Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session T26: Focus Session: Semiconductor Qubits - Charge Qubits, Measurement, and Noise |
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Sponsoring Units: GQI Chair: Matthew Borselli, HRL Laboratories, LLC Room: 328 |
Thursday, March 21, 2013 8:00AM - 8:36AM |
T26.00001: LeRoy Apker Award Lecture: Coherent control of a semiconductor charge qubit Invited Speaker: Yuliya Dovzhenko A charge qubit is formed in a GaAs double quantum dot containing one electron. The two basis states of the qubit correspond to the electron residing in either the left or the right dot. In order to drive coherent rotations of the qubit state, 100 ps timescale voltage pulses are applied to the depletion gates forming the double dot. The resulting charge state is detected by a nearby quantum point contact charge sensor. In contrast with previous work, where a single non-adiabatic pulse was applied for quantum control,\footnote{K. D. Petersson {\it et al.}, Phys. Rev. Lett. {\bf105}, 246804 (2010).} we apply multiple pulses working towards dynamic decoupling.\footnote{L. Viola {\it et al.}, Phys. Rev. Lett. {\bf82}, 2417 (1999).} Data for Ramsey and charge echo pulse sequences are obtained and compared with numerical simulations of the charge qubit evolution.\footnote{Y. Dovzhenko {\it et al.}, Phys. Rev. B {\bf84}, 161302(R) (2011).} Coherent multi-pulse control of a semiconductor charge qubit demonstrated in this experiment is an essential requirement for future work in understanding charge noise in semiconductor qubits and improving the fidelity of spin qubit operations. [Preview Abstract] |
Thursday, March 21, 2013 8:36AM - 8:48AM |
T26.00002: Spectroscopy of a many-electron InAs spin-orbit qubit J. Stehlik, M.D. Schroer, K.D. Petersson, M. Jung, J.R. Petta The ability to perform arbitrary single spin rotations is a crucial ingredient for solid state quantum computation using electron spins. However, achieving rapid and selective single spin rotations has been challenging. Strong spin-orbit materials are very promising in this regard, as the spin-orbit interaction can turn a periodic electric driving field into an effective oscillating magnetic field through a process called electric dipole spin resonance (EDSR). In this work we explore EDSR in an InAs nanowire spin-orbit qubit. The qubit is implemented using a many-electron double quantum dot (DQD) and is configured in Pauli-blockade, where electron transport is highly sensitive to processes that rotate spin. We use EDSR to probe the detailed level structure of the DQD. We find a strong current response in several regions of the parameter space, raising the prospects for fast spin rotations. [Preview Abstract] |
Thursday, March 21, 2013 8:48AM - 9:00AM |
T26.00003: Pulse-gated quantum dot hybrid qubit S.N. Coppersmith, Teck Seng Koh, John King Gamble, M.A. Eriksson, Mark Friesen A quantum dot hybrid qubit formed from three electrons in a double quantum dot has the potential for great speed, due to presence of level crossings where the qubit becomes charge-like. Here, we show how to exploit the level crossings to implement fast pulsed gating. We develop one- and two-qubit dc quantum gates that are simpler than the previously proposed ac gates [1]. We obtain closed-form solutions for the control sequences and show that the gates are fast (sub-nanosecond) and can achieve high fidelities. \\[4pt] [1] Z. Shi, et al., Phys. Rev. Lett. 108, 140503 (2012). [Preview Abstract] |
Thursday, March 21, 2013 9:00AM - 9:12AM |
T26.00004: Enhanced Coherence and High Figure of Merit in a Silicon Charge qubit Zhan Shi, Christie Simmons, Daniel Ward, Jonathan Prance, Teck Seng Koh, John Gamble, Xian Wu, Donald Savage, Max Lagally, Mark Friesen, Susan Coppersmith, Mark Eriksson Coherent manipulation of a charge qubit is an essential step in the use of pulsed gate voltages [1] to manipulate a quantum dot hybrid spin qubit [2]. Here, we demonstrate coherent manipulation of a charge qubit in Si/SiGe double quantum dot. We perform Larmor oscillations (x-rotations on the Bloch sphere) between the (2,1) and (1,2) charge states, measuring a T$_{2}$* time of 2.1 ns at the charge degeneracy point. We find an increased coherence time (3.7 ns) and higher figure of merit (37) away from the charge degeneracy point, arising from a second charge anti-crossing involving a low lying excited state in the right dot -- the desired structure for a hybrid spin qubit. We also observe Ramsey fringes (z-rotations on the Bloch sphere) and measure a T$_{2}$* of 179 ps at detunings away from any protective energy level structures.\\[4pt] [1] Teck Seng Koh, et al., e-print: http://arxiv.org/abs/1207.5581\\[0pt] [2] Zhan Shi, et al., \textit{Phys. Rev. Lett.} \textbf{108}, 140503 (2012). e-print: http:// arxiv.org/abs/1110.6622\\[0pt] [3] Zhan Shi, et al., e-print: http://arxiv.org/abs/1208.0519 [Preview Abstract] |
Thursday, March 21, 2013 9:12AM - 9:24AM |
T26.00005: Multi-electron double quantum dot spin qubits Erik Nielsen, Jason Kestner, Edwin Barnes, Sankar Das Sarma Double quantum dot (DQD) spin quits in a solid state environment typically consist of two electron spins confined to a DQD potential. We analyze the viability and potential advantages of DQD qubits which use greater then two electrons, and present results for six-electron qubits using full configuration interaction methods. The principal results of this work are that such six electron DQDs can retain an isolated low-energy qubit space that is more robust to charge noise due to screening. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 21, 2013 9:24AM - 9:36AM |
