. This talk will discuss the results of these experiments and its impact on our current understanding of this behavior. [Preview Abstract] |
Wednesday, March 15, 2006 3:42PM - 3:54PM |
R40.00005: Numerical modeling of the central spin problem using the spin coherent states P-representation V.V. Dobrovitski, K.A. Al-Hassanieh, E. Dagotto, B.N. Harmon We analyze decoherence of a central spin coupled to a spin bath (the central spin problem). Theoretical understanding of this process is important for many experiments, such as the recent study of decoherence of the electron spin by the nuclear spins in a quantum dot. To investigate the important non-perturbative decoherence regimes, we developed an efficient mean-field-based method for modeling the spin-bath decoherence. The method is based on the P-representation for the central spin density matrix, which is very useful in quantum optics, but has not been widely applied to quantum many-spin systems. In contrast with the standard time-dependent mean field theory, our method gives excellent agreement with the exact solution. We demonstrate performance of the method for longitudinal and transversal relaxation at different external fields. In particular, by modeling the quantum systems with up to 16000 bath spins, we make controlled predictions for the slow long-time decoherence of the central spin. We thank L.~Glazman, M.~Lukin, A.~Polkovnikov, and J.~Taylor for helpful discussions. This work was supported by NSA, ARDA, ARO, and NSF. [Preview Abstract] |
Wednesday, March 15, 2006 3:54PM - 4:06PM |
R40.00006: The sensitive electrical detection of spin coherence with pulsed electrically detected magnetic resonance Christoph Boehme, Klaus Lips We present a pulsed electrically detected magnetic resonance experiment which allows the very sensitive observation of the coherent evolution of localized electron spins in semiconductors. The experiment takes advantage of spin selection rules of electronic transport transitions which exist for tunneling through localized states of a 20 nm thick disordered silicon buffer layer between a crystalline silicon wafer and an 80 nm thick ZnO surface layer. When coherent spin states of defect pairs are prepared by means of pulsed electron spin resonance, the singlet content (= the projection of density matrix onto the singlet state) of these states is directly proportional to the additional charge which is transmitted by the interface. Experimental data and the sensitivity limits of this spin measurement technique will be presented. The applicability of this for the readout of silicon based spin quantum information concepts is discussed. [Preview Abstract] |
Wednesday, March 15, 2006 4:06PM - 4:18PM |
R40.00007: Polarization Requirements for Ensemble Implementations of Quantum Algorithms with a Single Bit Output Brandon Anderson, David Collins We compare the failure probabilities of ensemble implementations of quantum algorithms which use pseudo-pure initial states, quantified by their polarization, to those of competing classical probabilistic algorithms. Specifically we consider a class algorithms which require only one bit to output the solution to problems. For large ensemble sizes, we present a general scheme to determine a critical polarization beneath which the quantum algorithm fails with greater probability than its classical competitor. We apply this to the Deutsch-Jozsa algorithm to determine the critical polarization. [Preview Abstract] |
Wednesday, March 15, 2006 4:18PM - 4:30PM |
R40.00008: Optimizing quantum gate fidelities using energy-optimal control Sonia Schirmer Optimal control theory offers a promising framework for optimizing essential tasks in quantum computing from quantum state preparation to the implementation of quantum gates. It is applicable to a wide variety of systems from atoms and ions to quantum dots, different control mechanisms from all-optical to all-electronic, and allows implementation constraints and dissipation to be accommodated. Most existing approaches focus on the exact implementation of (unitary) quantum gates in a particular model and aim to optimize gate operation times subject to certain assumptions such as arbitrarily fast local operations, weak (inter-qubit) coupling and decoherence, etc. We consider a different paradigm for optimal control focussing on optimizing the overall gate fidelity for a desired gate and fixed gate operation time subject to physical and experimental contraints (including dissipative effects), which may be more appropriate for some systems such as electronically-controlled systems with non-weak, always-on inter-qubit coupling. We discuss the basic framework and illustrate the results using calculations for model systems. [Preview Abstract] |
Wednesday, March 15, 2006 4:30PM - 4:42PM |
