Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session J26: Semiconductor Qubits - Spin Measurement and Noise |
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Sponsoring Units: GQI Chair: Bill Coish, McGill University Room: 328 |
Tuesday, March 19, 2013 2:30PM - 3:06PM |
J26.00001: Taming spin decoherence in silicon Invited Speaker: Stephen Lyon Electron spins in semiconductor hosts have been candidate qubits since the early days of experimental quantum computing research, but it was generally assumed that the solid state environment would limit coherence to times much shorter than that seen in isolated atoms or ions. The longest measured electron spin coherence, measured in isotopically enriched silicon, was of order 1 ms. However, over the last 8 or 10 years the measured electron spin coherence times have steadily increased as materials and experimental techniques have improved. Much of the decoherence observed in the early ensemble Electron Spin Resonance (ESR) experiments arose from interactions amongst the spins being measured. In the most highly enriched bulk silicon measured to date, the residual silicon isotopes with nuclear magnetic moments affect the coherence of electrons bound to phosphorus donors on about a 1 second time scale. The remaining decoherence is still dominated by interactions between the donor spins, even in very lightly doped Si. Other decoherence processes have been shown to be at least an order of magnitude weaker. Recent work suggested that longer spin coherence would be obtained in bismuth doped Si, where magnetic-field insensitive ``clock transitions'' occur in the GHz frequency range. Recent experiments are bearing out these suggestions. [Preview Abstract] |
Tuesday, March 19, 2013 3:06PM - 3:18PM |
J26.00002: Spin-bath autocorrelation functions directly from quantum theory Wayne Witzel, Kevin Young, Sankar Das Sarma Cluster expansion techniques have enabled accurate modeling of the effects of a bath of local spins on solid state spin qubits with proven predictive power. These calculations are performed in the context of specific echo decay experiments (Hahn echo, CPMG, etc.). Classical noise, on the other hand, is described by a single autocorrelation function (or spectral density, equivalently) that is applicable to any control-specific experiment. Such a description is very useful in searching for optimal controls to produce high fidelity quantum logic gates using well-studied techniques. We demonstrate a cluster expansion method for directly computing autocorrelation functions as expectation values in the quantum spin-bath setting and show that it is a sufficient description of the noise effects for certain regimes, particularly in the high fidelity regime of interest. We use this approach to study the theoretical impact of using optimized pulse sequences tailored to individual qubits in enriched silicon. [Preview Abstract] |
Tuesday, March 19, 2013 3:18PM - 3:30PM |
J26.00003: Error in a spin-SWAP gate due to hyperfine interaction in a double quantum dot Jo-Tzu Hung, {\L}ukasz Cywi{\'n}ski, Xuedong Hu We study the SWAP gate for two exchange-coupled electron spins under the influence of hyperfine (hf) interaction in a double quantum dot. A gate error develops during such a gate because hf interaction causes dephasing between any pair of two-spin states. We find that this gate error is initial-state-dependent. For example, an initial state in the $S_z=0$ subspace suffers only from $S-T_0$ dephasing, leading to smaller gate error than in the case of other initial states. We calculate the gate fidelity for typical initial states, and compare the resulting gate errors. We also analyze the effects of inhomogeneous broadening on the gate fidelity in the presence of a random Overhauser field. [Preview Abstract] |
Tuesday, March 19, 2013 3:30PM - 3:42PM |
J26.00004: Electron Spin Relaxation and Coherence Times in Si/SiGe Quantum Dots R.M. Jock, Jianhua He, A.M. Tyryshkin, S.A. Lyon, C.-H. Lee, S.-H. Huang, C.W. Liu Single electron spin states in Si/SiGe quantum dots have shown promise as qubits for quantum information processing. Recently, electron spins in gated Si/SiGe quantum dots have displayed relaxation (T$_{\mathrm{1}})$ and coherence (T$_{\mathrm{2}})$ times of 250 $\mu $s at 350mK. The experiments used conventional X-band (10 GHz) pulsed Electron Spin Resonance (pESR) on a large area (3.5 x 20 mm$^{\mathrm{2}})$, double gated, undoped Si/SiGe heterostructure, which was patterned with 2 x 10$^{\mathrm{8}}$ quantum dots using e-beam lithography. Dots with 150 nm radii and 700 nm period are induced in a natural Si quantum well by the gates. Smaller dots are expected to reduce the effects of nearly degenerate valley states and spin-orbit coupling on the electron spin coherence. However, the small number of spins makes signal recovery extremely challenging. We have implemented a broadband cryogenic HEMT low-noise-amplifier and a high-speed single-pole double-throw switch operating at liquid helium temperatures. The switch and preamp have improved our signal to noise by an order of magnitude, allowing for smaller samples and shorter measurement times. We will describe these improvements and the data they have enabled. [Preview Abstract] |
