Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K28: Control and Calibration of Semiconducting QubitsFocus

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Sponsoring Units: DQI Chair: Thaddeus Ladd, HRL Laboratories, LLC Room: LACC 405 
Wednesday, March 7, 2018 8:00AM  8:36AM 
K28.00001: Dynamically corrected entangling gates for spin qubits Invited Speaker: Jason Kestner In this talk, I present a theoretical toolbox of control protocols designed explicitly to exploit the strengths of semiconductor spin qubits and circumvent their weaknesses while generating robust entangling gates. 
Wednesday, March 7, 2018 8:36AM  8:48AM 
K28.00002: Applying Machine Learning to QuantumDot Experiments: Generation of Training Datasets and Autotuning Sandesh Kalantre, Justyna Zwolak, Xingyao Wu, Steve Ragole, Jacob Taylor Arrays of gatedefined quantum dots provide a promising platform for the realisation of quantum computers. With experimental efforts moving towards such arrays, a new control challenge presents itself  determination of appropriate regions in the gate voltage space to allow efficient control and manipulation of the electrons. In the past, this challenge has been tackled with heuristic approaches. Machine learning tools have emerged as a practical toolkit for automated heuristics. I will describe our efforts to enable machine learning based autotuning of quantum dot arrays. A prerequisite is the availability of a training dataset that can qualitatively model the observed current and chargesensor outputs. We estimate capacitance and tunnelling models of arrays under the ThomasFermi and WKB approximations. We then describe the learning problems on these datasets and outline an architecture for autotuning. 
Wednesday, March 7, 2018 8:48AM  9:00AM 
K28.00003: Applying Machine Learning to QuantumDot Experiments: Learning from the Data Justyna Zwolak, Sandesh Kalantre, Xingyao Wu, Steve Ragole, Jacob Taylor

Wednesday, March 7, 2018 9:00AM  9:12AM 
K28.00004: A Geometrical Approach To Robust Quantum Control That Respects Pulse Constraints And Minimizes Gate Times Junkai Zeng, Edwin Barnes We extend our recently introduced geometrical method for designing driving fields that suppress quasistatic noise errors while performing singlequbit operations to incorporate realistic experimental constraints on the pulse shapes. We show that this approach can be made compatible with experimental restrictions on the pulse amplitude or rise time without sacrificing any of the robustness to noise. By leveraging the generality of our geometrical approach, we perform a variational analysis to derive the globally optimal driving pulse that obeys a given set of pulse shape constraints while minimizing the operation time and while cancelling noise errors to second order. In cases where the optimal pulses are not smooth, we provide a method based on our geometrical approach to obtain experimentally feasible smooth pulses that closely approximate the optimal ones without sacrificing the fidelity. We present systematic comparisons between our pulses and standard errorcorrecting pulse sequences to highlight the benefits of building experimental waveform constraints into dynamically corrected gate designs. 
Wednesday, March 7, 2018 9:12AM  9:24AM 
K28.00005: Hyperfineassisted Fast Electric Control of Dopant Nuclear Spins in Semiconductors Andras Palyi, Peter Boross, Gabor Szechenyi Nuclear spins of dopant atoms in semiconductors are promising candidates as quantum bits, due to the long lifetime of their quantum states. Conventionally, coherent control of nuclear spins is done using ac magnetic fields. Using the example of a phosphorus atom in silicon, we theoretically demonstrate that hyperfine interaction can enhance the speed of magnetic control. Based on that result, we show that hyperfine interaction also provides a means to control the nuclear spin efficiently using an ac electric field, in the presence of intrinsic or artifical spinorbit interaction. The electric control scheme we describe is especially fast in a hybrid dotdonor system subject to an inhomogeneous magnetic field. Reference: arXiv:1707.00581 
Wednesday, March 7, 2018 9:24AM  9:36AM 
K28.00006: Stroboscopically Robust Operating Points on IsingCoupled Qubits Ralph Kenneth Colmenar, Jason Kestner Recent work on Isingcoupled doublequantumdot spin qubits in GaAs with voltagecontrolled exchange interaction had shown improved twoqubit gate fidelities from the application of a.c. exchange gates along with a strong magnetic field gradient between adjacent dots [1]. By examining how noise propagates in the time evolution operator of the system, we find an optimal set of parameters that provides passive stroboscopic circumvention of errors in twoqubit gates to first order, provided that the fluctutations in the total qubit energy splitting is no greater than the interqubit coupling strength. We predict over 99% twoqubit gate fidelities for the case of quasistatic noise, which is an order of magnitude improvement over the typical unoptimzed implementation. The effects of 1/f noise are also taken into consideration. 
