Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F28: Architectures for Semiconducting Quantum ComputingFocus

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Sponsoring Units: DQI Chair: Andrew Dzurak, Univ of New South Wales Room: LACC 405 
Tuesday, March 6, 2018 11:15AM  11:51AM 
F28.00001: Longrange exchange coupling for spin qubits Invited Speaker: Filip Malinowski To scale the arrays of singleelectron quantum dots arrays it is necessary to realize longrange spin coupling. In the talk I will present our study of multielectron quantum dots and demonstrate they mediate exchange interaction. In addition I will discuss tunability of multielectron coupler and argue that it can also be used to transfer the electrons between nonneighboring dots. 
Tuesday, March 6, 2018 11:51AM  12:03PM 
F28.00002: Exchange coupling of spin qubits via a multielectron quantum dot Filip Malinowski, Frederico Martins, Thomas Smith, Stephen Bartlett, Andrew Doherty, Peter Nissen, Saeed Fallahi, Geoffrey Gardner, Michael Manfra, Charles Marcus, Ferdinand Kuemmeth We present a theoretical description for the effective exchange interaction between distant spin qubits mediated by a multielectron quantum dot. Our model is used to interpret experimentally observed exchange interactions between singlettriplet qubits in such a configuration. We show that a nonzero spin correlation energy for the mediator dot is required to explain key features of the measured exchange profile. The competition between the standard singletfavouring exchange and the tripletfavouring exchange results in a sweetspot where the interaction is resistant to charge noise on gate voltages. We argue that multielectron quantum dots are a promising avenue for fast, noiseresistant coupling of spin qubits. 
Tuesday, March 6, 2018 12:03PM  12:15PM 
F28.00003: Negative exchange interactions in fewelectron double quantum dots Kuangyin Deng, Fernando CalderonVargas, Nicholas Mayhall, Edwin Barnes In a recent experiment, it was shown that negative exchange interactions can arise when a singleelectron quantum dot is coupled to a larger quantum dot containing on the order of one hundred electrons. The origin of this negative exchange can be traced to the larger quantum dot exhibiting a spin triplet rather than singlet ground state. Here, we show using a microscopic model based on the configuration interaction (CI) method that both triplet and singlet ground states are realized depending on the number of electrons. In the case of only four electrons, a full CI calculation reveals that triplet ground states occur for sufficiently large dots. These results hold for symmetric and asymmetric quantum dots in both Si and GaAs, showing that negative exchange interactions are robust in fewelectron double quantum dots and do not require large numbers of electrons. 
Tuesday, March 6, 2018 12:15PM  12:27PM 
F28.00004: Synchronized HighFidelity TwoQubit Gates in Double Quantum Dots Guido Burkard, Maximilian Russ, Jacob Taylor, David Zajac, Anthony Sigillito, Felix Borjans, Jason Petta While singlequbit gates with fidelities exceeding 99% for spin qubits in natural silicon and 99.9% in isotopically purified silicon have been demonstrated [1], the fidelity of twoqubit gates is still subject to several limitations. Motivated by recent experiments [2], we theoretically describe a highfidelity controlledNOT gate using the exchange interaction between the spins in neighboring quantum dots subject to a magnetic field gradient. We find an optimal gate sequence and present a synchronization method which avoids detrimental spin flips when the control qubit is in state 0. Furthermore, we identify systematic phase mismatches accumulated during the gate. We show that by synchronizing the resonant and offresonant transitions and compensating unintended phases, the overall gate fidelity can be increased significantly. Numerical simulations also demonstrate a high tolerance against charge noise due to a partial intrinsic refocussing mechanism. 