Tuesday, March 19, 2013 3:42PM - 3:54PM |
J26.00005: Quantum theory of dynamic nuclear polarization in quantum dots Sophia Economou, Edwin Barnes Nuclear spins play a major role in the dynamics of spin qubits in III-V semiconductor quantum dots. Although the hyperfine interaction between nuclear and electron (or hole) spins is typically viewed as the leading source of decoherence in these qubits, understanding how to experimentally control the nuclear spin polarization can not only ameliorate this problem, but in fact turn the nuclear spins into a valuable resource for quantum computing. Beyond extending decoherence times, control of this polarization can enable universal quantum computation as shown in singlet-triplet qubits and, in addition, offers the possibility of repurposing the nuclear spins into a robust quantum memory. In [1], we took a first step toward taking advantage of this resource by developing a general, fully quantum theory of non-unitary electron-nuclear spin dynamics with a periodic train of delta-function pulses as the external control driving the electron spin. Here, we extend this approach to other types of controls and further expand on the predictions and physical insights that emerge from the theory. [1] Edwin Barnes and Sophia E. Economou, Phys. Rev. Lett. 107, 047601 (2011) [Preview Abstract] |
Tuesday, March 19, 2013 3:54PM - 4:06PM |
J26.00006: Enhanced spin-flip transport in a quantum dot spin-valve with uniform hyperfine coupling Stefano Chesi, William A. Coish We study the transport current and nuclear spin polarization dynamics in a quantum dot spin-valve, for which a strong enhancement of the spin-flip electron tunneling rates can be realized in the limit of uniform hyperfine interaction. We extend the analogy of transport to superradiance, directly applicable to a spin valve with half-metal leads and a maximally polarized nuclear system, to the more general situation of ferromagnetic contacts and a nuclear system initially fully dephased and partially polarized, as naturally realized at finite bias under stationary conditions. An analytic treatment of the dynamics in terms of simple rate equations becomes possible for very fast/slow nuclear dephasing. We recover these limiting results, as well as analyze the crossover regime, from a general master equation for the nuclear dynamics. We also present strategies to approach the limit of uniform hyperfine interaction in realistic heterostructures. [Preview Abstract] |
Tuesday, March 19, 2013 4:06PM - 4:18PM |
J26.00007: Single Electron Spin Resonance in a Si-MOS Double Quantum Dot Xiaojie Hao, Ming Xiao, Hongwen Jiang, Rusko Ruskov, Charles Tahan Pauli spin blockade is used as a means to detect the flip of spins in a silicon metal-oxide-semiconductor (MOS) based double quantum dot. Microwave driven electron spin resonance (ESR) signals, with a linewidth as narrow as 1.5 G, has been observed only in a narrow range of magnetic fields. ESR spectroscopy in the magnetic field - microwave frequency plane shows an unexpected level anti-crossing, with an energy gap of about 50 MHz. The spectral line gives an estimation of the lower bound for inhomogeneous phase decoherence time $T_{2}^{*}$ of about a couple of hundred ns for individual spins in the nano-structured system with a Si/SiO2 interface. We explain the anti-crossing gap as due to spin-orbit mixing with higher states, which is also responsible for the narrow-window visibility of the ESR signal in Si based double quantum dots. [Preview Abstract] |
Tuesday, March 19, 2013 4:18PM - 4:30PM |
J26.00008: Theory of Spin Relaxation in Two-Electron Laterally Coupled GaAs and Si Quantum Dots Martin Raith, Peter Stano, Jaroslav Fabian We present quantitative results of the phonon-induced spin relaxation in two-electron lateral double quantum dots for a wide range of tuning parameters. Both spin-orbit coupling and hyperfine coupling are taken into account. Our analysis of GaAs [1] and silicon [2] based dots includes the variation of the electric field (detuning), the exchange coupling, and the magnetic field strength and orientation. The focus is on experimentally important regimes. We find that even in strong magnetic fields, the hyperfine coupling can dominate the relaxation rate of the unpolarized triplet in a detuned double dot. Where the spin-orbit coupling dominates, the rate is strongly anisotropic and its maxima and minima are generated by an in-plane magnetic field either parallel or perpendicular to the dots' alignment dependent on specifics, such as spectral (anti-)crossings (spin hot spots), or the detuning strength. For all regimes, we give qualitative explanations of our observations. We emphasize the differences between GaAs and Si based dots. By understanding the spin lifetimes ($T_1$), this work marks a crucial step toward the realization of two-electron semiconductor qubits for quantum information processing.\\[4pt] [1] M. Raith et. al., PRL 108, 246602 (2012)\\[0pt] [2] M. Raith et. al., arXiv:1206.6906 [Preview Abstract] |
