Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L45: Semiconductor Qubits: Quantum Dot Entanglement and ControlFocus

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Sponsoring Units: GQI Chair: Matthew Rakher, HRL Laboratories Room: 348 
Wednesday, March 16, 2016 11:15AM  11:51AM 
L45.00001: One and twoqubit logic using siliconMOS quantum dots Invited Speaker: Andrew Dzurak Spin qubits in silicon are excellent candidates for scalable quantum information processing [1] due to their long coherence times and the enormous investment in silicon CMOS technology. While our Australian effort in Si QC has largely focused on spin qubits based upon phosphorus dopant atoms implanted in Si [2,3], we are also exploring spin qubits based on single electrons confined in SiMOS quantum dots [4]. Such qubits can have long spin lifetimes T1 $=$ 2 s, while electric field tuning of the conductionband valley splitting removes problems due to spinvalley mixing [5]. In isotopically enriched Si28 these SiMOS qubits have a control fidelity of 99.6{\%} [6], consistent with that required for faulttolerant QC. By gatevoltage tuning the electron g*factor, the ESR operation frequency can be Stark shifted by \textgreater 10 MHz [6], allowing individual addressability of many qubits. Most recently we have coupled two SiMOS qubits to realize a CNOT gate [7] using exchangebased controlled phase (CZ) operations. The speed of the twoqubit CZoperations is controlled electrically via the detuning energy and over 100 twoqubit gates can be performed within a coherence time of 8 $\mu $s. [1] D.D. Awschalom et al., ``Quantum Spintronics'', Science 339, 1174 (2013). [2] J.J. Pla et al., ``A singleatom electron spin qubit in silicon'', Nature 489, 541 (2012). [3] J.T. Muhonen et al., ``Storing quantum information for 30 seconds in a nanoelectronic device'', Nature Nanotechnology 9, 986 (2014). [4] S.J. Angus et al., ``Gatedefined quantum dots in intrinsic silicon'', Nano Lett. 7, 2051 (2007). [5] C.H. Yang et al., ``Spinvalley lifetimes in a silicon quantum dot with tunable valley splitting'', Nature Comm. 4, 2069 (2013). [6] M. Veldhorst et al., ``An addressable quantum dot qubit with faulttolerant control fidelity'', Nature Nanotechnology 9, 981 (2014). [7] M. Veldhorst et al., ``A twoqubit logic gate in silicon'', Nature 526, 410 (2015). [Preview Abstract] 
Wednesday, March 16, 2016 11:51AM  12:03PM 
L45.00002: Silicon quantum processor with robust longdistance qubit coupling Guilherme Tosi, Fahd A. Mohiyaddin, Stefanie Tenberg, Rajib Rahman, Gerhard Klimeck, Andrea Morello Recent demonstration of highfidelity quantum operations using donors in silicon [1] has ignited an urge in scaling up these systems to a multiqubit device. However, multiqubit operations and longdistance donor coupling remain a formidable challenge. We will present a novel scalable design for a silicon quantum processor [2] that allows for longdistance fast 2qubit gates and does not require precise donor placement. Quantum information is encoded into either the nuclearspin or the flipflop states of electron and nucleus. It can be manipulated by biasing the electron wavefunction to be shared between donor and interface, in such a way that the hyperfine interaction strongly depends on electric fields. The qubits are spaced by hundreds of nanometers and coupled through direct electric dipole interactions and/or photonic links. All operations are performed at secondorder clock transitions, preserving the qubits' outstanding coherence times. A large number of qubits can then be interconnected in a network robust against errors. Prototypical devices are fabricated to demonstrate the processor's basic units. [1] J. T. Muhonen, et.al. Nature Nanotechnol. 9, 986 (2014). [2] G. Tosi, et.al. arXiv:1509.08538 (2015). [Preview Abstract] 
Wednesday, March 16, 2016 12:03PM  12:15PM 
