Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B26: Focus Session: Semiconductor Qubits - Silicon and III-Vs |
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Sponsoring Units: GQI Chair: Jason Petta, Princeton University Room: D136 |
Monday, March 15, 2010 11:15AM - 11:51AM |
B26.00001: Silicon enhancement mode nanostructures for quantum computing Invited Speaker: Development of silicon, enhancement mode nanostructures for solid-state quantum computing will be described. A primary motivation of this research is the recent unprecedented manipulation of single electron spins in GaAs quantum dots, which has been used to demonstrate a quantum bit [1]. Long spin decoherence times are predicted possible in silicon qubits. This talk will focus on silicon enhancement mode quantum dot structures that emulate the GaAs lateral quantum dot qubit [1] but use an enhancement mode field effect transistor (FET) structure. One critical concern for silicon quantum dots that use oxides as insulators in the FET structure is that defects in the metal oxide semiconductor (MOS) stack can produce both detrimental electrostatic and paramagnetic effects on the qubit. Understanding the implications of defects in the Si MOS system is also relevant for other qubit architectures that have nearby dielectric passivated surfaces. Stable, lithographically defined, single-period Coulomb-blockade and single-electron charge sensing in a quantum dot nanostructure using a MOS stack will be presented. A combination of characterization of defects, modeling and consideration of modified approaches that incorporate SiGe or donors provides guidance about the enhancement mode MOS approach for future qubits and quantum circuit micro-architecture. [1] J. Petta et al., Science 309, 2180 (2005) We wish to acknowledge the research funding support provided by the laboratory directed research and development (LDRD) program at Sandia National Laboratories and the Laboratory of Physical Sciences. Sandia National Labs is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:03PM |
B26.00002: Ultra long coherence of GaAs electron spin qubits through dynamic decoupling from a spin bath Hendrik Bluhm, Sandra Foletti, Diana Mahalu, Vladimir Umansky, Amir Yacoby Semiconductor spin qubits are promising candidates for quantum computation because of their slow decoherence and potential for scalability. All fundamental single qubit operations have been demonstrated for GaAs based spin qubits, but they suffer from decoherence due to hyperfine coupling to nuclei. We show experimentally that this nuclear decoherence can be mitigated very effectively. Using CPMG decoupling pulses, we extended the coherence time of two-electron spin qubits in GaAs double quantum dots to more than 200 $\mu$s, two orders of magnitude larger than previously measured. For a Hahn echo with a single refocusing pulse, coherence persists for 30 $\mu$s. At low fields, the Hahn echo shows collapses and revivals associated with electron mediated spin-transfer between nuclei. They are in good agreement with recent theoretical work based on a quantum mechanical treatment of the nuclear spin bath. In conjunction with our quantum feedback technique that suppresses slow nuclear fluctuations, our results indicate that GaAs spin qubits are promising candidates for reaching the quantum error correction threshold. [Preview Abstract] |
Monday, March 15, 2010 12:03PM - 12:15PM |
B26.00003: Nuclear Polarization and its Influence on Spin Relaxation in Double Quantum Dots James Medford, Christian Barthel, Hendrik Bluhm, Morten Kjaergaard, Michael Stopa, Charles Marcus, Micah Hanson, Arthur Gossard High-fidelity repeated single-shot readout of the arrangement of two electrons in a GaAs double quantum dot with less than 1 $\mu$s repetition period is demonstrated experimentally. A radio frequency sensor quantum dot, fabricated next to the double dot and operated in the Coulomb blockade regime, is employed as a charge-sensor. Nuclear polarization created by electrical gate pulses is examined along with its decay as a function of field and pumping rate. The influence of the Overhauser field difference on the two electron spin relaxation is then investigated, yielding connections between the Overhauser field and the T1 of the qubit in the measurement position. [Preview Abstract] |
Monday, March 15, 2010 12:15PM - 12:27PM |
B26.00004: Comparison of Dynamical Decoupling Schemes in Double Quantum Dot Spin Qubits Christian Barthel, James Medford, Hendrik Bluhm, Charles Marcus, Micah Hanson, Arthur Gossard The decoherence and decoupling of a Singlet-Triplet spin qubit in a GaAs double quantum dot are studied, employing high-fidelity repeated single-shot readout of the charge arrangement with less than 1 $\mu$s repetition period. Coherence times of greater than $100~\mu$s are obtained for both Carr Purcell and Concatenated Dynamical Decoupling, opening the way for future fault tolerant error correction algorithms. In addition, long operation sequences combining single qubit gates and decoupling schemes are studied. Nuclear polarization build-up and feedback mechanisms are investigated and combined with single-shot measurement and control. [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 12:39PM |