Tuesday, March 19, 2013 4:30PM - 4:42PM |
J26.00009: Anomalous electron spin decoherence in an optically pumped quantum dot Xiaofeng Shi, L.J. Sham We study the nuclear-spin-fluctuation induced spin decoherence of an electron (SDE) in an optically pumped quantum dot. The SDE is computed in terms of the steady distribution of the nuclear field (SDNF) formed through the hyperfine interaction (HI) with two different nuclear species in the dot. A feedback loop between the optically driven electron spin and the nuclear spin ensemble determines the SDNF [W. Yang and L. J. Sham, Phy. Rev. B 85, 235319(2012)]. Different from that work and others reviewed therein, where a bilinear HI, $S_{\alpha}I_{\beta}$, between the electron (or hole) spin $\mathbf{S}$ and the nuclear spin $\mathbf{I}$ is used, we use an effective nonlinear interaction of the form $S_{\alpha}I_{\beta}I_{\gamma}$ derived from the Fermi-contact HI. Our feedback loop forms a multi-peak SDNF in which the SDE shows remarkable collapses and revivals in nanosecond time scale. Such an anomalous SDE results from a quantum interference effect of the electron Larmor precession in a multi-peak effective magnetic field. In the presence of a bilinear HI that suppresses the nuclear spin fluctuation, the non-Markovian SDE persists whenever there are finite Fermi contact interactions between two or more kinds of nuclei and the electron in the quantum dot. [Preview Abstract] |
Tuesday, March 19, 2013 4:42PM - 4:54PM |
J26.00010: Mechanisms for Electric Field Control of Single Spin Relaxation in Double Quantum Dots V. Srinivasa, K.C. Nowack, M. Shafiei, L.M.K. Vandersypen, J.M. Taylor We theoretically investigate electrically-tunable spin-flip transitions for a single electron confined within a double quantum dot. In the presence of spin-orbit and hyperfine interactions, the rate at which phonon-induced spin relaxation occurs depends non-monotonically on the detuning between the dots. We analyze this detuning dependence for both direct decay to the ground state and indirect decay via an intermediate excited state of the double dot. A description in terms of a simple toy model captures characteristic features of the relaxation rate recently measured for GaAs double quantum dots. Our results suggest that spin-orbit mediated relaxation via phonons serves as the dominant mechanism through which the electron spin-flip rate in these systems varies with detuning. [Preview Abstract] |
Tuesday, March 19, 2013 4:54PM - 5:06PM |
J26.00011: Enhanced hyperfine-induced spin dephasing in a magnetic field gradient Felix Beaudoin, William A. Coish Magnetic field gradients are important for single-site addressability and electric-dipole spin resonance of electrons in quantum dots or in donor impurities. We show that these advantages are offset by a potential reduction in coherence time. Although the magnetic field appears uniform to the electron, it provides a non-uniform field for the nuclear-spin bath. This leads to a finite bath correlation time, preventing the full recovery of electron-spin coherence. We apply our model to single electron spins in quantum dots and single donor impurities, singlet-triplet spin qubits, and consider both free-induction decay and spin-echo. This mechanism can dominate over known dephasing sources due to nuclear dipole-dipole interactions and hyperfine flip-flops. This result is especially important for systems requiring large magnetic field gradients, including spin qubits coupled to superconducting stripline resonators. [Preview Abstract] |
Tuesday, March 19, 2013 5:06PM - 5:18PM |
J26.00012: Spin Qubit Relaxation in a Moving Quantum Dot Peihao Huang, Xuedong Hu Long-range quantum communication for spin qubits is a significant open problem in the scale-up of spin qubit architectures. Among the many spin information transfer proposals, directly moving the electrons themselves is attractive because of its conceptual simplicity and its similarity to the conventional charge-coupled devices. Here we focus on electron spin decoherence when the quantum dot is in motion. Specifically, we study a spin decoherence mechanism for a moving but confined electron due to the spin-orbit interaction and an environmental random electric potential. We find that at the lowest order, the magnetic fluctuations experienced by the spin have only components transverse to the total magnetic field, so that the motion induced spin decoherence is a pure longitudinal relaxation channel. Our calculated spin relaxation time ranges from as fast as sub $\mu$s in GaAs to above ms in Si. Our results also clearly indicates how to reduce the decoherence effects of electron motion. [Preview Abstract] |
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