Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V27: Focus Session: Semiconductor Qubits- Dynamic Decoupling, Dephasing, and Relaxation |
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Sponsoring Units: GQI Chair: Hendrik Bluhm, Harvard University Room: C155 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V27.00001: Increasing Quantum Dot Electron Spin Coherence with Persistent Spin Narrowing Bo Sun, Colin Chow, Allan Bracker, Daniel Gammon, Lu Sham, Duncan Steel We demonstrate reproducible initialization of the Overhauser field in a single InAs self-assembled quantum dot using the hole assisted nuclear feedback mechanism. This fixes the mean the Overhauser field to a value determined by two pump lasers and dramatically reduces the statistical broadening of the electron spin resonance arising from averaging over the nuclear spin ensemble, (1/T2*). By initializing for tens of milliseconds, the prepared Overhauser field distribution lasts for well over a second even in the presence of a fluctuating electron spin. Furthermore, we find a mechanism which will initialize the nuclear spins using only a single laser, and that this mechanism involves the evolution of the nuclear spins ``in the dark'', that is, absent any optical field. This new method is directly compatible with the CW readout technique used in recent time-domain spin manipulation experiments. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V27.00002: Effects of Multi-pulse Dynamical Decoupling Schemes on Dephasing in a GaAs Spin Qubit James Medford, Christian Barthel, Charles Marcus, Micah Hanson, Arthur Gossard Coherence time ($T_2$) of a singlet-triplet qubit in a GaAs double quantum dot is studied as a function of the number of $\pi$-pulses in a Carr-Purcell-Meiborn-Gill (CPMG) dynamical decoupling sequence. In this system, the dominant forms of dephasing are expected to be hyperfine coupling to the nuclei and electrical noise. For $n_\pi$ ranging from 2 to 32, we find a power law dependence of $T_2$ with the number of pulses, $T_2 \propto n_\pi^\beta$, where $n_\pi$ is the number of pulses and $\beta \sim 0.7$ is a fit parameter. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V27.00003: The central spin problem: electron spin qubit evolution due to coherently evolving nuclear spins Izhar Neder, Mark Rudner, Hendrik Bluhm, Bertrand Halperin, Amir Yacoby In recent years, electron spin qubits in solid state quantum dots have emerged as promising candidates for the implementation of quantum information processing. We study the dephasing of two electron spins in a double quantum dot system due to the evolution of the underlying nuclear spins, as was measured in a recent spin echo experiments. We develop a semi-classical model for such a system, by treating the Overhauser field induced by the nuclear spins as a classical time-dependent vector. Comparing the outcome of this model to experimental echo signal of the electron qubit allows us to identify MNR-like signatures from the nuclear spin evolution, such as spin diffusion, coherent nuclear Larmor precession and the spread of the Larmor frequencies by various mechanisms. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V27.00004: Generating Entanglement and Squeezed States of Nuclear Spins in Quantum Dots Mark Rudner, Lieven Vandersypen, Vladan Vuletic, Leonid Levitov Entanglement generation and detection are two of the most sought-after goals in the field of quantum control. Besides offering a means to probe some of the most peculiar and fundamental aspects of quantum mechanics, entanglement in many-body systems can be used as a tool to reduce fluctuations below the standard quantum limit. For spins, or spin-like systems, such a reduction of fluctuations can be realized with so-called squeezed states [1]. Here we present a scheme for achieving coherent spin squeezing of nuclear spin states in single electron quantum dots [2]. This work represents a major shift from earlier studies, which have explored classical ``narrowing'' of the nuclear polarization distribution through feedback involving stochastic spin flips. In contrast, we use the nuclear-polarization-dependence of the electron spin resonance (ESR) line to provide a non-linearity which generates a non-trivial, area-preserving, ``twisting'' dynamics which squeezes and stretches the nuclear spin Wigner distribution without the need for nuclear spin flips. \\[4pt] [1] M. Kitagawa, M. Ueda, Phys. Rev. A 47, 5138 (1993). \\[0pt] [2] M. S. Rudner, L. V. M. Vandersypen, V. Vuletic, L. S. Levitov, to be published. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V27.00005: Dephasing of two-spin qubits due to their charge and nuclear environments Guy Ramon We consider dephasing of qubits encoded in the singlet and unpolarized triplet states of pairs of spins localized in biased double quantum dots. The charge environment is modeled by both two-center charge traps in the insulator (where electrons tunnel between the two centers), and single charge traps located near the gate electrodes and QPCs (where electrons charge and empty the trap). The couplings of these trapped charges to the qubits are calculated by considering their charge distributions within a multipole expansion. It is demonstrated that the summation over these random telegraph processes in mesoscopic devices results in non-Markovian and non-Gaussian noise. For the nuclear environment we consider hyperfine-induced electron-spin dephasing in a nuclear spin bath with narrowed distribution. Nuclear state preparation using dynamical polarization cycles was experimentally achieved recently, and it is also essential to enable $X$-rotations for two-spin qubits. Our analysis is performed for both free induction and echo signals. The scaling of these dephasing mechanisms with the number of qubits is also discussed. