Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session Y17: Spin Dynamics in Semiconducting Materials and Quantum Dots |
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Sponsoring Units: DCMP Chair: Janica Whitaker, Naval Research Lab Room: LACC 404B |
Friday, March 25, 2005 11:15AM - 11:27AM |
Y17.00001: Phonon Decoherence of a Double Quantum Dot Charge Qubit Eduardo R. Mucciolo, Serguei Vorojtsov, Harold U. Baranger The decoherence of a lateral double quantum dot charge qubit due to coupling to piezoelectric acoustic phonons has been investigated within the Born-Markov approximation. After including appropriate form factors, we have found that phonon decoherence rates are one to two orders of magnitude weaker than earlier predictions based on the spin-boson model. We have calculated the dependence of the Q-factor on lattice temperature, quantum dot size, and interdot coupling. Our results suggest that mechanisms other than phonon decoherence play a more significant role in current experimental setups. [Preview Abstract] |
Friday, March 25, 2005 11:27AM - 11:39AM |
Y17.00002: Fidelity of spin ensemble memory for mesoscopic quantum bits V.V. Dobrovitski, J.M. Taylor, M.D. Lukin Development of techniques for coherently manipulating electron spins in quantum dots is important for future applications in spintronics and in quantum information processing. In this work we study the quantum memory protocol suggested recently [1] for storage and retrieval of the electron spin states in the lattice nuclear spins. We report detailed studies of this technique in the presence of imperfections, such as the incomplete polarization of the nuclear spins and the spread in the hyperfine couplings between the electron and the nuclei. We numerically simulate the memory protocol by solving the time-dependent Schr\"odinger equation for the system comprising the electron spin and the bath spins [2]. We find that the memory operation is robust with respect to these relalistic imperfections and that high fidelity operation is possible with realistic values of nuclear spin polarization. This work was supported by the NSA, ARDA and ARO.\\ 1. J. M. Taylor, C. M. Marcus, and M. D. Lukin, Phys. Rev. Lett. {\bf 90}, 206803 (2003).\\ 2. V. V. Dobrovitski and H. A. De Raedt, Phys. Rev. E {\bf 67}, 056702 (2003) [Preview Abstract] |
Friday, March 25, 2005 11:39AM - 11:51AM |
Y17.00003: Controlling decoherence due to nuclear spins in III-V compounds: Which price do we pay? Rogerio de Sousa, Neil Shenvi, K. Birgitta Whaley Nuclear spins of the host lattice are the dominant source of decoherence in semiconductor donor and quantum dot spin qubits. There are two channels for nuclear induced decoherence: (1) Loss of visibility arising from the non-secular hyperfine coupling; (2) Spectral diffusion arising from the combined effect of inter-nuclear dipolar coupling and the secular hyperfine term. We performed numerical calculations to show that application of a moderate static magnetic field ($\sim$ 2 Tesla) is enough to suppress mechanism (1) within the $10^{-4}$ criteria of quantum error correction. On the other hand a much greater overhead is required to control mechanism (2). We consider the Carr-Purcell-Meiboom-Gill sequence as a means to control (2) and provide a realistic assessment of the required overhead in number of qubit $\pi$-pulses. We show that the required rate of $\pi$-pulsing is proportional to the nuclear spin quantum number squared, showing that robust coherent manipulation in the large spin environments characteristic of the III-V compounds is possible without resorting to nuclear spin polarization. [Preview Abstract] |
Friday, March 25, 2005 11:51AM - 12:03PM |
Y17.00004: Electron Spin Dynamics of Hyperfine Interaction in a Quantum Dot Channgxue Deng, Xuedong Hu We investigate spin dynamics of electrons in a quantum dot interacting with nuclei through the inhomogeneous hyperfine coupling. The problem has been studied previously using either perturbation theory (fails at zero nuclear spin polarization) or treating the system semi classically (Markovian approximation). In this paper we study the system systematically with a large N (effective number of nuclear spins in the dot) expansion technique which is valid at arbitrary nuclear polarization and external magnetic field. The coherent oscillations and the decoherence, represented by the poles and the branch cuts respectively, are treated in a unified way within this non-perturbative method. Our calculations reproduce previous results of highly polarized nuclear spin configuration. On the other hand we find some new features of the real time dynamics for the unpolarized nuclear spins which has not been obtained. [Preview Abstract] |
Friday, March 25, 2005 12:03PM - 12:15PM |
