Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H35: Semiconducting Qubits: Characterization of Electron and Hole Spin QubitsFocus

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Sponsoring Units: DQI Chair: Andrew Pan, HRL Laboratories, LLC Room: BCEC 205B 
Tuesday, March 5, 2019 2:30PM  2:42PM 
H35.00001: Fourqubit quantum processor in isotopically enriched silicon Anthony Sigillito, James Loy, David Zajac, Felix Borjans, Michael Gullans, Jason R Petta Quantum processors based on spins in semiconductors are rapidly becoming a strong contender in the race to build a quantum computer. When coupled with micromagnets, these devices offer high fidelity singlequbit [1] and twoqubit [2,3] control while maintaining long coherence times. Until now, silicon spin qubit devices incorporating micromagnets have been limited to one and two qubit demonstrations, even though a onedimensional extended array of silicon quantum dots has been demonstrated [4]. In this presentation, we will present a fourspinqubit device architecture. This device is fabricated on an isotopically enriched ^{28}Si/SiGe quantum well and offers four individually addressable spin qubits. Multiqubit gates can be implemented by gating nearestneighbor exchange interactions. Preliminary data demonstrating single and multiqubit operation will be presented. 
Tuesday, March 5, 2019 2:42PM  2:54PM 
H35.00002: Characterization of gate fidelities in a Si/SiGe twoqubit device Xiao Xue, Thomas F Watson, Jonas Helsen, Daniel Ward, Donald E Savage, Max G Lagally, Susan Coppersmith, Mark Alan Eriksson, Stephanie Wehner, Lieven Vandersypen Various candidate implementations for future quantum computers have been investigated over the past twenty years. Silicon spin qubits show great promise [1] for their long coherence times and integration using semiconductor technology but there have been very few quantitative studies of the fidelities of twoqubit gates. Here we characterize the gate fidelities of a CPhase gate using randomized benchmarking. For singlequbit gates, we perform randomized benchmarking on each spin by itself and also on both spins simultaneously, to probe crosstalk effects. Furthermore, we developed and experimentally verified a new method called character randomized benchmarking [2,3], which combines the advantages of simultaneous and interleaved randomized benchmarking. With this new method, we characterized the fidelity of a CPhase gate with tighter bounds than those in the traditional approach. 
Tuesday, March 5, 2019 2:54PM  3:06PM 
H35.00003: Benchmarking of twoqubit gates for singlettriplet qubits Chengxian Zhang, Xin Wang We perform benchmarking to evaluate the performance of twoqubit dynamically corrected gates (DCGs) in the singlettriplet spin qubit system. We execute two types of circuits, i.e. the DeutschJozsa algorithm and Grover algorithm, each of which is carried out with DCGs and gates not immune to noise. The benchmarking process is performed by repeating 100 times of the respective circuit under 1/f noises with different noise exponents ($\alpha$). The average error per execution of one algorithm is compared between DCGs and nonnoisecorrecting ones, and whether DCGs offer improvement depends on the noise exponent, $\alpha$. We have found that when $\alpha<0.8$ the DCGs would not offer improvements, but DCGs can outperform the noncorrecting ones otherwise. The fact that DCGs can offer improvement for a wide range of $\alpha$, on one hand, suggests using DCGs on twoqubit gates can reduce the error, but on the other hand also indicates that quantum algorithms involving twoqubit gates suffer more heavily from noises than those only involving singlequbit gates if no correction to noise is performed. 
Tuesday, March 5, 2019 3:06PM  3:18PM 
H35.00004: Noise correlations in a twoqubit Si/SiGe quantum dot device Jelmer Boter, Xiao Xue, Thomas F Watson, Tobias Krähenmann, Vickram Premakumar, Daniel Ward, Donald E Savage, Max G Lagally, Mark G Friesen, Susan Coppersmith, Mark Alan Eriksson, Robert James Joynt, Lieven Vandersypen Qubits are affected by noise in their environment, but conversely can also be used to probe this noise and study properties such as spectral density and correlations. We use a twoqubit device in a Si/SiGe hetrostructure [1] to investigate noise correlations by studying the decay of two Bell states. These Bell states are insensitive to either correlated or anticorrelated noise, resembling the concept of decoherencefree subspaces, which allows us to extract the uncorrelated, correlated and anticorrelated contributions to the noise affecting the qubits from the decay times for the different initial states. Knowledge about the noise properties makes it possible to design operations that are less sensitive to this noise, and yields information on the noise source, which potentially makes it possible to reduce the noise. We demonstrate this method by artificially adding (anti) correlated noise and use it characterize the noise in our system. 