Wednesday, March 7, 2018 9:36AM  9:48AM 
K28.00007: Floquet state spectroscopy of a semiconductor charge qubit with a microwave resonator Jonne Koski, Andreas Landig, Andras Palyi, Pasquale Scarlino, Christian Reichl, Werner Wegscheider, Guido Burkard, Andreas Wallraff, Thomas Ihn, Klaus Ensslin Applying a strong periodic drive to a qubit is a useful method to manipulate its state and gives rise to intricate physics, such as the ac Stark effect and LandauZenerStückelberg interference [1], characterized by the emergence of Floquet states. Here, we measure a strongly driven GaAs double quantum dot charge qubit with a weakly coupled superconducting microwave resonator [2]. In contrast to earlier experiments [1, 3, 4], we probe the Floquet states by tracing the qubit  resonator resonance condition as a function of the drive amplitude and the detuning between the (2,1) and (1,2) charge configurations of the qubit. For large drive amplitudes, multiple resonances emerge corresponding to processes involving a single resonator photon and multiple drive field photons. Furthermore, we observe LandauZenerStückelberg interference arising only for those resonances that involve at least a single photon absorbed from the drive. 
Wednesday, March 7, 2018 9:48AM  10:00AM 
K28.00008: Scalable DC Filter System for Semiconductor Spin Qubits James Loy, Anthony Sigillito, Jason Petta Over the past couple of years the complexity of gate defined quantum dots has dramatically increased. A 9 qubit device has been fabricated and tested in the Si/SiGe accumulationmode device platform^{1}, but the overlapping gate design requires independent voltage biasing of thirty nine gate electrodes. Electrical noise on the gates increases the electron temperature and may limit coherence times on devices incorporating micromagnets for spin control^{2}. We develop a scalable lowpass filter system that supports up to 150 dc lines. The filter design is modular, can be readily expanded, and allows for easy repairs without complete disassembly of the filter system. The filters are located at the mixing chamber plate, have a roll off frequency of 2 MHz, and provide 15 dB of attenuation above 10 MHz. 
Wednesday, March 7, 2018 10:00AM  10:12AM 
K28.00009: Controlling Hole Spin in Quantum Dots: Alloy Effects Garnett Bryant, Arthur Lin, Xiangyu Ma, Matthew Doty Hole spins in semiconductor quantum dots (QD) are promising qubits. The Zeemansplit states form twolevel systems with splitting determined by the physical spin of the state. Due to strong spinorbit coupling, the hole spin orientation is locked to the QD axis for magnetic fields B away from the Voigt configuration. However, in Voigt configuration, the hole spin is nearly fully suppressed. Application of an electric field parallel or antiparallel to B in the Voigt configuration restores the hole spin and provides exquisite control of the spin orientation over a wide range of angles. This control is disrupted if the QD is an alloy. Tightbinding theory is used to describe InGaAs alloy quantum dots. The disorder of a random alloy configuration scrambles the hole distribution at the atomic scale. Applying a lateral electric field with B in Voigt configuration can restore the z component of the spin even with alloy disorder, but the inplane spin remains small when the QD is disordered. We provide several examples to illustrate this alloy induced spin scrambling, show the dependence on alloy concentration, and discuss the effect on hole Zeeman splitting, gfactors, and the resulting spin control. 
Wednesday, March 7, 2018 10:12AM  10:24AM 
K28.00010: Controlling Hole Spin in Quantum Dot Molecules: Random Alloy GaBiAs Interdot Barriers Arthur Lin, Matthew Doty, Garnett Bryant Hole spins in InAs quantum dots (QDs) allow alloptical control and have long lifetimes, qualifying them as excellent qubit candidates. Furthermore, quantum dot molecules (QDMs) created by vertically stacking two QDs introduce an interdot barrier, which can be used to mediate spinmixing between respective dot states. As spinmixing is essential for welldesigned qubits, we seek to further enhance hole spinmixing in QDM systems by introducing a low concentration of Bi in the interdot region. Barriers containing GaBiAs, compared to GaAs, have a higher hole tunneling rate while minimally affecting electrons or split off bands. Using an atomistic tightbinding model, we show that a threefold increase in hole tunnel coupling is achieved for welldesigned barriers. We also show that this leads to a significant enhancement in hole spinmixing, and allows for control schemes that are not possible with pure GaAs barriers. Finally, we present a breakdown of various compounding effects, such as strain, Bi electronic structure, or spinorbit coupling, and how they contribute to the final hole tunnelcoupling and spinmixing. 
Wednesday, March 7, 2018 10:24AM  11:00AM 
K28.00011: Circuit qed enhanced magnetic resonance Invited Speaker: Patrice Bertet The detection and characterization of paramagnetic species by electronspin resonance (ESR) spectroscopy has numerous applications. Most ESR spectrometers rely on the inductive detection of the small microwave signals emitted by the spins during their Larmor precession into a microwave resonator in which they are embedded. Using the tools offered by circuit Quantum Electrodynamics (QED), namely high quality factor superconducting microresonators and Josephson parametric amplifiers that operate at the quantum limit when cooled at 20mK [1], we investigate magnetic resonance in a new regime where the quantum nature of the microwave field plays a role. In particular, the spin detection sensitivity is strongly enhanced [2,3] and spin relaxation is governed by spontaneous emission through the cavity [4]. In this talk I will discuss applications of this new regime to highsensitivity nuclear spin detection. 
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