Y17.00005: Electron-Nuclear Spin Transfer in Triple Quantum Dot Networks Marta Prada, Ryan Toonen, Robert Blick, Paul Harrison We investigate the conductance spectra of coupled quantum dots to study systematically the nuclear spin relaxation of delta- and y-junction networks and observe spin blockade dependence on the electronic configurations. We derive the conductance using the Beenakker approach generalised to an array of quantum dots where we consider the nuclear spin transfer to electrons by hyperfine coupling. This allows us to predict the relevant memory effects on the different electronic states by studying the evolution of the single electron resonances in presence of nuclear spin relaxation. We find that the gradual depolarisation of the nuclear system is imprinted in the conductance spectra of the multidot system. Our calculations of the temporal evolution of the conductance resonance reveal that spin blockade can be lifted by hyperfine coupling. [Preview Abstract] |
Friday, March 25, 2005 12:15PM - 12:27PM |
Y17.00006: Large Magnetic Field Gradients for Crystal Lattice Quantum Computing Jonathan Goldman, Thaddeus Ladd, Charles Santori, Shinichi Koseki, Glenn Solomon, Bingyang Zhang, Yoshinori Matsumoto, Fumiko Yamaguchi, Yoshihisa Yamamoto A quantum computer using nuclear spins in a crystal lattice requires a method for addressing individual quantum bits. This identification can be achieved with a spatially varying magnetic field. Spins at different lattice sites can have distinguishable Zeeman frequencies allowing initialization, logic operations, and measurements to be performed through radio frequency (rf) pulse techniques. Here, we present magnet designs that have gradients between 1 and 20 G/Angstrom, which are necessary to realize quantum computation with particular crystals. [Preview Abstract] |
Friday, March 25, 2005 12:27PM - 12:39PM |
Y17.00007: Experimental and Theoretical Limits on Pulse Quality in Silicon NMR Experiments Rona Ramos, Kenneth MacLean, Yanqun Dong, Dale Li, Anatoly Dementyev, Sean Barrett Previous NMR experiments on Silicon involving multiple pulses showed long lived spin echoes [A.E. Dementyev, D. Li, K. MacLean, S.E. Barrett, Phys. Rev. B, 68, 153302 (2003)], an anomalous behavior that disagrees with conventional NMR theory. The application of several pulses to a many spin system warrants the understanding of pulse quality in fine detail. A series of experiments to improve the pulse characteristics and to approach the limit of delta function, spatially homogeneous pulses were performed. These experiments and detailed calculations of the pulse fields involved will be discussed, as well as the implications in understanding the anomalous long lived behavior of previous experiments. [Preview Abstract] |
Friday, March 25, 2005 12:39PM - 12:51PM |
Y17.00008: Quantum and Classical Spin Dynamics in Silicon NMR Yanqun Dong, Rona Ramos, Kenneth MacLean, Anatoly Dementyev, Dale Li, Sean Barrett Recent Si29 NMR measurements [A.E. Dementyev, D. Li, K. MacLean, S.E. Barrett, Phys. Rev. B, 68, 153302(2003)] revealed several surprises, such as unexpectedly long tails and even-odd asymmetry in CPMG data. To understand these phenomena, we implemented a series of simulations with increasing levels of sophistication designed to reflect the actual experimental conditions (e.g. including H1 field inhomogeneity, finite pulse duration, etc.). I will present the simulations, compare them with our data and discuss the implications. [Preview Abstract] |
Friday, March 25, 2005 12:51PM - 1:03PM |
Y17.00009: Puzzles in Silicon NMR: Pushing the limits of pulse quality Dale Li, Anatoly Dementyev, Yanqun Dong, Rona Ramos, Sean Barrett Recent observations of anomalously long-lived spin echoes in Silicon are still not understood [A.E. Dementyev, D.Li, K. MacLean, S.E. Barrett, Phys. Rev. B 68, 153302 (2003).]. This talk will discuss our most recent experimental efforts to understand the basis of this effect. In particular, detailed studies of the effect of pulse quality (including spatial uniformity across our many-spin system) will be reported. [Preview Abstract] |
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Y17.00010: Relaxation of the electron spin in a quantum dot due to interaction with nuclear spin bath Khaled Al-Hassanieh, V.V. Dobrovitski, E. Dagotto, B.N. Harmon Understanding the dynamics of electron spins in semiconducting nanostructures is important for novel applications in spintronics and in quantum information processing. An electron spin in a quantum dot is strongly affected by its interaction with the environmental degrees of freedom, in particular, with the nuclear spins. In this work we study the longitudinal relaxation of the electron spin component $S^z$ by the nuclear spin bath for different applied fields and initial polarizations of the bath. We numerically simulate the motion of the compound system (the electron spin plus the bath) by explicitly solving the corresponding time-dependent Schr\"odinger equation using the method described in [1]. Typically, $S^z$ exhibits a pronounced oscillation with subsequent saturation; at high fields, several such oscillations are observed. We compare our numerical results with the earlier analytical predictions [2], and discuss the agreements and differences.\\[4pt] [1] V. V. Dobrovitski and H. A. De Raedt, Phys. Rev. E {\bf 67}, 056702 (2003)\\[0pt] [2] A. V. Khaetskii, D. Loss, and L. Glazman, Phys. Rev. Lett. {\bf 88}, 186802 (2002); I. A. Merkulov, E. L. Efros, and M. Rosen, Phys. Rev. B {\bf 65}, 205309 (2002). [Preview Abstract] |
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