T26.00006: Charge noise and dynamical decoupling in singlet-triplet spin qubits Guy Ramon We consider theoretically the effects of an ensemble of fluctuating charges on the coherence of a singlet-triplet qubit in gate-defined double quantum dots. We predict a crossover behavior of the system between non-Gaussian noise and 1/f spectrum, going from mesoscopic single-qubit devices to multi-qubit larger devices. With increasing size of the fluctuator ensemble we find a narrowed distribution of qubit dephasing times that result from random sets of fluctuators. At the same time the noise becomes Markovian with a characteristic Gaussian spectrum and it is dominated by a large collection of weakly-coupled fluctuators. The efficiency of dynamical decoupling pulse sequences in restoring coherence is examined as a function of the qubit's working position and the fluctuator ensemble size. Analytical solutions for qubit dephasing in the limits of weak and strong qubit-fluctuator coupling shed light on the distinct dynamics at different parameter regimes. [Preview Abstract] |
Thursday, March 21, 2013 9:36AM - 9:48AM |
T26.00007: Relaxation in quantum dots due to evanescent-wave Johnson noise from a metallic backgate Luke Langsjoen, Amrit Poudel, Maxim Vavilov, Robert Joynt This talk will present a study of decoherence in charge and spin qubits due to evanescent-wave Johnson noise (EWJN) in a laterally coupled double quantum dot and single quantum dot, respectively. The high density of evanescent modes in the vicinity of metallic gates causes energy relaxation and a loss of phase coherence of electrons trapped in quantum dots. These energy relaxation rates are derived, and EWJN is shown to be a dominant source of decoherence for spin qubits held at low magnetic fields. Previous studies in this field approximated the charge or spin qubit as a point dipole. Ignoring the finite size of the quantum dot in this way leads to a spurious divergence in the relaxation rate as the qubit approaches the metal. Our approach goes beyond the dipole approximation and remedies this unphysical divergence by taking into account the finite size of the quantum dot. [Preview Abstract] |
Thursday, March 21, 2013 9:48AM - 10:00AM |
T26.00008: Charge noise and spin noise in a semiconductor qubit Richard Warburton, Andreas Kuhlmann, Julien Houel, Arne Ludwig, Andreas Wieck Developing semiconductor spin qubits involves dealing with noise. Spin noise arising from the fluctuating nuclear spins results in electron spin dephasing and decoherence. Charge noise also results in dephasing and decoherence via the spin-orbit interaction and the electric field dependence of the g-factors. We have used resonance fluorescence from a single optically-active quantum dot as a local, minimally-invasive probe of the noise. Our technique is sensitive to 4 decades of noise over 6 decades of frequency. We present a method which allows us to distinguish between charge noise (a fluctuating electrostatic potential) and spin noise (a fluctuating effective magnetic field): we show how the two noise sources result in different optical signatures. The charge noise dominates at low frequencies, the spin noise at higher frequencies. The charge noise spectrum following neither a Lorentzian nor a 1/f-behaviour can be understood by considering an ensemble of 2-level fluctuators located close to the quantum dot. Crucially, both sources of noise decrease rapidly with increasing frequency. The consequences for the quantum dot are profound: at high frequencies (above 10 kHz) the noise is sufficiently small that we achieve ideal optical linewidths (the Fourier transform limit). [Preview Abstract] |
Thursday, March 21, 2013 10:00AM - 10:12AM |
T26.00009: Leakage-current lineshapes from inelastic cotunneling in the Pauli spin blockade regime Farzad Qassemi, Bill Coish We find the leakage current through a double quantum dot in the Pauli spin blockade regime accounting for inelastic (spin-flip) cotunneling processes. Taking the energy-dependence of this spin-flip mechanism into account allows for an accurate description of the current as a function of applied magnetic fields, gate voltages, and an inter-dot tunnel coupling. In the presence of an additional local dephasing process or nonuniform magnetic field, we obtain a simple closed-form analytical expression for the leakage current giving the full dependence on an applied magnetic field and energy detuning. This work is important for understanding the nature of leakage, especially in systems where other spin-flip mechanisms (due, e.g., to hyperfine coupling to nuclear spins or spin-orbit coupling) are weak, including silicon and carbon-nanotube or graphene quantum dots. \\ W. A. Coish and F. Qassemi, Phys. Rev. B 84, 245407 (2011), http://arxiv.org/abs/1109.4445, [Preview Abstract] |
Thursday, March 21, 2013 10:12AM - 10:24AM |
