Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z17: Spin Qubit Coherence and Control 
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Sponsoring Units: GQI Chair: Paola Cappellaro, Harvard University Room: 318 

Z17.00001: ABSTRACT WITHDRAWN 
Friday, March 20, 2009 11:27AM  11:39AM 
Z17.00002: Dephasing of exchange coupled spin qubits by electronphonon coupling: Effect of phonon relaxation Xuedong Hu Electronphonon interaction leads to pure dephasing between twoelectron singlet and triplet states for two exchangecoupled spin qubits in a semiconductor double quantum dot because of the Coulombic nature of exchange interaction. Here we clarify the effect of phonon relaxation, whether via boundary scattering or phonon anharmonicity, on phononinduced twospin dephasing. Specifically, we show that within a spinboson model, phonon relaxation leads to a complete exponential decay of the phase between the singlet and triplet twospin states, with the decoherence rate proportional to the phonon relaxation rate. We examine the relevant parameter regimes and clarify the importance of this decoherence effect. [Preview Abstract] 
Friday, March 20, 2009 11:39AM  11:51AM 
Z17.00003: Softpulse refocusing in the presence of Markovian dephasing Gregory D. Quiroz, Leonid P. Pryadko We consider the effect of Markovian decoherence on the performance of refocusing sequences. This is relevant if dynamical decoupling is to be concatenated with quantum error correcting codes as the first stage of decoherence protection. The basic effect is that an asymmetric decoherence can cause a change in the direction of polarization of a quantum system. For example, dephasing of a single qubit reduces transverse components of the spin polarization vector, thus shifting it towards the $z$ axis. In this work we construct perturbation expansions of effective decoherence operators for generic shaped pulses, and for several sequences of $\pi$ and $\pi/2$pulses. While in general the performance of soft pulses is worse that that of the ideal $\delta$pulses, the detrimental effect of dephasing can be reduced by pulse shaping. [Preview Abstract] 
Friday, March 20, 2009 11:51AM  12:03PM 
Z17.00004: FockSpace Coherence in Quantum Dots Eduardo Vaz, Jordan Kyriakidis We investigate the nonMarkovian time evolution of the Fockspace coherence between states with different particle numbers in a multilevel quantum dot. By analyzing the off diagonal density matrix elements for a model where the dominant relaxation mechanism is through sequential tunneling transport, we observe a decoupling between the evolution of the Fockspace coherence and that of the population probabilities for the dot states. When tunneling rates to distinct orbitals differ  a common occurrence  the decoherence time of the Fockspace elements of the density matrix can be dramatically increased even when the Hilbertspace coherence between states with same particle number decreases. This is an example of how a manybody coherence can remain robust even in the presence of rather large singleparticle noise. [Preview Abstract] 
Friday, March 20, 2009 12:03PM  12:15PM 
Z17.00005: Randomized Benchmarking of Superconducting Qubits Jerry M. Chow, Jay Gambetta, Lars Tornberg, Jens Koch, Lev Bishop, Andrew Houck, Steven Girvin, Robert Schoelkopf We present measurements of average gate errors for a superconducting qubit using randomized benchmarking [1]. The results are compared with gate errors obtained from a double $\pi$ pulse experiment and quantum process tomography. Randomized benchmarking reveals a minimum average gate error of $1.1\pm0.3\%$ and a simple exponential dependence of fidelity on the number of applied gates. It shows that the limits on gate fidelity are primarily imposed by qubit decoherence and finite gate lengths (constrained by qubit anharmonicity), in agreement with theory. \\[3pt] [1] E. Knill et al., Phys. Rev. A. \textbf{77}, 012307(2008). [Preview Abstract] 
Friday, March 20, 2009 12:15PM  12:27PM 
Z17.00006: Simulating quantum spin systems with superconducting electrical circuits Emily Pritchett, Michael Geller There is currently great interest in the simulation of quantum spin and lattice models using cold atoms. Motivated by experimental progress in the fabrication and control of superconducting electrical circuits and their use in quantum information processing, we investigate their use as simulators of quantum spin systems, and consider a wide family of spin models that can be simulated with existing Josephson junction devices. [Preview Abstract] 
Friday, March 20, 2009 12:27PM  12:39PM 
Z17.00007: Electromagnetic radiation emanating from the molecular nanomagnet Fe$_{8}$ Amit Keren, Oren Shafir Photons emitted by transition between the discrete levels of single molecular magnets have an interesting property: their wave length can be similar to that of the sample size. This is the elementary condition for Dicke's superradiance. In this radiative process a short intense pulse of light from a molecular system appears as a result of enhanced spontaneous emission rate due to interactions via the electromagnetic field. Consequently, several investigators have been looking for this type of radiation in the molecular magnet Mn$_{12}$, where energy bursts were reported after magnetic avalanches. We investigate the same phenomenon in the Fe$_{8}$ molecule. Unlike in Mn$_{12}$ we found energy bursts each time there is a jump in the magnetization, confirming their quantum nature. A series of tests indicated that photons carry out the energy. These photons obey the elementary conditions for superradiance. [Preview Abstract] 
Friday, March 20, 2009 12:39PM  12:51PM 
Z17.00008: Asymmetries in electron spin resonance signal of magnetized spin chains and quantum wires due to spinorbital interactions Suhas Gangadharaiah, Oleg Starykh We discuss consequences of the symmetry breaking Zeeman and uniform DzyaloshinskiiMoriya (DM) terms for the electronspinresonance (ESR) measurements in a spin1/2 Heisenberg antiferromagnetic spin chain. At the noninteracting level, a nonorthogonal orientation of the magnetic field and DM vector leads to a sharp deltafunction ESR signal for the right and left moving excitations. The peak positions and their intensities generally differ, and can serve as a possible chiral probe for the two excitations. Similar results hold for a magnetized quantum wire with spinorbit terms. Including a momentum dependent fluctuations in the spinorbit coupling smears the deltafunction and instead results in an asymmetric square root singularity. We discuss the role of temperature and interactions in the further modification of the ESR signal. [Preview Abstract] 
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