Tuesday, March 5, 2019 3:18PM  3:30PM 
H35.00005: MagneticField Effects on Error and Leakage in Randomized Benchmarking of a Si/SiGe Tripledot Qubit Cody Jones The exchangeonly, tripledot qubit is fully controllable through voltagebased modulation of the exchange interaction between neighboring dot pairs, requiring no magnetic gradients or magnetic resonance. Nevertheless, several magnetic effects play important roles in errors and leakage of the qubit under operation, whether by dephasing for an idle qubit or during a multipulse experiment such as randomized benchmarking. This talk discusses the magneticfielddependent impacts of three types of magnetic gradients: magnetic screening gradients due to the Meissner effect in superconducting gates, interfacespinorbit effects, and contact hyperfine interactions including the full vector of nuclear magnetization. While Meissner and spinorbit gradients are suppressed by operating near zero magnetic field, we find that gradients oriented in directions transverse to the field, in particular the flipflop terms of the hyperfine component, play a more dominant role. Isolating the effects of magnetic gradients on qubit performance is aided by modifications to the randomized benchmarking protocol that allow reliable extraction of the proportion of total error due to leakage, which is a signature of magnetic gradient effects in exchangeonly qubits. 
Tuesday, March 5, 2019 3:30PM  3:42PM 
H35.00006: Analyzing the fidelity of a singlettriplet spinorbit qubit in silicon using gate set tomography Chloe BureauOxton, Kenneth Rudinger, Noah T Jacobson, Daniel Ward, John Anderson, Ronald P. Manginell, Joel R. Wendt, Tammy Pluym, Michael P Lilly, Michel PioroLadriere, Dwight R Luhman, Malcolm S. Carroll It has been recently demonstrated that spinorbit effects observed in silicon quantum dots are much larger than what is expected for bulk silicon [13]. These spinorbit effects can be used to achieve allelectrical universal control of a double quantum dot singlettriplet qubit without the need for any external components, such as micromagnets or microwave resonators, to produce a magnetic field gradient [4]. In this work, we use gate set tomography to analyze the fidelity of these gates. We also explore the possibility of using AC control, both in the weak and strong driving regimes, to improve the fidelity of qubit operations. 
Tuesday, March 5, 2019 3:42PM  3:54PM 
H35.00007: Measurements of capacitive coupling in two double dots in Si/SiGe for application in two qubit gates Samuel Neyens, Evan R MacQuarrie, John Dodson, Nathan Holman, Brandur Thorgrimsson, Thomas McJunkin, Joelle Corrigan, Mario Palma, Lisa Edge, Mark G Friesen, Susan Coppersmith, Mark Alan Eriksson We present measurements of a Si/SiGe quantum dot device made with overlapping selfoxidized Al gates. The device includes a linear array of four quantum dots and two auxiliary dots for charge sensing. The dots have a lithographic pitch of 130 nm and are separated from the gates by a 30 nm SiGe spacer and 5 nm of aluminum oxide deposited by ALD. We measure the charge noise in all six dots and observe a power spectral density that is approximately 1/f in the 1200 Hz range with a chemical potential noise amplitude of 12 μeV/Hz^{1/2} at 1 Hz. We tune the device to form two tunnelcoupled double dots and observe the capacitive coupling with both double dots at the (0,1)(1,0) polarization line. We measure a detuning shift of ~8 GHz due to the change in polarization of the adjacent double dot, demonstrating the potential of the system for coherent twoqubit coupling via capacitive interaction. 