T26.00010: Reweighting of charge occupation in charge stability diagrams due to finite temperature effect and asymmetric tunnel rates in a silicon MOS double quantum dot Khoi Nguyen, Michael Lilly, Nathaniel Bishop, Erik Nielsen, Rajib Rahman, Joel Wendt, Jason Dominguez, Tammy Pluym, Jeff Stevens, Greg Ten Eyck, Malcolm Carroll The combination of asymmetric tunnel rates and finite temperature can shift the average charge occupation within a double quantum dot (DQD) stability diagram. DQD charge sensing shows the transitions in electron occupation dependence on gate bias. Applied source-drain bias further introduces shifts in the charge transition lines including the formation of bias triangles. In some material systems, tunnel barrier uniformity can be difficult to achieve. Asymmetry in tunnel barriers can lead to vanishingly small transitions in regions. Finite temperature effects with asymmetric barriers further leads to kinks in the stability diagram. In this talk we present measurements of DQDs with asymmetric barriers and compare them to simulation of stability diagrams using a capacitance network including the rate equation and temperature dependent tunneling. The model provides quantitative insight about finite temperature effects as well as the vanishing charge transition lines that is not readily available in the literature. [Preview Abstract] |
Thursday, March 21, 2013 10:24AM - 10:36AM |
T26.00011: Integration of on-chip FET switches with dopantless Si/SiGe quantum dot structures for high throughput testing Daniel Ward, Donald Savage, Max Lagally, Susan Coppersmith, Mark Eriksson In the last few years, significant research on dopantless Si/SiGe planar quantum dot structures has occurred. One of the limiting factors is that typically only a single double-dot structure can be cooled down in a dilution refrigerator at time due to the limited number of electrical connections available. We report on our recent work to create samples with four sets of double-dots on a single chip that can all be tested in a single cool down through the introduction of on-chip FET switches. In our samples the four double-dot structures have their depletion gates and ohmic contacts connected in parallel, minimizing the number of connections. We energize accumulation gates for the device under test such that the other dot structures do not contribute to the measurements. Our double-dot structures require five accumulation gates, which limits scaling due to limited fridge wiring capacity. To alleviate this problem and to test integration approaches for cryogenic quantum dot devices we fabricated a series of on-chip FET switches to form a multiplexer for the accumulation gates. Using the multiplexer we can wire up four double-dot structures using just 23 connections instead of the 34 required without it. As more devices are added the scaling benefits increase exponentially. [Preview Abstract] |
Thursday, March 21, 2013 10:36AM - 10:48AM |
T26.00012: Electron transport on ultra thin helium Maika Takita, E.Y. Huang, S.A. Lyon Electrons floating on the surface of superfluid helium have been suggested as promising mobile spin qubits, and they have shown extremely efficient transport above micron-sized helium-filled channels. While the calculated spin decoherence and relaxation times on helium are long, no experimental measurements have been made. Efficient thermalization of the spins is necessary for ESR measurements of their coherence, and a lack of thermalization has hindered these experiments. Bringing electrons onto a thin helium film above a metallic layer will speed spin relaxation due to Johnson noise current in the metal. At the same time, higher electron densities can be supported by thin helium films. Ideally, the electrons could be thermalized on the thin helium film coating a metal surface, and then moved to a helium-filled channel for electrical measurements of their density and the spin measurements. However roughness of the metal surface severely limits the electron mobility. Preliminary work show that electrons can be transported from one channel, across a helium-coated metal layer, and to the neighboring channel, by creating a smooth transition from the channel to the thin film. [Preview Abstract] |
Thursday, March 21, 2013 10:48AM - 11:00AM |
T26.00013: Implementation and test of an Levitov's n-electron coherent source D. Christian Glattli, Julie Dubois, Thibaut Jullien, Preden Roulleau, Fabien Portier, P. Roche Injecting a controlled number of electrons in a quantum conductor opens the way to new quantum experiment. It is known that a voltage biased contact applied on a single mode quantum conductor, such as a perfectly transmitting Quantum Point Contact (QPC), continuously injects single electrons at a rate \textit{eV/h}. Here we consider the injection of n electrons using a short time voltage pulse with $\smallint $\textit{eV(t)dt }$=$\textit{ nh}. When the voltage pulse has a Lorentzian shape, L. Levitov et al. [1] have shown that the n-electron injection is free of extra neutral electron-hole pairs and is a minimal excitation state. We present the first realization of Levitov's proposal. Using periodic voltage pulses applied on a contact of a 2DEG, a coherent train of n-electrons is send to a QPC which acts as an electron beam splitter. By measuring the shot noise resulting from the partitioning of all excitations we demonstrate that Lorentzian pulses are minimal excitation states. This is complemented by energy domain study of the excitations using shot noise spectroscopy and by a time-domain study using shot noise in a Hong-Ou-Mandel like n-electron collision experiment.\\[4pt] [1] H-W Lee {\&} L. Levitov, cond-mat: 9312013; J. Keeling, I. Klich, and L. Levitov, Phys. Rev. Lett. 97, 116403 (2006).\\[0pt] [2] J. Dubois, T. Jullien, P. Roulleau, F. Portier, P. Roche, W. Wegscheider and D.C. Glattli, submitted. [Preview Abstract] |
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