L45.00003: Towards optimizing twoqubit operations in threeelectron double quantum dots Adam Frees, John King Gamble, Sebastian Mehl, Mark Friesen, S.N. Coppersmith The successful implementation of singlequbit gates in the quantum dot hybrid qubit motivates our interest in developing a high fidelity twoqubit gate protocol. Recently, extensive work has been done to characterize the theoretical limitations and advantages in performing twoqubit operations at an operation point located in the charge transition region. Additionally, there is evidence to support that singlequbit gate fidelities improve while operating in the socalled ``fardetuned" region, away from the charge transition. Here we explore the possibility of performing twoqubit gates in this region, considering the challenges and the benefits that may present themselves while implementing such an operational paradigm. This work was supported in part by ARO (W911NF120607) (W911NF12R0012), NSF (PHY1104660), ONR (N000141510029). 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 is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the US Department of Energy's National Nuclear Security Administration under Contract No. DEAC0494AL85000. [Preview Abstract] 
Wednesday, March 16, 2016 12:15PM  12:27PM 
L45.00004: HighFidelity Entangling Gates for TwoElectron Spin Qubits Pascal Cerfontaine, Sebastian Mehl, David P. DiVincenzo, Hendrik Bluhm High fidelity gate operations for manipulating individual and multiple qubits are a prerequisite for faulttolerant quantum information processing. Recently, we have shown that singlequbit gates for singlettriplet qubits in GaAs can be pulseengineered to reduce systematic errors and mitigate magnetic field fluctuations from the abundant nuclear spins, leading to experimentally demonstrated gate fidelities of 98.5\% [1]. We expect that a similar approach will be successful for twoqubit gates. We now describe short gating sequences for exchangebased twoqubit gates, showing that gate infidelities below 0.1\% can be reached in realistic quantum dot setups [2]. Additionally, we perform numerical pulse optimization to fully take the experimentally important imperfections into account, minimizing systematic errors and noise sensitivity. Since transferring the optimal control pulses to an experimental setting will inevitably incur systematic errors, we discuss how these errors can be calibrated on the experiment. [1] P. Cerfontaine, T. Botzem, D. Schuh, D. Bougeard, H. Bluhm, in preparation. [2] S. Mehl, H. Bluhm, D. P. DiVincenzo, PRB 90 (2014). [Preview Abstract] 
Wednesday, March 16, 2016 12:27PM  12:39PM 
L45.00005: Errorreducing sequence for capacitively coupled singlettriplet qubits Fernando CalderonVargas, Jason Kestner Twoqubit gates can be implemented by capacitively coupling singlettriplet qubits, which has been experimentally demonstrated to be capable of generating entangling operations. However, the fidelity of the entangling twoqubit gates is still far from optimum. In this light, we propose a twoqubit entangling echo sequence that reduces drastically the twoqubit decoherence due to the Overhauser field fluctuation and improves the fidelity of twoqubit gates under charge noise. [Preview Abstract] 
Wednesday, March 16, 2016 12:39PM  12:51PM 
L45.00006: Decoherence of two electron spin qubit in Si double quantum dot with gfactor modulations Peihao Huang, Garnett Bryant The rapid progress in the manipulation and detection of semiconductor spin qubits enables the experimental demonstration of high fidelity two qubit gates that are necessary for universal quantum computing. Here, we consider the decoherence of two electron spin due to phonon emission in a Si double quantum dot (DQD). In the large detuning regime, where the two qubit gate is operated, we find that the decoherence depends strongly on the gfactor modulation and the asymmetry of the two dots. The estimated two qubit decoherence rate is comparable to the experimental measured results. We discuss the impact of the decoherence on the single/two qubit operations and ways to reduce the gate errors for the addressable semiconductor spin qubit. [Preview Abstract] 
Wednesday, March 16, 2016 12:51PM  1:03PM 