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V27.00006: Noisy (spin) neighbors of a solid state (spin) qubit Wayne Witzel, Malcolm Carroll, Lukasz Cywinski, Sankar Das Sarma Powerful computational methods have been developed in recent years for understanding decoherence induced by environmental spins. Specifically, the cluster correlation expansion [Phys. Rev. B 78, 085315 (2008)] and adaptations [Phys. Rev. Lett. 105, 187602 (2010)] provide successive approximations that approach the solution to the full quantum mechanical problem for small and large spin baths with good efficiency. We present our findings from these computations. These have implications for solid state spin qubit fabrication and materials choices. In silicon where nuclear spins may be eliminated through isotopic enrichment, we consider other sources of bath spins in the bulk and near interfaces. We also investigate the conditions under which we may abstract out an approximate noise model that is independent of operations applied to the qubit. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V27.00007: Competing effects of hyperfine and spin-orbit interactions in two-electron spin qubits Ernesto Cota, Sergio Ulloa We analyze the dynamics of a double quantum dot system with two electrons in a uniform magnetic field, taking into account the hyperfine interaction as well as the interdot tunneling-induced Rashba spin-orbit coupling. The former mixes the singlet and triplet (1,1) states while the latter accounts for mixing triplet states and the doubly occupied (0,2) singlet. We focus on the effects on experimental results in GaAs dots [1], involving the generation and control of a nuclear field gradient, necessary for full quantum control of this electron spin qubit. Using a complete description of the quantum states involved in the dynamics and numerical solution of the time-dependent Schr\"{o}dinger equation, we study different pumping processes used to polarize (and read) the nuclear system, creating a large inhomogeneous nuclear field. We evaluate the fidelity of gate operations involving the two-electron qubit in the presence of competing spin-flip interactions as well as the implementation of these operations in quantum computation with characteristic experimental dot systems. \\[4pt] [1] S. Foletti et al., Nature Physics 5, 903 (2009) [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V27.00008: Two-eletron spin relaxation in double quantum dots and P donors Chia-Wei Huang, Massoud Borhani, Xuedong Hu We study singlet-triplet relaxation of two electrons confined in a double quantum dot or bound to P donors in Silicon. Hyperfine interaction of the electrons with the host/phosphorus nuclei, in combination with the electron-phonon interaction, leads to relaxation of the triplet states. We calculate the triplet relaxation rates in the presence of an applied magnetic field. This relaxation mechanism affects, for example, the resonance peaks in current Electron Spin Resonance (ESR) experiments on P-dimers. Moreover, the estimated time scales for the spin decay put an upper bound on the gate pulses needed to perform fault-tolerant two-qubit operations in spin-based quantum computers. We have found the optimal regimes, which mitigate this relaxation mechanism, yet permit sufficiently fast two-qubit operations. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V27.00009: Spin-orbit induced two-electron spin relaxation in double quantum dots Massoud Borhani, Xuedong Hu We study the spin decay of two electrons confined in a double quantum dots via the spin-orbit interaction and acoustic phonons. We have obtained a generic form for the spin Hamiltonian for two electrons confined in (elliptic) harmonic potentials in doubles dots and in the presence of an arbitrary applied magnetic field. Our focus is on the interdot bias regime where singlet-triplet splitting is small, in contrast to the spin-blockade regime. Our results clarify the spin-orbit mediated two-spin relaxation in lateral/nanowire quantum dots, particularly when the confining potentials are different in each dot. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V27.00010: Impurity effects on coupled quantum dot spin qubits in semiconductors Nga Nguyen, Sankar Das Sarma Localized electron spins confined in semiconductor quantum dots are being studied by many groups as possible elementary qubits for solid-state quantum computation. We theoretically consider the effects of having unintentional charged impurities in laterally coupled two-dimensional double (GaAs) quantum dot systems, where each dot contains one or two electrons and a single charged impurity in the presence of an external magnetic field. We calculate the effect of the impurity on the 2-electron energy spectrum of each individual dot as well as on the spectrum of the coupled-double-dot 2-electron system. We find that the singlet-triplet exchange splitting between the two lowest energy states, both for the individual dots and the coupled dot system, depends sensitively on the location of the impurity and its coupling strength (i.e. the effective charge). We comment on the impurity effect in spin qubit operations in the double dot system based on our numerical results. This work is supported by LPS-CMTC and CNAM. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V27.00011: Theory of anisotropic exchange in laterally coupled quantum dots Fabio Baruffa, Peter Stano, Jaroslav Fabian We consider an interacting pair of quantum dot electron spin qubits (a two electron double quantum dot). In this setup, two-qubit operations are generated by the (isotropic) exchange interaction, which results from the tunable inter-dot coupling. In the presence of spin-orbit interactions, additional effective inter-qubit coupling arises, termed anisotropic exchange. We show that in GaAs, where spin-orbit interactions are weak, the magnitude of the anisotropic exchange is proportional to the external magnetic field and therefore directly controllable, boosting prospects for spin-based quantum computing. We show how the form of anisotropic exchange follows from its spin-orbit origin and that its magnitude can be traced down to dipole moment matrix elements. Based on this findings, we propose an effective spin Hamiltonian suitable for practical modeling of two-electron spin dynamics. We prove the effective Hamiltonian quantitative accuracy confronting it with a microscopic numerical model.\\[4pt] [1] F. Baruffa, P. Stano, J. Fabian, Phys. Rev. Lett. 104, 126401 (2010)\\[0pt] [2] F. Baruffa, P. Stano, J. Fabian, Phys. Rev. B 82, 045311 (2010) [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V27.00012: Many-body singlets of nuclear spins Wang Yao We show that dynamic spin polarization by collective raising and lowering operators can drive a spin ensemble from arbitrary initial state to many-body singlets, the zero-collective-spin states with large scale entanglement. For an ensemble of $N$ arbitrary spins, both the variance of the collective spin and the number of unentangled spins can be reduced to $O(1)$ (versus the typical value of $O(N)$), and many-body singlets can be occupied with a population of $\sim 20 \%$ independent of the ensemble size. We implement this approach in a mesoscopic ensemble of nuclear spins through dynamic nuclear spin polarization by an electron. The result is of two-fold significance for spin quantum technology: (1) a resource of entanglement for nuclear spin based quantum information processing; (2) a cleaner surrounding and less quantum noise for the electron spin as the environmental spin moments are effectively annihilated. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V27.00013: Atomistic theory of spin relaxation in self-assembled (In, Ga)As/GaAs quantum dots at zero magnetic field Lixin He, Hai Wei, Ming Gong, G.-C. Guo We investigated the spin-flip time (T$_{1}$) of electrons and holes mediated by acoustic phonons in self-assembled In(Ga)As/GaAs quantum dots at zero magnetic field, using an atomistic pseudopotential method. At low magnetic field, the first-order process is suppressed, and the second-order process becomes dominant. We find that the spin-phonon-interaction induced spin relaxation time is 40 - 80 s for electrons, and 1 -20 ms for holes at 4.2 K. The calculated hole-spin relaxation times are in good agreement with recent experiments, which suggests that the two-phonon process is the main relaxation mechanism for hole-spin relaxation in the self-assembled quantum dots at zero field. We further clarify the structural and alloy composition effects on the spin relaxation in the quantum dots. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V27.00014: Optically controlled electron-nuclear spin dynamics in a quantum dot Edwin Barnes, Sophia Economou In recent years, a large number of experiments involving coherent and incoherent control of electron spins in quantum dots have revealed the important role of the nuclear spins of the host material. Experiments with optical controls, both pulsed and continuous wave, have shown that the feedback of the nuclear spins on the electron spin strongly affects the electron spin response. However, a microscopic theory of this mechanism is not available at present. We introduce a formalism that allows us to investigate this system without invoking any phenomenological spin-flip rates for the nuclei. We derive the electron-nuclear dynamics under the influence of external periodic pulsed control to second order in the electron-nuclear hyperfine coupling. Our formalism should have wide applications in both coherently and incoherently driven electron spins interacting with a nuclear spin bath, including self-assembled and gated quantum dots. [Preview Abstract] |
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