Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T36: Focus Session: Semiconductor Qubits: Magnetic Control & Nuclear Dynamics 
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Sponsoring Units: GQI Chair: Jason Petta, Princeton University Room: 703 
Thursday, March 6, 2014 11:15AM  11:27AM 
T36.00001: Spin measurement in an undoped Si/SiGe double quantum dot incorporating a micromagnet Xian Wu, Daniel Ward, Jonathan Prance, Dohun Kim, Zhan Shi, Robert Mohr, John Gamble, Donald Savage, Max Lagally, Mark Friesen, Susan Coppersmith, Mark Eriksson We present measurements on a double dot formed in an accumulationmode undoped Si/SiGe heterostructure. The double dot incorporates a proximal micromagnet to generate a stable magnetic field difference between the quantum dots. The gate design incorporates two layers of gates, and the upper layer of gates is split into five different sections to decrease crosstalk between different gates. A novel pattern of the lower layer gates enhances the tunability of tunnel rates. We will describe our attempts to create a singlettriplet qubit in this device. This work was supported in part by ARO(W911NF120607), NSF(DMR1206915), and the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government. [Preview Abstract] 
Thursday, March 6, 2014 11:27AM  11:39AM 
T36.00002: Singlespin manipulation via exchange interaction in a double quantum dot with micromagnet Stefano Chesi, YingDan Wang, Daniel Loss The manipulation of single spins in double quantum dots by making use of the exchange interaction and a highly inhomogenous magnetic field was discussed in W. A. Coish and D. Loss, Phys. Rev. B 75, 161302 (2007). Given that such large inhomogeneity of the magnetic field is difficult to achieve, we examine an analogous scheme applicable to current double quantum dot setups in the presence of the stray field of a neighboring micromagnet. We estimate typical gate times realized at the singlettriplet anticrossing induced by the micromagnet field, and discuss the optimization of the singlespin gates through suitable pulse shapes and orientation of the micromagnet magnetization. We also examine the effect of several decoherence sources, as in particular the Overhauser field induced by nuclear spins and charge noise from the electric gates, and characterize the corresponding decay of the Rabi oscillations. Our results suggest that this scheme is a promising approach for the realization of fast singlespin operations. [Preview Abstract] 
Thursday, March 6, 2014 11:39AM  11:51AM 
T36.00003: Strongly excited electric dipole spin resonance with field gradient Yasuhiro Tokura Coherent manipulation of the qubit is the essential part of the quantum information processing. Traditionally, spin manipulation is realized by electron spin resonance, where timedependent transverse magnetic field of frequency close to the Zeeman energy by the external static magnetic field. The idea of electric dipole spin resonance, which uses oscillating electric field, instead of magnetic field, had been proposed. Electron spin dipole itself is independent of the electric field, while the charge (orbital) degree of freedom in a quantum dot (QD) is efficiently coupled to it. With the gradient of the static magnetic field coupling the orbital degree with the spin, the spin can be manipulated. Rabi frequency characterizes the driving speed of the spin, which is usually regarded as linearly proportional to the electric field amplitude. We had studied the Rabi frequency in two models. One is that the orbital state is also twolevel system [1], which may be corresponding to the lowest levels in the coupled QDs. The other is that the electron is in anharmonic potential. In both cases, we predict a clear deviation of the Rabi frequency from the linear dependence for large electric field.\\[4pt] [1] Y. Tokura T. Kubo, and W. J. Munro, to appear in J. Phys. Soc. Jpn. (arXiv: 1308.0071). [Preview Abstract] 
Thursday, March 6, 2014 11:51AM  12:27PM 
T36.00004: Control and coherence of LossDiVincenzo qubits in Si/SiGe quantum dots Invited Speaker: Pasquale Scarlino Electron spins in Si/SiGe quantum dots are one of the most promising candidates for a quantum~bit~for their potential to scale up and their long dephasing time. We report for the first time the experimental realization of single electron spin rotations in a single quantum dot (QD) defined in a Si/SiGe 2D electron gas. The electron spin is read out in singleshot mode by a QD charge sensor. Spin rotations are achieved by applying microwave excitation to one of the gates, which oscillates the electron wave function back and forth in the gradient field produced by cobalt micromagnets fabricated near the dot. By measuring the electron spin resonance frequency as a function of the external magnetic field, the electron gfactor of 1.994 $\pm$ 0.007 is determined. A dephasing time of T2*$=$850 ns, about 20 times longer than that in GaAs quantum dots, is extracted from the linewidth of the electron spin resonance peak. We observe spin Rabi oscillations with Rabi frequencies up to 5 MHz. Because the coherence time can be longer than the spin manipulation time, we are able to rotate the electron spin even when detuned in frequency, giving the typical chevron pattern when sweeping detuning and microwave burst time. We also realized Ramsey interference experiments, giving a free induction decay T2* $=$ 800 ns. Looking closely, all these data exhibit interference patterns resulting from the contribution of two resonances separated by a frequency difference $\Delta $f $=$24 MHz. We tentatively interpret these two resonances as intravalley spin resonance for two different valley states. Due to the valleyorbit mixing, the orbital wavefunction of each valley state is slightly different, which yields a different Zeeman splitting for each valley state. Finally, we perform Hahnecho measurement and deduce, for the first time in Si/SiGe, a single spin T2$=$37$\mu $s.