Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session E35: ExchangeBased Spin QubitsFocus

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Sponsoring Units: DQI Chair: Pascal Cerfontaine, RWTH Aachen University Room: BCEC 205B 
Tuesday, March 5, 2019 8:00AM  8:36AM 
E35.00001: Spin qubits made of quantum dots and donors in silicon Invited Speaker: Patrick HarveyCollard In this talk, I will present our recent work on control and readout of electron spin qubits in silicon MOS. Valley splitting is an issue that can induce control variability or limit readout fidelity. We show how this splitting is tunable in MOS devices [1], and introduce a shellfilling trick to overcome potential readout challenges. Readout fidelity has lagged behind that of control. We demonstrate that an enhanced latching mechanism can be used to improve the signal and lifetime of the spinblockade readout while preserving its speed, achieving singleshot readout fidelities of > 99.86% [2]. The best spin qubit in the solid state is the nuclear spin of donors, but it is difficult to couple it to other qubits. We show coherent coupling of a donor to a quantum dot, a milestone for nuclear spin quantum computing [3]. We also study spinorbit interaction for electrons in silicon quantum dots. We show that it is sufficiently large to drive a singlettriplet qubit, allowing universal control without external elements [4]. We study the mechanisms behind this interaction and find three different effects [5]. Our work identifies crystallographic and magnetic field anisotropies that can be used to enhance or suppress these effects. 
Tuesday, March 5, 2019 8:36AM  8:48AM 
E35.00002: Decoherence of a donordot hybrid qubit in Si John Truong, Xuedong Hu A recent proposal for a scalable donorbased quantum computer scheme promises excellent coherence properties and fast qubit couplings [1]. The system consists of two types of qubits per donor: a flipflop qubit consisting of the electron and nuclear spins, and a charge qubit of the donor electron tunneling between the donor and an interface quantum dot. The proposal identifies a parameter regime where flipflop qubit dephasing due to electrical noise is strongly suppressed. 
Tuesday, March 5, 2019 8:48AM  9:00AM 
E35.00003: Probing exchange interaction for gatedefined double quantum dots Patrick Bethke, Michael A Wolfe, Robert P. G. McNeil, Arne Ludwig, Andreas D. Wieck, Hendrik Bluhm The exchange interaction J is vital for gating in electronspin qubits. Therefore, understanding its behavior is crucial for highfidelity gating, especially at low J where an inability to turn it off completely may impact the fidelity of parallel qubit operations. Historically, the exchange interaction (controlled via detuning ε between dots) measured in experiment, J(ε) ∝ exp(−ε/ ε_{0}) [12], has deviated from Hubbardbased predictions, J(ε) ∝ 1/ε. 
Tuesday, March 5, 2019 9:00AM  9:12AM 
E35.00004: Computational modeling of exchange splitting in Si/SiO_{2} double quantum dots Harshad Sahasrabudhe, Rajib Rahman One and two qubit operations have been demonstrated using electrons trapped in quantum dots at the interface of isotopically purified Si/SiO_{2} materials. We present a computational study of Si/SiO_{2} based double quantum dot qubits which could help in scaling the device design. Exchange splitting between the lowest singlet and triplet states, which plays an important role in two qubit operations, is calculated using the full configuration interaction (FCI) method. The single electron wavefunctions used in FCI are calculated using 20band sp^{3}d^{5}s* tight binding (TB) Hamiltonian, which accurately represents the conductionband X valley and spin orbit coupling in quantum dots. The electrostatic potential needed in TB is calculated using selfconsistent effectivemass SchrodingerPoisson (SP) simulations on finite element meshes resembling the actual device geometry. Energy spectrum of SP and TB simulations shows a good match at low detuning between the dots. At high detuning close to the (1,1)(0,2) transition, where the two qubit operations take place, higher Xvalleys along x, y and z axes are found to play a role in the exchange splitting, which could impact the response to charge noise. 