R40.00009: Exact solution of qubit decoherence models by a transfer matrix method. Diu Nghiem, Robert Joynt A new method for the solution of the behavior of an enesemble of qubits in a random time-dependent external field was found. In this method the forward evolution in time is governed by a transfer matrix whose eigenvalues determine the various decoherence times. The method provides an exact solution in cases where the noise is piecewise constant in time. It can apply to a realistic model of decoherence of electron spins in semiconductors as well. Results are obtained for the non-perturbative regimes of the models, and we see a transition from weak relaxation to overdamped behavior as a function of noise anisotropy. [Preview Abstract] |
Wednesday, March 15, 2006 4:42PM - 4:54PM |
R40.00010: Quantum kinetics of dynamical decoupling Leonid P. Pryadko, Pinaki Sengupta In an ideal world, coherent control could be made perfect by running infinitely fast sequences of infinitely short pulses. In practice, in each system there are obvious spectral limitations. There is also a large-time limit set by decoherence due to environment coupling. Altogether, this makes pulse shape and sequence design an extremely complicated optimization problem. A systematic way to approach this problem is to consider a cumulant expansion of the evolution operator, treating the strong control fields exactly. The cumulants give the expansion of the effective Hamiltonian in powers of the system Hamiltonian. The locality of the cumulant expansion ensures that the classification by sequence order remains meaningful even for large systems. The corresponding calculation can be done efficiently by constructing a time-dependent perturbation theory expansion on small clusters [1]. Intuitively, refocusing should also remain effective in the presence of low-frequency environment, as long as the parameters of the system Hamiltonian are varying slowly compared to the refocusing period $\tau$. A systematic study of this effect will be presented, based on the Floquet analysis of the non-Markovian quantum kinetic (master) equation for the open multi-qubit system in the presence of continuous refocusing fields exact up to 2nd order in the cumulant expansion [2]. [1]~P.~Sengupta and L. P. Pryadko, Phys. Rev. Lett. {\bf 95}, 037202 (2005). [2]~L.~P.~Pryadko and P. Sengupta, quant-ph/0510001 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 4:54PM - 5:06PM |
R40.00011: Enhanced Convergence and Robust Performance of Randomized Dynamical Decoupling Lea Santos, Lorenza Viola Dynamical decoupling methods consist of repetitive sequences of control operations, whose net effect is to coherently modify the natural target dynamics to a desired one. In addition to standard deterministic schemes, randomized decoupling strategies have been recently introduced. Here, we exhibit clear evidence of the benefits of randomization in reducing the effects of undesirable couplings. For control systems which are either time-varying or require decoupling cycles involving a large number of operations, we find that simple randomized protocols offer superior convergence and stability as compared to high-level deterministic designs, including combinatorial and concatenated methods. We also show how significant improvements may be achieved for long interaction times by combining deterministic and stochastic features into new hybrid decoupling schemes. [Preview Abstract] |
Wednesday, March 15, 2006 5:06PM - 5:18PM |
R40.00012: Resurrection of Schr\"{o}dinger Cat Jae-Seung Lee, Anatoly Khitrin The most striking difference between quantum and classical systems is the ability of quantum objects to be in a superposition state. A system in a superposition of macroscopically distinct states (\textit{alive} and \textit{dead} states of the ``Schr\"{o}dinger cat'') would demonstrate highly unusual behavior. Cat states are the central elements in recent proposals on high-precision spectroscopy, amplified quantum detection and measurement. Quantum decoherence is the major obstacle in building practical devices which could revolutionize high-precision measurements or information processing. Here we experimentally demonstrate that quantum state of a system can be recovered after the state is destroyed by uncontrollable natural decoherence. The physical system used in this experiment is a cluster of seven dipolar-coupled nuclear spins of single-labeled $^{13}$C-benzene oriented in liquid crystal. After decoherence of the cat state, superposition of states with all spins up (\textit{alive}) and all spins down (\textit{dead}), information stored in a single ancillary spin ($^{13}$C) is used to bring the protons subsystem into the \textit{alive} state, while the excess entropy produced by decoherence is transferred to the ancillary spin. [Preview Abstract] |
Wednesday, March 15, 2006 5:18PM - 5:30PM |
R40.00013: Statistical performance of ensemble quantum computers in search algorithms David Collins, Tomasz Kott We consider the statistical performance of ensemble quantum computers applied to search algorithms. In particular we consider algorithms implemented on pseudo pure initial states and determine the initial polarization required so that the quantum algorithm outperforms classical probabilistic competitors in terms of failure probabilities. [Preview Abstract] |
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