Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session B35: Silicon Spin QubitsFocus

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Sponsoring Units: DQI Chair: Emily Pritchett, HRL Laboratories Room: BCEC 205B 
Monday, March 4, 2019 11:15AM  11:51AM 
B35.00001: Spin Qubits at Intel Invited Speaker: Jim Clarke Intel is developing a 300mm process line for spin qubit devices using stateoftheart immersion lithography and isotopically pure epitaxial silicon layers. Both SiMOS and Si/SiGe devices are being evaluated in this multilayer integration scheme. In this talk, we will be sharing our current progress towards spin qubits starting with substrate characterization. Transistors and quantum dot devices are then cofabricated on the same wafer and allow calibration to Intel’s internal transistor processes. Electrical characterization and feedback is accomplished through wafer scale testing at both room temperature and 1.6K prior to millikelvin testing. Accelerated testing across a 300mm wafer provides a vast amount of data that can be used for continuous improvement in both performance and variability. This removes one of the bottlenecks towards a large scale system: trying to deliver an exponentially fast compute technology with a slow and linear characterization scheme using only dilution refrigerators. 
Monday, March 4, 2019 11:51AM  12:03PM 
B35.00002: A novel Si/SiGe heterostructure for quantum dot spin qubits Thomas McJunkin, Evan R MacQuarrie, Samuel Neyens, Brandur Thorgrimsson, Joelle Corrigan, John Dodson, Donald E Savage, Max G Lagally, Mark G Friesen, Susan Coppersmith, Mark Alan Eriksson Motivated by a desire to increase the energy splitting between the two lowlying valley states in a silicon quantum well, a Si/SiGe heterostructure is grown via UHVCVD with a ~1 nm layer of SiGe buried ~2 nm beneath the upper interface of a silicon quantum well. High resolution STEM measurements confirm that this thin epitaxial layer is abrupt on the side facing the substrate and gradual on the side facing the surface. We report Shubnikovde Haas and quantum Hall measurements, finding a transport mobility in excess of 100,000 cm^{2}/(V s) at 6 x 10^{11} cm^{2} carrier density and a series of stable oscillations in measurements of the longitudinal voltage as a function of magnetic field and carrier density (a fan diagram). We report both activation energy measurements in the quantum Hall regime and measurements of the excited state spectrum in gatedefined quantum dots fabricated in this material. 
Monday, March 4, 2019 12:03PM  12:15PM 
B35.00003: Coherent control of a semiconductor quantum dot qubit, encoded by valleystates in Si Nicholas Penthorn, Joshua S Schoenfield, HongWen Jiang Traditionally, spin and charge of individual electrons are used to encode a qubit in gateddefined semiconductor quantum dots. Valley states of electrons in silicon represents another degree of freedom in addition to spin and charge degree of freedoms. In this talk, we demonstrate the coherent control of a semiconductor quantum dot qubit, encoded by valleystates in Si. A double quantum dot device, fabricated on a SiGe heterostructure, is used for the experiment. We found that either one of the quantum dots can be used to encode a qubit. The left dot has a valley splitting of 5.3 GHz (or 22 ueV), while the right dot has a different splitting of 8.2 GHz (or 34 ueV). The xaxis control of the qubit is done by either timed xrotation near the anticrossing point of the two valley eigenstates, or by the LandauZener effect using a slowly rising/falling pulse. The zaxis control is done at the region when the energy separation of the two valley levels is nearly detuning independent, which offers a protection against charge noise. Fast qubit operations, in the range of a few GHz, for both x and z rotations, are realized. 
Monday, March 4, 2019 12:15PM  12:27PM 
B35.00004: SpinBlockade Spectroscopy of Si/SiGe Quantum Dots Edward Chen We introduce a technique for measuring the singlettriplet energy splitting in undoped, accumulationmode Si/SiGe quantum dots [1]. We find that the measured splitting varies smoothly as a function of confinement gate biases and are also consistent to those obtained using photonassisted tunneling (PAT) spectroscopy at the (2,0)(1,1) charge transition. Because our technique operates in the limits of both large and small singlettriplet energy splittings, we are able to show that the splitting can be limited by the lateral orbital excitation energy rather than solely by the valley splitting in the silicon well. 