Tuesday, March 5, 2019 9:12AM  9:24AM 
E35.00005: Achieving A High Fidelity ControlledNOT Gate Between A Pair Of ExchangedCoupled Silicon DoubleQuantumDot Hybrid Qubits YuanChi Yang, Mark G Friesen, Susan Coppersmith It has been shown that operating qubits by varying exchange couplings while operating at sweet spots against detuning noise can improve gate fidelities. For doublequantumdot hybrid qubits, this requires keeping the system in the largedetuning regime where dephasing is greatly suppressed. Here we show that, in a pair of exchangecoupled doublequantumdot hybrid qubits, it is possible to exploit the largedetuning regime to achieve a sizeable exchange interaction between the qubits while suppressing leakage and dephasing, yielding a highfidelity controlled phase gate with a gate time less than 1 ns. We find that the fidelity of a CNOT gate can be above 99.9%, in the presence of charge noise typical for semiconductor devices. 
Tuesday, March 5, 2019 9:24AM  9:36AM 
E35.00006: Quadrupolar exchangeonly spin qubit Maximilian Russ, Jason R Petta, Guido Burkard We propose a quadrupolar exchangeonly (QUEX) spin qubit that is highly robust against charge noise and nuclear spin dephasing, the dominant decoherence mechanisms in quantum dots [1]. Building on ideas developed for the exchangeonly qubit [2], the hybrid qubit [3], and the exchangeonly singletonly spin qubit [4], the QUEX the qubit consists of four electrons trapped in three quantum dots, and operates in a decoherencefree subspace to mitigate dephasing due to nuclear spins. To reduce sensitivity to charge noise, the qubit can be completely operated at an extended charge noise sweet spot that is firstorder insensitive to electrical fluctuations. Due to on site exchange mediated by the Coulomb interaction, the qubit energy splitting is electrically controllable and can amount to several GHz even in the "off" configuration, making it compatible with conventional microwave cavities. A symmetric readout and initialization protocol can be used to perform fast and high fidelity measurements. 
Tuesday, March 5, 2019 9:36AM  9:48AM 
E35.00007: Strong Microwave Photon Coupling to the Electron Quadrupole Moment Jonne Koski, Andreas Landig, Pasquale Scarlino, Maximilian Russ, David Van Woerkom, Christian Reichl, Werner Wegscheider, Guido Burkard, Andreas Wallraff, Thomas Ihn, Klaus Ensslin The implementation of circuit quantum electrodynamics (cQED) allows coupling of distant qubits by microwave photons hosted in onchip resonators. Typically, the qubitphoton interaction is realized by coupling the photons to the electrical dipole moment of the qubit. A recent proposal [1] suggests storing the quantum information in the quadrupole moment of an electron in a triple quantum dot. This type of qubit is expected to have an improved coherence since the qubit does not have a dipole moment and is consequently better protected from electric noise. We report the experimental realization of such a quadrupole qubit hosted in a triple quantum dot in a GaAs/AlGaAs heterostructure. A highimpedance microwave resonator is capacitively coupled to the middle of the triple dot to realize interaction with the qubit quadrupole moment. We demonstrate strong quadrupole qubitphoton coupling with a qubitphoton coupling strength of g / 2π ≈ 130 MHz and a qubit decoherence rate of γ_{2} / 2π ≈ 30 MHz. Furthermore, we observe improved coherence properties of the qubit when operating in the parameter space where the dipole coupling vanishes. 
Tuesday, March 5, 2019 9:48AM  10:00AM 
E35.00008: Suppression of Leakage for a Charge Quadrupole Qubit in Triangular Geometry Guo Xuan Chan, Xin Wang The relatively weak coupling between spin states in spin qubits has spurred a revival of interest in charge qubits, which promises stronger coupling between multiple charge qubits due to stronger longrange Coulomb interaction. Among the proposed charge qubits, charge quadrupole (CQ) qubit is suggested to provide a relatively robust quantum computation by virtue of the logical bases residing in a decoherence free subspace such that leakage state decouples from the manifold [1]. Conventionally, CQ qubit is realized with an electron residing in a lateral triple quantum dot device, yet any fluctuation in tunneling and detuning control causes significant leakage. We propose a strategy to mitigate such destructive coupling by simply implementing the CQ qubit in a triangular triple quantum dot, where the tunneling between the two dots on the edge strongly suppresses leakage, eliminating the need of complex pulse sequences. The reduction of leakage is demonstrated from molecular orbital calculation, in corporation with numerical simulations. 