B26.00005: Review of spin and orbital relaxation in silicon quantum dot qubits Charles Tahan, Robert Joynt We give updated results for orbital and spin relaxation times in single-spin, silicon quantum dots. We find marked differences from donor-confined electrons and GaAs quantum dots. The dominant spin qubit 1/T1 relaxation rates are due to inversion asymmetry-induced spin-orbit mixing and are found to have a dependence proportional to the seventh power of the magnetic field, in contrast to both the P:Si donor qubit and GaAs quantum dot situations (for different reasons). Recent results have suggested that a Dresselhaus-like spin-orbit mixing contribution is nonzero and indeed can exceed that of the Rashba contribution for parameter regimes of interest. We include contributions from both terms to the spin relaxation rate with the latest estimates of their quantitative contributions. Other relaxation mechanisms, including those inherited from the bulk and from spin-valley mixing, are calculated to be smaller in most situations. Results for relaxation rates of other excited states are also given. We relate our results to current and future experiments and discuss implications for future quantum computer architectures in silicon. [Preview Abstract] |
Monday, March 15, 2010 12:39PM - 12:51PM |
B26.00006: Pulsed-gate manipulation and real-time readout of one electron in Si/SiGe quantum dots C. B. Simmons, Madhu Thalakulam, B. M. Rosemeyer, B. J. Van Bael, D. E. Savage, Mark Friesen, S. N. Coppersmith, M. A. Eriksson We present the results of recent charge-sensing measurements of electrons in a Si/SiGe double quantum dot, demonstrating controlled occupation of the (0,0), (1,0), (0,1), and (1,1) charge states. Pulsed gate voltages can be used to determine the tunnel rates on and off such dots. We present the results of recent pulsed-gate experiments on both single and double quantum dots. Electron tunneling at MHz rates in the one-electron state is observed. We observe energy-dependent tunneling in these quantum dots: we consistently find significant reductions in tunnel rates as the energy eigenstates on the quantum dot drop below the Fermi level of the leads, and we discuss the utility of this effect for readout and initialization. We also present real-time readout of the occupation of the quantum dot, enabling the single-shot measurement of electron tunneling events. This work was supported by ARO and LPS, DOD, and NSF. [Preview Abstract] |
Monday, March 15, 2010 12:51PM - 1:03PM |
B26.00007: Excited-state spectroscopy in a Si/SiGe quantum dot using charge sensing and pulsed gate voltages Madhu Thalakulam, C.B. Simmons, B.M. Rosemeyer, B.J. Van Bael, D.E. Savage, Mark Friesen, S.N. Coppersmith, M.A. Eriksson Excited states in semiconductor quantum dots are often measured using transport spectroscopy, in which the differential conductance through the quantum dot is measured as a function of source-drain bias. Such spectroscopy can be very difficult in the few electron regime. We describe the results of spectroscopy on a Si/SiGe single quantum dot using charge sensing and pulsed gate voltages, thus avoiding the need for transport through the quantum dot. The quantum dot and the charge sensing quantum point contact are defined by top-gates on a Si/SiGe heterostructure. Multiple excited states are observed as a function of increasing amplitude of the voltage pulse. We demonstrate a method to calibrate the ratio of gate voltage to dot energy without using transport through the quantum dot. The result is a quantitative spectroscopy that is efficient even in the absence of any measurable transport through the quantum dot itself. This work was supported in part by ARO and LPS (W911NF-08-1-0482), by NSF (DMR-0805045), by DOD, and by DOE (DE-FG02-03ER46028). This research utilized NSF-supported shared facilities at the University of Wisconsin-Madison. [Preview Abstract] |
Monday, March 15, 2010 1:03PM - 1:15PM |
B26.00008: Spectra of Accumulation-Mode Few-Electron Si/SiGe Quantum Dots R.S. Ross, A.A. Kiselev, M.G. Borselli, B.M. Maune, A.T. Hunter, M.F. Gyure, C.R. Anderson We present the results of electronic structure calculations of the ground and excited state spectra of accumulation-mode semiconductor quantum dots (QD) in both the Si/SiGe and InAlAs/InGaAs material systems. Devices are modeled using a real-space Poisson-Schr\"odinger code coupled to a full configuration interaction (FCI) method in which both spin and valley degrees of freedom are explicitly included. Good agreement is found with measured ground state addition spectra allowing us to conclude that valleys play an essential role in Si QDs and that we have conclusively demonstrated single electron quantum dots in Si/SiGe. Calculations of the multi-electron excited-state spectra for both III-V and Si/SiGe accumulation-mode quantum dots will be presented along with predictions for transverse magneto-spectroscopy and comparisons with recent experimental data. Sponsored by United States Department of Defense Approved for Public Release, Distribution Unlimited. [Preview Abstract] |
Monday, March 15, 2010 1:15PM - 1:27PM |