Tuesday, March 5, 2019 3:54PM  4:06PM 
H35.00008: Detuning dependence of capacitive coupling for quantum dot hybrid qubits Arman Setser, Jason Paul Kestner

Tuesday, March 5, 2019 4:06PM  4:18PM 
H35.00009: Induced quantum dot probe for qubit and material characterization Charles Tahan, YunPil Shim, Rusko Ruskov, Hilary Hurst We propose a nondestructive means of characterizing quantum dot parameters across a semiconductor wafer by inducing a quantum dot on the material system of interest with a separate probe chip that can also house the measurement circuitry. We show that a single wire can create the dot, determine if an electron is present, and be used to measure critical device parameters. Adding more wires enables more complicated setup and measurements. As one application for this concept we consider silicon metaloxidesemiconductor and silicon/silicongermanium quantum dot qubits relevant to quantum computing and show how to measure lowlying excited states (socalled `valley' states) in a novel way. The approach provides a simple and flexible method for characterization applicable to various quantum systems. 
Tuesday, March 5, 2019 4:18PM  4:30PM 
H35.00010: Devicelevel modeling of hole quantum dot qubits in germanium Mitchell Brickson, Andrew Baczewski, Will Hardy, Noah T Jacobson, TzuMing Lu, Leon Maurer, Dwight R Luhman Holes in Ge quantum wells in Ge/SiGe heterostructures have a number of promising properties that have made them the target of growing interest for qubit applications. These include strong intrinsic spinorbit coupling, the absence of valley degeneracy, and small effective masses. In this talk, we will describe ongoing work towards developing comprehensive models of these systems to facilitate the design and optimization of devices and the rationalization of experiments. We make use of electrostatic/strain modeling to compute the potential landscape and capacitances of a given device structure. The potential landscape is fed into a multiband effective mass theory, with other parameters drawn from first principles models, to extract a qubit Hamiltonian. We will report on work describing ongoing quantum dot experiments and make projections pertaining to qubit tunability and performance. 
Tuesday, March 5, 2019 4:30PM  4:42PM 
H35.00011: EDSR of a single heavy hole in a lateral GaAs/AlGaAs quantum dot qubit Sergei Studenikin, Motoi Takahashi, Guy Austing, Alex Bogan, Louis Gaudreau, Marek J Korkusinski, Piotr Zawadzki, Andrew Sachrajda, Lisa A Tracy, John Reno, Terry Hargett Single holes are attractive as spin qubits due to their advantageous properties which include a reduced hyperfine interaction, a strong spinorbit coupling for subnanosecond spin rotations, and the absence of valley complications. 
Tuesday, March 5, 2019 4:42PM  4:54PM 
H35.00012: Origins and enhancement of hole spinmixing in InAs quantum dot molecules Arthur Lin, Matthew F Doty, Garnett Bryant Hole spins in selfassembled InAs quantum dots molecules (QDMs) are a strong candidate for qubit architecture due to their alloptical operation and enhanced tunability via voltage bias across the two dots. In order to capitalize on the alloptical operation and the enhanced tunability, we exploit the spinmixing provided by the “moleculelike” coupled hole states. Through an atomistic tightbinding model and perturbative field analysis, we discuss the origin of spinmixing and compare it with previous models using effective Hamiltonians. We then apply our understanding to the case where the GaAs interdot region is alloyed with a dilute amount of Bi. Replacing As atoms with the heavier Bi atoms enhances spinorbit effects and, in turn, increases spinmixing. However, it complicates the model, as atomistic alloying effects have to be considered, something not explored by prior models. Finally, we briefly discuss the practical operation of InAs/GaBiAs QDM qubits with the enhanced spinmixing. 
Tuesday, March 5, 2019 4:54PM  5:06PM 
H35.00013: Controlling hole spin in quantum dots: Rashba or not Rashba Garnett Bryant, Arthur Lin Hole spins in semiconductor quantum dots (QD) are promising qubits. Zeemansplit states form twolevel systems with splitting determined by the physical spin of the hole. Due to strong spinorbit coupling, 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 displays significant texture across the dot but is weakly polarized. Application of an electric field parallel or antiparallel to B in the Voigt configuration restores the hole spin. This spin control can be related to the QD geometry. The question remains whether this can be explained as a Rashba effect originating from interface fields or is inherent to an atomistic description of hole spins in QDs. Tightbinding theory is used to study GaAs/AlAs QDs with a graded alloy describing the QD interface to minimize Rashba effects of sharp interfaces. The results are compared with results for QDs with sharp interfaces. Several examples illustrate how a graded interface influences the spin locking seen for QDs with sharp interfaces and how this changes spin texture and spin polarization in the Voigt configuration. The results are used to assess the contribution of Rashba effects. 
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