Tuesday, March 5, 2019 10:00AM  10:12AM 
E35.00009: A HighFidelity Gateset for ExchangeCoupled SingletTriplet Qubits Rene Otten, Pascal Cerfontaine, Michael A Wolfe, Hendrik Bluhm A key ingredient for a quantum computer is the accurate manipulation of qubits in order to generate highfidelity gates. For ST_{0} qubits in semiconductor quantum dots, which allow purely electric control with moderate bandwidth requirements, singlequbit gates with fidelities above the error correction threshold were demonstrated, whereas twoqubit operations have not reached the required fidelity [12]. 
Tuesday, March 5, 2019 10:12AM  10:24AM 
E35.00010: Fast highfidelity entangling gates in Si double quantum dots Fernando CalderonVargas, George Barron, Xiuhao Deng, Edwin Barnes, Sophia Economou Highfidelity twoqubit gates remain a challenge for spin qubits in quantum dots, partly due to the exchange coupling’s sensitivity to charge noise and relatively longer gate times. Here we show that, by using simple smooth pulses to control the amplitude of the oscillating magnetic field, entangling gates locally equivalent to controlNOT and controlZ gates can be implemented in times as low as 26 ns and with fidelities up to 99.99%. Moreover, we provide the particular singlequbit gates necessary to make the twoqubit gates exactly equal to the CNOT and CZ gates. 
Tuesday, March 5, 2019 10:24AM  10:36AM 
E35.00011: Robust implementation of onequbit gates despite alwayson exchange coupling in silicon double quantum dots Utkan Gungordu, Jason Paul Kestner In a silicon twoqubit device, bandwidth constraints on exchange coupling and limitations on ESR power are important obstacles for the realization of strictly local gates, which are themselves necessary components of robust nonlocal gating schemes [1]. Here we show that, even in the presence of an exchange coupling stronger than onequbit Rabi frequencies, spins can be addressed separately to implement a particular class of onequbit gates optionally surrounded by exchange gates, and the crosstalk between them can be eliminated stroboscopically. We also show how to make such gates robust against quasistatic charge and hyperfinenoise. 
Tuesday, March 5, 2019 10:36AM  10:48AM 
E35.00012: Adiabatic twoqubit gates of capacitively coupled quantum dot hybrid qubits Adam Frees, John Gamble, Mark G Friesen, Susan Coppersmith Semiconductor quantum dot qubits have progressed greatly over the past several years, with twoqubit gates realized by several groups. So far, the fidelities reported for these gates are still below the error correction threshold. Here, we model a system consisting of two capacitivelycoupled quantum dot hybrid qubits, and optimize the adiabatic electrical pulses used to entangle these qubits. We find a simple pulse that yields a CZ gate with greater than 99% fidelity in the presence of a quasistatic charge noise distribution with standard deviation 1 µeV. Further, we introduce the concept of a “dynamical sweet spot” which can be used to develop pulses that decrease the infidelity by a factor of >5. 
Tuesday, March 5, 2019 10:48AM  11:00AM 
E35.00013: Minimal nonorthogonal gate decomposition for qubits with limited control Xiaoming Zhang, Jianan Li, Xin Wang, ManHong Yung How an arbitrary unitary transformation can be decomposed into minimum number of elementary rotations, subject to particular physical constraints, is a fundamental and practical question. Here, we consider two important scenarios. The first one is when rotation axes are allowed to vary in a range of a plane, which corresponds to the SingletTriplet (ST) qubit in quantumdot systems; the second one is when rotation axes can only along two fixed directions, corresponding to the ExchangeOnly (EO) qubits. For both scenarios, we provide the criteria for determining the minimal number of pieces and explicit gate construction procedure for each unitary gate. Our results analytically explain the fourgate decomposition of EO qubits, previously determined numerically by Divincenzo et al. Moreover, our approaches can reduce both gate time and gate fidelity dramatically for ST qubits, compared with Ramon sequence and its variant. 
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