Monday, March 4, 2019 12:27PM  12:39PM 
B35.00005: Valley and orbital state spectroscopy of a Si/SiGe triple quantum dot John Dodson, Joelle Baer, Jose Carlos AbadilloUriel, Nathan Holman, Trevor Knapp, Brandur Thorgrimsson, Evan R MacQuarrie, Samuel Neyens, Thomas McJunkin, Ryan Foote, Lisa Edge, Mark G Friesen, Susan Coppersmith, Mark Alan Eriksson Valley and orbital states have important consequences for silicon based qubits, and a thorough understanding of their interaction in the few electron regime is crucial in forming robust qubits. While many spinbased qubits operate best with large valley and orbital splittings, valley qubits such as the quantum dot hybrid qubit [1] utilize the valley degree of freedom for its logical states. Here we present measurements of valley and orbital energies using excited state spectroscopy in a triple quantum dot fabricated using an AlAl_{x}O_{1x}Al overlapping gate design. Valley splitting is shown to vary as a function of electron occupancy in the N=1 to N=4 regime between 35 and 70 µeV. We observe that higher lying orbital levels have a larger valley splitting, and we present tight binding simulations consistent with this observation. The spatial dependence of the valley splitting in a linear array of quantum dots is also investigated. Finally, we observe anomalously low orbital splittings of 200400 µeV that could have potential applications to new readout mechanisms for certain silicon based qubits. 
Monday, March 4, 2019 12:39PM  12:51PM 
B35.00006: Energy Spectra of FewElectron Si Quantum Dots Ekmel Ercan, Susan Coppersmith, Mark G Friesen In this talk, we theoretically study the energy spectra of multielectron dots in Si/SiGe quantum wells, including valley effects and disorder at the quantum well interface. Our computational method combines tightbinding (TB) calculations with a full configuration interaction (FCI) scheme to study silicon quantum dots. While TB provides an accurate description of single electron wavefunctions by taking microscopic effects like interface disorder into account, and captures the valley physics of silicon, FCI allows us to calculate multielectron energies and corresponding wavefunctions by including the effects of electronelectron interactions. We use this computational tool to investigate the properties of valley and orbital states in Si/SiGe quantum dots in the experimentally relevant regime with the goal of being able to tune these devices in a more predictable way. 
Monday, March 4, 2019 12:51PM  1:03PM 
B35.00007: Effect of an Interface step on Tunnel Coupling and Energy Spectrum in a Si/SiGe double quantum dot Bilal Tariq, Xuedong Hu Understanding the effect of a rough interface on valleyorbit coupling in Si quantum dots is crucial in determining its suitability for application in quantum information processing. Interface roughness, such as an atomic step at the interface of Si and SiGe alloy leads to a change in the magnitude and phase of the valleyorbit coupling in a single quantum dot. Here we study the effect of interface steps on the spectrum and particularly tunnel coupling in a double quantum dot. More specifically, we explore how intervalley tunneling depends on the location and orientation of the step(s). We also investigate the interplay between a magnetic field and an interface step, and how it affects the state of an electron. Our results show that the geometry of a step is an important factor in determining the energy spectrum and tunneling coupling of a single electron in a Si double quantum dot. 
Monday, March 4, 2019 1:03PM  1:15PM 
B35.00008: Nuclear Dynamics in Isotopically Enhanced Silicon Spin Qubits Thaddeus Ladd Nuclear dipoledipole dynamics have long been identified as a problem for silicon qubits. Early NMR and spinqubit experiments in dense nuclear lattices such as GaAs characterized nuclear dynamics as classical spin diffusion, amounting to “Brownian motion” of the nuclear magnetization as seen by an electron spin. However, recent experiments in Si/SiGe qubits, phosphorus impurity qubits, and MOS qubits in both isotopically natural and isotopically purified samples point to more complex behavior, showing a noise spectrum with a 1/f spectrum at low frequency and some indications of measurement backaction. This talk will survey these various results and provide rough models for these observations, and will include a discussion of the challenges for efforts to model complex dipoledipole behavior using more quantitative manybody spin models. 