B26.00009: Si/SiGe Depletion-mode and Accumulation-Mode Few-Electron Quantum Dots M.G. Borselli, R.R. Hayes, A.A. Kiselev, R.S. Ross, E.T. Croke, P.W. Deelman, W.S. Wong, I. Alvarado-Rodriguez, I. Milosavljevic, A.E. Schmitz, M. Sokolich, M.F. Gyure, A.T. Hunter We have measured charging spectra and charge dynamics of few-electron quantum dots made using Si/SiGe heterostructures. In the standard depletion-mode design, an excited state with Zeeman splitting consistent with a g-factor of 2.0$\pm$0.1 was identified on the lowest observed transition. The lifetime was 615 msec at 1.2T and had close to a B7 dependence on magnetic field, in good agreement with T1 spin relaxation estimates. We have also developed Si/SiGe accumulation-mode dots based on a double-well heterostructure in which electrons are localized in the top, nominally empty well by forward biasing a small gate. We have measured charging spectra from N=0 up to N=15, with addition energies as high as 4.5 meV. Magnetospectroscopy and charge dynamics are utilized to characterize valley splitting in these devices. Sponsored by United States Department of Defense Approved for Public Release, Distribution Unlimited. [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B26.00010: Effects of valley degeneracy and valley mixing in SiGe quantum dot structures A.A. Kiselev, R.S. Ross, M.F. Gyure We have analyzed the effects of valley degeneracy and valley mixing on single- and few-electron states in (001) electrostatically defined SiGe quantum dots (QDs), focusing on those most sensitive to macro- and microscopic characteristics of Si/SiGe interface. Theoretical analysis suggests that interface steps, variations in interface quality, and especially intentional interface engineering can dramatically modify valley-induced effects; this notion is further supported by our numerical simulations, where we explicitly allow for an arbitrary and spatially inhomogeneous stacking of heterolayers in the active area of the device. A correspondence with recent experimental data on SiGe QDs is critically examined. We have also considered intervalley relaxation in (001) SiGe QDs and identified an ``admixture'' mechanism, based on valley-orbit mixing, as a possible leading candidate. We evaluated its characteristic times for a number of relevant scenarios. The admixture mechanism is strongly suppressed in a truly planar geometry, whereas it can provide a fast relaxation channel for structures with macroscopically inhomogeneous interfaces. [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B26.00011: Strained-Si/SiGe enhancement mode structures for quantum computing Nathaniel Bishop, Donald Savage, Gregory Ten Eyck, Michael Lilly, Malcolm Carroll Silicon is an ideal system for investigating single electron or isolated donor spins for quantum computation, due to long spin coherence times. Enhancement mode strained-silicon / silicon germanium (sSi/SiGe) devices would offer an as-yet untried path toward electron or electron/donor quantum dot systems. Thin, undoped SiGe dielectrics allow tight electrostatic confinement, as well as potential Land\'e g-factor engineered spin manipulation. In this talk we summarize recent progress toward sSi/SiGe enhancement mode devices on sSi on insulator, including characterization with X-ray diffraction and atomic force microscopy, as well as challenges faced and progress on integration of either top-down and bottom-up donor placement approaches in a sSi/SiGe enhancement mode structure. This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B26.00012: Cryogenic CMOS circuits for single charge digital readout Kevin Eng, T. M. Gurrieri, J. Hamlet, M. S. Carroll The readout of a solid state qubit often relies on single charge sensitive electrometry. However the combination of fast and accurate measurements is non trivial due to large RC time constants due to the electrometers resistance and shunt capacitance from wires between the cold stage and room temperature. Currently fast sensitive measurements are accomplished through rf reflectrometry. I will present an alternative single charge readout technique based on cryogenic CMOS circuits in hopes to improve speed, signal-to-noise, power consumption and simplicity in implementation. The readout circuit is based on a current comparator where changes in current from an electrometer will trigger a digital output. These circuits were fabricated using Sandia's 0.35$\mu $m CMOS foundry process. Initial measurements of comparators with an addition a current amplifier have displayed current sensitivities of $<$ 1nA at 4.2K, switching speeds up to $\sim $120ns, while consuming $\sim $10$\mu $W. I will also discuss an investigation of noise characterization of our CMOS process in hopes to obtain a better understanding of the ultimate limit in signal to noise performance. [Preview Abstract] |
Monday, March 15, 2010 2:03PM - 2:15PM |
B26.00013: Double Dot Induced by a Single Defect in a Silicon Nanowire Ted Thorbeck, Neil Zimmerman, Akira Fujiwara, Yukinori Ono, Yasuo Takahashi, Hiroshi Inokawa Double quantum dots are an essential feature for several schemes of semiconductor quantum computation. We have seen both intentional, gate defined, double quantum dots as well as unintentional, defect induced, double quantum dots in our devices, which consist of a silicon nanowire with two layers of poly-silicon gates. This talk will give evidence, including SIMON simulations, that both of the dots we have seen are caused by one physical defect, where one dot corresponds to the defect and the other dot is induced by the defect at the interface of the silicon and the silicon dioxide. It has been previously proposed that one dopant can induce a quantum dot at the interface; we hope to go beyond this and show that the defect and the induced dot can act as a double quantum dot. This work could be useful for recent proposals for quantum computation using defects in silicon such as dopants and dangling bonds. [Preview Abstract] |
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