Tuesday, March 6, 2018 12:27PM  12:39PM 
F28.00005: Entangling silicon spin qubits in a hybrid dotdonor system Vanita Srinivasa, N. Tobias Jacobson, Andrew Baczewski, John Gamble, Ryan Jock, Patrick HarveyCollard, Martin Rudolph, Wayne Witzel, Malcolm Carroll Quantum bits realized in hybrid systems harness the optimal features of multiple types of quantum systems. Silicon provides a particularly attractive platform for realizing hybrid semiconductor qubits, as it supports both nuclear and electron spin qubits in wellisolated atomic impurities as well as highly tunable quantum dotbased spin qubits. Recent experiments [1,2] combine the favorable properties of both systems to demonstrate full coherent control of a hybrid singlettriplet qubit via exchange and an intrinsic magnetic gradient provided by the donor hyperfine interaction. In order to couple these qubits, we theoretically consider the capacitive interaction in the presence of hyperfine coupling and identify regimes of operation relevant for experiments. We also extend this analysis to investigate the electron spinmediated interaction between the nuclear spins as a potential mechanism for entangling gates. 
Tuesday, March 6, 2018 12:39PM  12:51PM 
F28.00006: HighFidelity and Robust TwoQubit Gates for QuantumDot Spin Qubits in Silicon ChiaHsien Huang, C. H. Yang, ChienChang Chen, Andrew Dzurak, HsiSheng Goan A twoqubit controlledNOT (CNOT) gate, realized by a controlledphase (Cphase) gate together with some singlequbit gates, has been experimentally implemented recently for quantumdot spin qubits in isotopically purified silicon, a promising solidstate system for practical quantum computation. In the experiments, the singlequbit gates have been demonstrated with faulttolerant controlfidelity, but the infidelity of the twoqubit Cphase gate is, primarily due to the electrical noise, still higher than the required error threshold for faulttolerant quantum computation (FTQC). Here, by taking the realistic system parameters and the experimental constraints on the control pulses into account, we construct experimentally realizable highfidelity CNOT gates robust against the electrical noise with the experimentally measured 1/ f^{1.01} noise spectrum and against the uncertainty in the interdot tunnel coupling amplitude. Our optimal CNOT gate has about two orders of magnitude improvement in gate infidelity over the ideal Cphase gate. Furthermore, within the same control framework, highfidelity and robust singlequbit gates can also be constructed, paving the way for largescale FTQC. 
Tuesday, March 6, 2018 12:51PM  1:03PM 
F28.00007: A pulse sequence designed for robust CNOT gates in SiMOS quantum dots Utkan Güngördü, Jason Kestner We theoretically analyze the errors in one and twoqubit gates in a silicon MOS semiconductor double quantum dot setup [1], and present a pulse sequence which can suppress the errors in exchange coupling due to charge noise to within the acceptable rate for faulttolerance using ideal local rotations. In practice, the overall fidelity of the pulse sequence will be limited only by the quality of the singlequbit gates available: the CNOT infidelity comes out to be ~10x the infidelity of the singlequbit operations. 
Tuesday, March 6, 2018 1:03PM  1:15PM 
F28.00008: Achieving high fidelity single qubit gates in strongly driven silicon quantum dot qubits YuanChi Yang, Mark Friesen, Susan Coppersmith Performing qubit gate operations as quickly as possible can be important for minimizing the effects of decoherence. For resonant gating, this requires applying a strong ac drive. However, strong driving can present control challenges by causing leakage as well as theoretical challenges because the rotatingwave approximation can break down. Here we analyze resonant X rotations of silicon quantum double dot hybrid and charge qubits in the presence of 1/f charge noise, typical for semiconducting devices. We obtain analytical formulas for optimal driving parameters, as well as the system evolution. We show that, by exploiting strong driving, gate fidelities in both qubits can be above 99.9%. 
Tuesday, March 6, 2018 1:15PM  1:27PM 
F28.00009: Compressed Optimization of Device Architectures (CODA) for Semiconductor Quantum Devices Adam Frees, John Gamble, Daniel Ward, Robin BlumeKohout, M. A. Eriksson, Mark Friesen, Susan Coppersmith Recent advances in nanotechnology have enabled researchers to control quantum mechanical objects with unprecedented accuracy. As these devices become larger and more complex, the ability to design them such that they can be simply controlled becomes a daunting task. Here, we introduce a protocol for the Compressed Optimization of Device Architectures (CODA) and apply it to semiconducting quantum dot qubit devices. CODA leads to a metric for benchmarking device performance and optimizing device designs, and provides a scheme for automating the control of gate operations. We demonstrate the CODA protocol through simulations of up to eight quantum dots in devices. This work was supported in part by ARO (W911NF1210607, W911NF1710274), and NSF (PHY1104660). The authors gratefully acknowledge support from the Sandia National Laboratories Truman Fellowship Program, which is funded by the Laboratory Directed Research and Development (LDRD) Program. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for DOE's National Nuclear Security Administration under contract DENA0003525, SAND Number: SAND201711688 A. 
Tuesday, March 6, 2018 1:27PM  1:39PM 
F28.00010: Measurementfree Implementations of SmallScale Surface Codes for Quantum Dot Qubits Ekmel Ercan, Joydip Ghosh, Daniel Crow, Vickram Premakumar, Robert Joynt, Mark Friesen, Susan Coppersmith In quantum dot spin qubits, readout is typically much slower than gate operations, and conventional surface code implementations that rely on syndrome measurements could therefore be challenging. However, fast and accurate reset of quantum dot qubits can be achieved via tunneling to a reservoir. Here, we propose smallscale surface code implementations for which syndrome measurements are replaced by a combination of Toffoli gates and qubit reset. For quantum dot qubits, this enables much faster error correction than measurementbased schemes, but requires additional ancilla qubits and nonlocal interactions. We have performed numerical simulations of two different coding schemes, obtaining error thresholds on the orders of 10^{2} for a 1D architecture that only corrects bitflip errors, and 10^{4} for a 2D architecture that corrects bit and phaseflip errors. We have also demonstrated that when the measurement time is about 10 times the gate time, our method begins producing a higher threshold than the measurementbased method. 
Tuesday, March 6, 2018 1:39PM  1:51PM 
F28.00011: Demonstration of a hole quantum dot in Ge/SiGe Dwight Luhman, Jonathan Moussa, Leon Maurer, Y. Chuang, JiunYun Li, TzuMing Lu Heavy holes in Ge/SiGe quantum well heterostructures have potential advantages as spinbased qubits including all electrical control through intrinsic spinorbit coupling, the possibility of isotopic enrichment, and the absence of valley states. Recently highmobility material suitable to host spin qubits has become available. We will report on experiments which demonstrate the formation of a gatecontrolled lateral quantum dot in undoped Ge/SiGe heterostructure quantum wells. Details of the device fabrication and lowtemperature characterization of the quantum dot will be presented. These results, demonstrating confinement of holes in this material system, are the first step toward achieving heavy holebased spin qubits in a lateral quantum dot device design in germanium. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DENA0003525. 
Tuesday, March 6, 2018 1:51PM  2:03PM 
F28.00012: Lightmass holespins in a Ge quantum well as qubits Rodrigo Capaz, Luis Terrazos, Andre Saraiva, Mark Friesen, Susan Coppersmith, Belita Koiller Electrons bound to quantum dots provide well defined (spin 1/2) physical qubits. Under strong spinorbit coupling spin qubits are expected to suffer rapid dephasing from charge noise; however, recent experiments have effectively employed spinorbit coupling in coherent quantum gates. Here we show that hole spins in a Si_{1}_{x}Ge_{x}/Ge/ Si_{1x}Ge_{x} quantum well possess highly desirable properties as qubits, including a high natural abundance of nuclear spin0 isotopes, a large (~100 meV) intrinsic splitting between the light and heavy hole bands, and a very light (~0.05m_{0}) effective mass. We employ ab initio methods to determine the band structure of strained Ge quantum wells as a function of x, and we discuss the advantages and challenges of using Ge hole spins as qubits. 
Tuesday, March 6, 2018 2:03PM  2:15PM 
F28.00013: A Gatedriven Entanglement Switch, Magic Angles, and a Decoherence Free Subspace in Acceptor Spin Qubits in Si Maria Calderon, Jose Carlos AbadilloUriel, Joseph Salfi, Xuedong Hu, Sven Rogge, Dimitrie Culcer

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