\\[4pt] This work has been done in collaboration with E. Kawakami, Kavli Institute of Nanoscience, TU Delft, Lorentzweg 1, 2628 CJ Delft, The Netherlands; D. Ward, University of WisconsinMadison, Madison, Wisconsin 53706, USA; F.R. Braakman, Kavli Institute of Nanoscience, TU Delft; D. E. Savage, M. G. Lagally, S.N. Coppersmith, M. A. Eriksson, University of WisconsinMadison; and L. M. K. Vandersypen, Kavli Institute of Nanoscience, TU Delft. [Preview Abstract] 
Thursday, March 6, 2014 12:27PM  12:39PM 
T36.00005: Spinorbit effects on nuclear state preparation at the $ST_{+}$ anticrossing in double quantum dots Marko Rancic, Guido Burkard We explore the interplay of spinorbit and hyperfine effects on the nuclear preparation schemes in twoelectron double quantum dots, e.g. in GaAs. The quantity of utmost interest is the electron spin decoherence time $T_{2}^{*}$ in dependence of the number of sweeps through the electron spin singlet $S$ triplet $T_{+}$ anticrossing. Decoherence of the electron spin is caused by the difference field induced by the nuclear spins. We study the case where a singlet $S(2,0)$ is initialized, in which both electrons are in the left dot. Subsequently, the system is driven repeatedly through the anticrossing and back using linear electrical bias sweeps. Our model describes the passage through the anticrossing with a large number of equally spaced, steplike parameter increments. We develop a numerical method describing the nuclear spins fully quantum mechanically, which allows us to track their dynamics. Both Rashba and Dresselhaus spinorbit terms do depend on the angle $\theta$ between the $[110]$ crystallographic and the interdot axis. Our results show that the suppression of decoherence (and therefore the enhancement of $T_{2}^{*}$) is inversely proportional to the strength of the spinorbit interaction, which is tuned by varying the angle $\theta$. [Preview Abstract] 
Thursday, March 6, 2014 12:39PM  12:51PM 
T36.00006: Nuclear spin dynamics and spin orbit effects in LandauZener sweep correlations at the S$\mathrm{T_+}$ Transition Christian Dickel, Sandra Foletti, Amir Yacoby, Diana Mahalu, Vladimir Umansky, Hendrik Bluhm In GaAsbased double quantum dot spin qubits, nuclear spins have been used for qubit control, but are also an important source of decoherence. The S and $\mathrm{T_+}$ levels exhibit a small avoided crossing as a function of detuning. It has been used for S$\mathrm{T_+}$ qubit control and for dynamic nuclear polarization (DNP). The transition matrix element contains the nuclear Overhauser fields perpendicular to the external Bfield and spinorbit coupling. We show, both theoretically and experimentally, that nuclear spin dynamics can be seen in the temporal correlation of singleshot measurements after LandauZener sweeps across this transition. A semiclassical model of the nuclear spins is sufficient. The dynamics consist of the relative Larmor precession of the three GaAs nuclear spin species in the external Bfield and dephasing of the oscillations due to local field fluctuations. Theoretically, it is expected that the absolute Larmor precessions also become visible in the presence of spinorbit coupling. This can be used to qualitatively and quantitatively observe spinorbit coupling and to distinguish it from the nuclear spin contribution. Understanding these dynamics is relevant for the fidelity of S$\mathrm{T_+}$ qubit operations and the effectiveness of DNP. [Preview Abstract] 
Thursday, March 6, 2014 12:51PM  1:03PM 
T36.00007: Mechanisms of $ST_+$ coupling in singlettriplet qubits John Nichol, Michael Shulman, Shannon Harvey, Vladimir Umansky, Amir Yacoby Semiconductor quantum dots provide a unique environment for studying a variety of problems, from fewparticle systems to the central spin problem. Investigating these systems furthers our understanding of fundamental physics and advances efforts to achieve semiconductor spinbased quantum information processing. We study two electron spins in a semiconducting double quantum dot and measure the spin singlet to $m_s=1$ triplet ($ST_+$) avoided crossing. Our results suggest that several processes, including the hyperfine interaction between the electrons and the host nuclei and spinorbit coupling in the quantum dots, compete to drive the $ST_+$ transition. This work gives insight into the poorly understood nuclear dynamics in these systems and provides a path forward for improving nuclear pumping efficiency and therefore coherence times in semiconducting spin qubits. [Preview Abstract] 
Thursday, March 6, 2014 1:03PM  1:15PM 
