Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S36: Focus Session: Semiconductor Qubits: Device, Control, and Measurement System Engineering |
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Sponsoring Units: GQI Chair: Matt Borselli, HRL Room: 703 |
Thursday, March 6, 2014 8:00AM - 8:36AM |
S36.00001: Quantum hardware Invited Speaker: David Reilly |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S36.00002: SiGe HBT cryogenic preamplification for higher bandwidth donor spin read-out Matthew Curry, Stephen Carr, Greg Ten-Eyck, Joel Wendt, Tammy Pluym, Michael Lilly, Malcolm Carroll Single-shot read-out of a donor spin can be performed using the response of a single-electron-transistor (SET). This technique can produce relatively large changes in current, on the order of 1 (nA), to distinguish between the spin states. Despite the relatively large signal, the read-out time resolution has been limited to approximately 100 (kHz) of bandwidth because of noise. Cryogenic pre-amplification has been shown to extend the response of certain detection circuits to shorter time resolution and thus higher bandwidth. We examine a SiGe HBT circuit configuration for cryogenic preamplification, which has potential advantages over commonly used HEMT configurations. Here we present 4 (K) measurements of a circuit consisting of a Silicon-SET inline with a Heterojunction-Bipolar-Transistor (HBT). We compare the measured bandwidth with and without the HBT inline and find that at higher frequencies the signal-to-noise-ratio (SNR) with the HBT inline exceeds the SNR without the HBT inline. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S36.00003: Optimal post-processing for a generic single-shot qubit readout Benjamin D'Anjou, William A. Coish We analyze three different post-processing methods applied to a single-shot qubit readout: the average-signal (boxcar filter), peak-signal, and maximum-likelihood methods. In contrast to previous work, we account for a stochastic turn-on time $t_i$ associated with the leading edge of a pulse signaling one of the qubit states. This model is relevant to spin-qubit readouts based on spin-to-charge conversion and would be generically reached in the limit of large signal-to-noise ratio $r$ for several other physical systems, including fluorescence-based readouts of ion-trap qubits and nitrogen-vacancy center spins. We find that the peak-signal method outperforms the boxcar filter significantly when $t_i$ is stochastic, but is only marginally better for deterministic $t_i$. We generalize the theoretically optimal maximum-likelihood method to stochastic $t_i$ and show numerically that a stochastic turn-on time $t_i$ will always result in a larger single-shot error rate. Based on this observation, we propose a general strategy to improve the quality of single-shot readouts by forcing $t_i$ to be deterministic. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S36.00004: Electron transport in double quantum dots: Pauli spin blockade and an ultrasmall magnetic field effect Jeroen Danon We consider electron transport in a double quantum dot tuned to the Pauli spin blockade regime. We revisit the role of the random nuclear fields in the two dots and develop a theory going beyond the usual master-equation approach. This allows us to take into account a dark state that forms when the average effective field in the two dots is zero. We show that this small-field dark state can survive averaging over the random fields, most noticeably at intermediate interdot tunnel coupling strength. Besides deepening our understanding of the electron dynamics in double quantum dots, our results might help explaining a so-called ultrasmall magnetic field effect observed in some organic semiconductors. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S36.00005: Field-effect-induced two-dimensional electron gas utilizing modulation doping for improved ohmic contacts Sumit Mondal, Geoff Gardner, John Watson, Micheal J. Manfra Recently there has been a significant interest in the use of GaAs-based quantum dots for spin qubits. Progress is hindered by the presence of charge noise in modulation doped heterostructures where fluctuations occurring in the remote ionized dopant layer couple to the qubit. In this work we demonstrate the experimental realization of a new field effect transistor (FET) device where the active channel region is locally devoid of the silicon doping layer and hence precludes the possibility of charge fluctuations on ionized dopants causing instability. The underlying heterostructure was grown by molecular beam epitaxy and is designed with an etch-stop between the silicon delta-doping layer and single interface GaAs/AlGaAs heterojunction that facilitates removal of the modulation doping at precise locations defined by lithography. The resulting 2DEG is induced by a field-effect and the density is tunable in a wide range of 6X10$^{\mathrm{10}}$ cm$^{\mathrm{-2}}$ to 2.7X10$^{\mathrm{11}}$ cm$^{\mathrm{-2}}$. The design, fabrication, and operation of these devices along with low temperature (T $=$ 0.3K) transport data is presented. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S36.00006: Accumulation-Only Device Architecture for Si/SiGe Quantum Dots K. Wang, D. Zajac, T. Hazard, C. Payette, J.R. Petta Silicon is one of the most promising candidates for ultra coherent quantum bits due to its relatively weak spin-orbit coupling and the absence of nuclear spin in its naturally abundant isotope [1]. High quality charge and spin qubits have been demonstrated with a dual-gate device geometry [1] [2]. Due to the larger effective mass of electrons in Si, it is desirable to have a more tightly confined quantum dot to increase the orbital level spacings. Here we demonstrate a new silicon quantum dot device architecture. The quantum dot and potential barriers are individually formed by corresponding accumulation gates, potentially allowing more precise control over electron occupation and tunnel coupling. The gate geometry can also be scaled up to create multiple quantum dot devices. [1] B. W. Maune \emph{et al.}, Nature \textbf{481}, 344 (2012). [2] K. Wang \emph{et al.}, Phys. Rev. Lett. \textbf{111}, 046801 (2013). [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S36.00007: Si/SiGe quadruple quantum dots with direct barrier gates Daniel Ward, John Gamble, Ryan Foote, Donald Savage, Max Lagally, Susan Coppersmith, Mark Eriksson We have fabricated a quadruple quantum dot in a Si/SiGe heterostructure with the aim of demonstrating a two-qubit quantum gate. This device makes use of direct barrier gates, in which individual gates are placed directly over the quantum dots and tunnel barriers. This design enables rational control of both energies and tunnel rates in coupled quantum dots. In this talk we discuss the design, fabrication, and initial characterization of the device. This work was supported in part by ARO (W911NF-12-0607), NSF (DMR-1206915), and the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S36.00008: Design considerations for multielectron double quantum dot qubits in silicon Erik Nielsen, Edwin Barnes, Jason Kestner Solid state double quantum dot (DQD) spin qubits can be created by confining two electrons to a DQD potential. We present results showing the viability and potential advantages of creating a DQD spin qubit with greater than two electrons, and which suggest that silicon devices which could realize these advantages are experimentally possible. Our analysis of a six-electron DQD uses full configuration interaction methods and shows an isolated qubit space in regimes which 3D quantum device simulations indicate are accessible experimentally. 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 6, 2014 10:00AM - 10:12AM |
S36.00009: Charge Offset Stability in Si Single Electron Devices with Al Gates M.D. Stewart, Jr., Chih-Hwan Yang, Nai Shyan Lai, Wee Han Lim, Andrew Dzurak, Neil Zimmerman The charge offset drift (time stability) is an important real-world issue in single electron devices (SEDs). For use as current standards for electrical metrology, we require time stability over long periods of time. For use as qubits, we require time stability for device integration and because, on short timescales, the charge offset drift can contribute to dephasing. Recently, workers have shown excellent qubit performance using aluminum gates on bulk Si wafers [1]. We report on the charge offset drift in these devices: the value (0.15 $e$) is intermediate between that of Al/AlO$_x$/Al tunnel junctions (greater than 1 $e$) and Si SEDs defined with Si gates (0.01 $e$). This range of values suggests that defects in the AlO$_x$ are the main cause of the charge offset drift instability. \\[4pt] [1] J. J. Pla, K. Tan, J. Dehollain,W. Lim, J. Morton, D. Jamieson, A. Dzurak, and A. Morello, Nature 489, 541 (2012). [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S36.00010: Epitaxial growth of$^{28}$Si enriched \textit{in situ} to 99.9998{\%} for quantum information Kevin Dwyer, Joshua Pomeroy, David Simons In support of quantum information devices, we epitaxially deposit \textgreater 100 nm $^{28}$Si films enriched \textit{in situ} to \textgreater 99.9998 {\%} isotope fraction at high temperature. Using our silicon enrichment ion beam deposition source, we explore electrical and structural properties of our $^{28}$Si films using \textit{in situ }reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM) and electrical measurements including capacitance--voltage profiling. Secondary ion mass spectrometry (SIMS) is used to show $^{28}$Si films have residual $^{29}$Si isotope fractions \textless 1 ppm (40 times less than previously reported $^{28}$Si sources). We also demonstrate the ability to produce isotope heterostructures with applications including $^{28}$Si/$^{28}$Si$^{74}$Ge quantum wells. $^{28}$Si is a critical material for quantum computing as removal of $^{29}$Si spins means qubits such as phosphorous atoms can have nuclear coherence (T$_{2})$ times of minutes even up to room temperature and can be addressed optically due to hyperfine transitions not normally resolvable in natural Si. Despite these advantages, $^{28}$Si is quite scarce making it clear that an alternate source such as the one we demonstrate is needed. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S36.00011: Cavity Exciton-Polariton mediated, Single-Shot Quantum Non-Demolition measurement of a Quantum Dot Electron Spin Shruti Puri, Peter McMahon, Yoshihisa Yamamoto The quantum non-demolition (QND) measurement of a single electron spin is of great importance in measurement-based quantum computing schemes. The current single-shot readout demonstrations exhibit substantial spin-flip backaction. We propose a QND readout scheme for quantum dot (QD) electron spins in Faraday geometry, which differs from previous proposals and implementations in that it relies on a novel physical mechanism: the spin-dependent Coulomb exchange interaction between a QD spin and optically-excited quantum well (QW) microcavity exciton-polaritons. The Coulomb exchange interaction causes a spin-dependent shift in the resonance energy of the polarized polaritons, thus causing the phase and intensity response of left circularly polarized light to be different to that of the right circularly polarized light. As a result the QD electron's spin can be inferred from the response to a linearly polarized probe. We show that by a careful design of the system, any spin-flip backaction can be eliminated and a QND measurement of the QD electron spin can be performed within a few 10's of nanoseconds with fidelity 99:95\%. This improves upon current optical QD spin readout techniques across multiple metrics, including fidelity, speed and scalability. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S36.00012: Simulation of electrical control of a solid-state flying qubit Seyed Mostafa Akrami Solid-state approaches to quantum information technology are attractive because they are scalable. The coherent transport of quantum information over large distances, as required for a practical quantum computer, has been demonstrated by coupling solid-state qubits to photons. However, there have been no demonstrations to date of techniques that can coherently transfer scalable qubits and perform quantum operations on them at the same time. The resulting so-called flying qubits are attractive because they allow for control over qubit separation and non-local entanglement with static gate voltages, which is a significant advantage over other solid-state qubits in confined systems for integration of quantum circuits. Here a numerical investigation has been performed over the transportation of coherent electrons. The simulation has been done in order to model a quantum point contact (QPC), tunnel-coupled wire, Aharonov-Bohm ring and finally a complete system for control of a solid-state flying qubit by means of Landauer-Buttiker formalism. The flying qubit state is defined by the presence of a travelling electron in either channel of the wire, and can be controlled without a magnetic field. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S36.00013: Dynamical spin-spin coupling of quantum dots Vahram Grigoryan, Jiang Xiao We carried out a nested Schrieffer-Wolff transformation of an Anderson two-impurity Hamiltonian to study the spin-spin coupling between two dynamical quantum dots under the influence of rotating transverse magnetic field. As a result of the rotating field, we predict a novel Ising type spin-spin coupling mechanism between quantum dots, whose strength is tunable via the magnitude of the rotating field. Due to its dynamical origin, this new coupling mechanism is qualitatively different from the all existing static couplings such as RKKY, while the strength could be comparable to the strength of the RKKY coupling. The dynamical coupling with the intristic RKKY coupling enables to construct a four level system of maximally entangled Bell states in a controllable manner. [Preview Abstract] |
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