L45.00007: Valley dependent gfactor anisotropy in Silicon quantum dots Rifat Ferdous, Erika Kawakami, Pasquale Scarlino, Michal Nowak, Gerhard Klimeck, Mark Friesen, Susan N. Coppersmith, Mark A. Eriksson, Lieven M. K. Vandersypen, Rajib Rahman Silicon (Si) quantum dots (QD) provide a promising platform for a spin based quantum computer, because of the exceptionally long spin coherence times in Si and the existing industrial infrastructure. Due to the presence of an interface and a vertical electric field, the two lowest energy states of a Si QD are primarily composed of two conduction band valleys. Confinement by the interface and the Efield not only affect the charge properties of these states, but also their spin properties through the spinorbit interaction (SO), which differs significantly from the SO in bulk Si. Recent experiments have found that the gfactors of these states are different and dependent on the direction of the Bfield. Using an atomistic tightbinding model, we investigate the electric and magnetic field dependence of the electron gfactor of the valley states in a Si QD. We find that the gfactors are valley dependent and show 180degree periodicity as a function of an inplane magnetic field orientation. However, atomic scale roughness can strongly affect the anisotropic gfactors. Our study helps to reconcile disparate experimental observations and to achieve better external control over electron spins in Si QD, by electric and magnetic fields. [Preview Abstract] 
Wednesday, March 16, 2016 1:03PM  1:15PM 
L45.00008: Strong spin relaxation anisotropy in a singleelectron quantum dot Liuqi Yu, L. C. Camenzind, D. E. F. Biesinger, J. Zimmerman, A. C. Gossard, D. M. Zumbühl Spin coherence and relaxation is of crucial importance in operating spin based qubits. In a magnetic field, spins relax predominately through spinphonon coupling mediated by spinorbit interaction (SOI) [1]. Here we present measurements of the spin relaxation rate anisotropy in a gate defined singleelectron GaAs quantum dot. The spin relaxation rate W is measured at applied magnetic fields of 4 T in the plane of the 2D electron gas. W exhibits strong anisotropy: a sinusoidal dependence on the Bfield angle $\varphi $ with a period of 180 degrees, as reported recently [2]. The extrema are observed at fields pointing nearly along the [110] and [110] crystal axes, modulated by a factor of about 14 from minimum to maximum. The periodicity is attributed to the interplay of Rashba and Dresselhaus SOIs. To decipher the role of SOI, we perform pulsedgate spectroscopy to extract orbital excitedstate energies, and obtain very good agreement with theory also for the angular dependence W($\varphi )$, indicating that $\alpha $ and $\beta $, Rashba and Dresselhaus coefficients respectively, have the same relative sign and are within 20{\%} of each other. With controllable manipulations of the dot orbitals by varying gate voltages, it is possible to precisely extract values of $\alpha $ and $\beta $. Meanwhile, top and back gates have been implemented on the device structure, which allows full electrical control over the Rashba SOI in the 2D electron gas [3]. [1] V. N. Golovach et al., Phys. Rev. Lett. \textbf{93}, 016601 (2004). [2] P. Scarlino et al., Phys. Rev. Lett. \textbf{113}, 256802 (2014). [3] F. Dettwiler et al., arXiv:1403.3518 (2014). [Preview Abstract] 
Wednesday, March 16, 2016 1:15PM  1:27PM 
L45.00009: Electron Spin Resonance Characterization of Damage and Recovery of Si/SiO$_{2}$ Interfaces from Electron Beam Lithography JinSung Kim, Alexei Tyryshkin, Stephen Lyon Electron beam lithography (EBL) is an essential tool for the fabrication of few electron silicon quantum devices. However, highenergy electrons and photons from the EBL process create shallow traps and other defects at the Si/SiO$_{2}$ interface, inhibiting the control of electron populations through electrostatic gating. To reduce defect densities, high temperature and forming gas anneals are commonly used. We studied the effect of these anneals on the reduction of shallow traps created by EBL by fabricating two sets of large area ($\sim$1cm$^{2}$) MOSFETs and characterizing them using transport and electron spin resonance (ESR) measurements. One set was exposed to a typical EBL dosage (10kV, 40$\mu$C/cm$^{2}$) and the other remained unexposed. All MOSFETs were fabricated from the same commercially grown gate stack (30nm dry thermal oxide, 200nm amorphous silicon gate layer) and were annealed at 900C in N$_{2}$ and at 435C in forming gas. Our transport data indicate that these annealing steps recover the EBL exposed sample's low temperature (4.2K) peak mobility to 85$\%$ of the unexposed sample's. Additionally, our ESR data indicate that annealing the EBL exposed sample reduces its density of shallow traps (24 meV) to the same density as the unexposed sample. [Preview Abstract] 