Monday, March 4, 2019 1:15PM  1:27PM 
B35.00009: Spin relaxation and dephasing in a ^{28}SiGe QD with nanomagnet Tom Struck, Arne Hollmann, Veit Langrock, Tim Leonhardt, Andreas Schmidbauer, Floyd Schauer, Christian Neumann, Nikolay V. Abrosimov, Dominique Bougeard, Lars R. Schreiber The isotopical purification of Si has significantly improved coherent control of a single electron spin in electrostatically defined ^{28}Si/SiGe quantum dots ^{[1]}. The question to what extent the spin coherence can be improved by further purification is currently connected to the open question whether electrical noise in the device, in combination with the stray field of a micromagnet dominantly limits the spin dephasing time. 
Monday, March 4, 2019 1:27PM  1:39PM 
B35.00010: Charge noise induced spin decoherence in a double quantum dot: Effects of a micromagnet Xinyu Zhao, Xuedong Hu Charge noise is one of the largest error source preventing the highfidelity quantum computing in semiconductor systems. We study the decoherence of an electron spin in a double quantum dot in the presence of an inhomogeneous magnetic field and induced by a nonMarkovian charge noise. We derive a master equation based on the stochastic Schrodinger equation. By analyzing the physical process represented by each term in the master equation, we show how the properties of the charge noise affect spin decoherence and how an orbital charge noise affects the spin dynamics through the inhomogeneous magnetic field. We find that a longer correlation time can slow down spin decoherence, particularly during the early stage of an evolution. A relation between the spin relaxation rate and the gradient of the magnetic field is given. The stochastic approach used to derive master equation can be also extended to other semiconductor systems in the presence of charge noise. Our results present a systematic approach to study decoherence processes caused by charge noise, particularly for quantum dots in an inhomogeneous magnetic field. 
Monday, March 4, 2019 1:39PM  1:51PM 
B35.00011: Spin decoherence in a quantum dot due to micromagnets Peihao Huang, Xuedong Hu A spin qubit in a semiconductor quantum dot is a promising candidate for quantum information processing for scalability and miniaturization. Micromagnets have been proven to be effective to mediate the coupling of a spin qubit to photon and electrical control field. In this work, we study spin decoherence in a quantum dot mediated by the magnetic field gradient created by micromagnets. The spin relaxation mediated by magnetic field gradient shows different magnetic field dependence compare with spin relaxation mediated by spinorbit coupling. Furthermore, to the first order of the magnetic field gradient, there is finite spin dephasing due to 1/f charge noise. We discuss the consequence of the new spin decoherence mechanisms on the number of qubit operations. 
Monday, March 4, 2019 1:51PM  2:03PM 
B35.00012: Targeted enrichment of ^{28}Si for quantum computing Ke Tang, Hyunsoo Kim, Aruna Ramanayaka, David S Simons, Joshua Pomeroy We report on the growth of isotopically enriched ^{28}Si epitaxial films with precisely controlled enrichment levels, ranging from natural abundance ratio of 92.2% all the way to 99.99987% (0.832ppm ^{29}Si). Isotopically enriched ^{28}Si is regarded as an ideal host material for semiconducting quantum computing due to the lack of ^{29}Si nuclear spins. However, the detailed mechanisms for quantum decoherence and the exact level of enrichment needed remain unknown. Here we use hyperthermal energy ion beam deposition with silane gas to deposit epitaxial ^{28}Si. In the meantime, we switch the mass selective magnetic field periodically to control the ^{29}Si concentration. A model predicting the residual ^{29}Si isotope fraction and the corresponding secondary ion mass spectrometry (SIMS) analysis are presented. Crosssectional SEM/TEM will also been shown for the deposited ^{28}Si film throughout the range of enrichments. 
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