T36.00008: Precise Measurement of Nuclear Magnetic Fields with Gallium Arsenide Spin Qubits Shannon Harvey, Michael Shulman, John Nichol, Vladimir Umansky, Amir Yacoby Qubits that can be easily initialized and read out hold promise both for metrology and for quantum information processing. In particular, spin qubits in semiconductor quantum dots are able probe their rich magnetic and electric environment and study spin and charge dynamics in semiconductors. In this talk, we present measurements using a singlettriplet (ST$_{\mathrm{0}})$ qubit in a gallium arsenide double quantum dot to measure the nuclear magnetic field gradient surrounding it. This nuclear bath has slow dynamics compared to the timescale of qubit operations and measurement, so precise sensing of nuclear fields can be done with repeated projective measurement. We compare three techniques for rapidly estimating the nuclear gradient, and verify it to within 1 MHz in 800 us of real time for the best performing scheme. This level of precision offers the prospect of performing realtime monitoring of the nuclear gradient, which could both yield insight into quantum manybody problems such as nuclear spin diffusion and be used to drive the qubit adaptively in a way that is insensitive to nuclear noise. [Preview Abstract] 
Thursday, March 6, 2014 1:15PM  1:27PM 
T36.00009: Suppression of Dephasing using Real Time Environmental Monitoring Michael Shulman, Shannon Harvey, John Nichol, Vladimir Umansky, Amir Yacoby Electron spins in semiconductor quantum dots are promising candidates for the building blocks of a quantum information processor due to their potential for scalability and miniaturization. However, interactions between the electrons and a fluctuating nuclear bath cause these qubits to dephase in tens of nanoseconds. These ill effects can be partially mitigated by nuclear programming or dynamical decoupling; however, these techniques are limited by nuclear pumping efficiency and the complexity of decoupled sequences. Here we present a new scheme that stabilizes the qubit by exploiting the slow nuclear dynamics. The qubit measures the size of the nuclear splitting, which is used in real time to feed back on the control of the qubit. We employ this technique on a singlettriplet qubit operated in the rotating frame in a regime where it is sensitive to fluctuating nuclear magnetic fields, and we show that this feedback increases qubit coherence times. This feedback is distinct from schemes that constantly monitor a qubit through weak measurement and can improve arbitrary qubit operations in all qubits that suffer dephasing from slow environmental fluctuations, including all spin qubits. [Preview Abstract] 
Thursday, March 6, 2014 1:27PM  1:39PM 
T36.00010: Quantum limit for nuclear spin polarization in semiconductor quantum dots Julia Hildmann, Eleftheria Kavousanaki, Guido Burkard, Hugo Ribeiro One of main sources of decoherence for spin qubits confined in semiconductor quantum dots comes from hyperfine interaction of the electron spin with the nuclear spins. By polarizing the nuclear spins to 100\% it is possible to extend coherence times. A recent experiment [E. A. Chekhovich \emph{et al.}, Phys. Rev. Lett. \textbf{104}, 066804 (2010)] has demonstrated that high nuclear spin polarization can be achieved in selfassembled quantum dots by exploiting an optically forbidden transition between a heavy hole and a trion state. However, a fully polarized state is not obtained as expected from a classical rate equation. We theoretically investigate this problem with the help of a quantum master equation and we demonstrate that a fully polarized state cannot be reached due to formation of a nuclear dark state. We also show that the maximal degree of polarization depends on the form of the electron envelope wave function inside of the quantum dot. [Preview Abstract] 

T36.00011: ABSTRACT WITHDRAWN 
Thursday, March 6, 2014 1:51PM  2:03PM 
T36.00012: Feedback control of nuclear spin bath for a single hole spin in a quantum dot Hongliang Pang, Zhirui Gong, Wang Yao In a semiconductor quantum dot, the nuclear spin bath plays an important role as the ultimate environment of an electron or hole spin at low temperature. Through dynamic nuclear spin polarization driven by an oscillating electric field, we show that feedback controls can be implemented on the nuclear spin bath of a single hole spin. The feedback controls utilize the anisotropic hyperfine interaction between the hole spin and the nuclear spins. The negative feedback can suppress the statistical fluctuations of the nuclear hyperfine field and lead to longer coherence time of the hole spin. Positive feedback can possibly lead to cat like state of nuclear spin bath. The efficiency of the controls schemes is investigated under different parameters and control strategies. [Preview Abstract] 
Thursday, March 6, 2014 2:03PM  2:15PM 
T36.00013: Optical nuclear spin polarization in the presence of heavy hole hyperfine interactions Hugo Ribeiro, Franziska Maier, Daniel Loss In selfassembled quantum dots, the form of the effective hyperfine Hamiltonian for heavy holes states is still under debate. The first theories suggested an Isinglike type of interaction with a strength on the order of 10\% of the one of the electron and with opposite sign. Consequently, flipflop terms similar to those of the electronic hyperfine Hamiltonian are very weak and do not provide an efficient mechanism for exchange of angular momentum. However, due to band mixing, matrix elements of the hyperfine Hamiltonian taken with the same effective heavy hole state are nonzero and can lead to transitions in the nuclear spin state. Here, we propose an experiment aiming at detecting and simultaneously cancel the effective hyperfine heavy hole noncollinear interaction. Although its relative strength is in average three orders of magnitude smaller than the electronic hyperfine coupling constant, the effective noncollinear interaction is efficient at polarizing nuclear spins. Our results force a complete reinterpretation of experiments dealing with nuclear spins in optically active quantum dots. [Preview Abstract] 
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