Wednesday, March 16, 2016 1:27PM  1:39PM 
L45.00010: Assessing MOS Interface Quality for Silicon Quantum Dot Device Fabrication Ryan Stein, JinSung Kim, Steve Lyon, Neil M. Zimmerman, M. D. Stewart, Jr. Defects at the SiSiO2 interface are capable of trapping electrons and degrading the operation of siliconbased quantum dot devices. To improve device performance, we are working to characterize the interface quality in MOSCAPs and MOSFETs fabricated at NIST by comparing industry standard defect measurements, such as capacitancevoltage (CV), conductance, and mobility, to electron spin resonance (ESR) measurements. This comparison will give insight into the relative role of defects near the band edge and those distributed throughout the gap in degrading device performance. We will discuss our progress toward this goal as well as our latest data and interpretations. [Preview Abstract] 
Wednesday, March 16, 2016 1:39PM  1:51PM 
L45.00011: Firstprinciples hyperfine tensors for electrons and holes in silicon and GaAs Pericles Philippopoulos, Stefano Chesi, William Coish Knowing (and controlling) hyperfine interactions in silicon and IIIV semiconductor nanostructures is important for quantum information processing with electron and nuclear spin states. We have performed densityfunctional theory (DFT) calculations that fully account for spin structure of the Bloch states (in contast with approaches that rely on the density alone). Using this method, we confirm the known value for the contact hyperfine coupling in the conduction band of silicon, but find a significant deviation in the value for the conduction band of GaAs relative to the accepted value, estimated in ref. [1]. Moreover, this method can be used to calculate the full hyperfine tensor for the valence band, where spinorbit effects may be strong, precluding methods that determine hyperfine couplings from the density alone. This general method can be applied to a broad class of materials with strong combined spinorbit and hyperfine interactions. [1] D. Paget, G. Lampel, B. Sapoval, and V. I. Safarov Phys. Rev. B 15, 5780 (1977) [Preview Abstract] 
Wednesday, March 16, 2016 1:51PM  2:03PM 
L45.00012: Quantum quench dynamics of a centralspin system Alessandro Ricottone, William Coish, Stefano Chesi, Yinan Fang Quantum effects can significantly influence equilibration dynamics. In quantum annealing, a local tunneling mechanism may accelerate the approach to equilibrium. Similarly, longrange quantum coherence can allow for rapid transitions between macroscopically distinct states of a quantum system. An experimentally relevant example of this is given by a 'central' electron spin coupled to an ensemble of nuclear spins in a quantum dot. This system admits a superradiancelike burst of current through ferromagnetic leads due to longrange nuclear spin coherence [1] with a simultaneous inversion of the nuclearspin polarization. Here, we study this system coupled to normal leads. In particular, we study quench dynamics of the nuclear spin polarization after passing through a quantum phase transition controlled by an applied magnetic field. As a function of dephasing controlled by a magnetic field gradient, we find a crossover from rapid equilibration via collective states to slow dynamics described by classical (productstate) spin configurations. This understanding may allow us to better control dynamic nuclear spin polarization processes in quantum dots and to control more general quantum states of nuclearspin ensembles. [1] S. Chesi and W. A. Coish PRB 91, 245306 (2015) [Preview Abstract] 
Wednesday, March 16, 2016 2:03PM  2:15PM 
L45.00013: ABSTRACT WITHDRAWN 
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