Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H17: Focus Session: Semiconducting Qubit Approaches |
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Sponsoring Units: GQI Chair: Jeremy Levy, University of Pittsburgh Room: 318 |
Tuesday, March 17, 2009 8:00AM - 8:36AM |
H17.00001: Solid state quantum memory using 31P spins in silicon: advantages of hybrid qubit systems Invited Speaker: |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H17.00002: Radio-frequency single-electron transistor coupled to a few-electron double quantum dot Feng Pan, Joel Sttentheim, Mustafa Bal, Mingyun Yuan, Alex Rimberg, Vladimir Umansky The radio frequency single-electron transistor (rf-SET) has been shown to be an ultra fast and highly sensitive electrometer, and can be potentially operated close to the quantum noise limit as a qubit readout device [1]. The interplay between the rf-SET electrometer and a two-level system offers an interesting system for study. Here we report our progress on investigating rf-SETs capacitively coupled to few-electron double quantum dots (DQDs). We fabricate lateral-defined DQDs from an AlGaAs/GaAs heterostructure and the rf-SET from superconducting aluminum embedded in a tank circuit. The sensitivity and bandwidth of on-chip rf-SET electrometer can be used to probe DQD operated in the few-electron regime. We have observed coupling between SET and DQD and have optimized our device design to enhance coupling in the few-electron limit. Recent experimental results will be discussed. [1] M. H. Devoret and R. J. Schoelkopf, Nature, \textbf{406}, 1039 (2000). [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H17.00003: 3D Acoustic Modes, Shot Noise and Strain Displacements in a Radio Frequency Quantum Point Contact J. Stettenheim, M. Thalakulam, F. Pan, M. Bal, L.N. Pfeiffer, K.W. West, A.J. Rimberg As previously reported, our broadband frequency resolved measurements of shot noise in a radio frequency QPC (RF-QPC) reveal a remarkable frequency dependence absent from theoretical predictions. Based on piezoelectric coupling in GaAs, our data suggest a feedback loop in which shot noise drives resonant acoustic vibrations that in turn create correlations in electron tunneling. The feedback concentrates the initially white noise in a narrow band around the sample's resonant frequency, allowing shot noise spectrum engineering. We solve for the 3D acoustic modes of our samples, finding close correspondence with measured frequencies. We have determined that the geometry and magnitude of the polarization field selects the acoustic mode excited. As polarization fields and strain displacements are linked in GaAs, we estimate the ultimate mechanical displacement sensitivity of our RF-QPC. [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H17.00004: Radio-frequency quantum point contact in a silicon/silicon-germanium two-dimensional electron system Madhu Thalakulam, Christie Simmons, Eric Sackmann, Bjorn Van Bael, D.E. Savage, Max Lagally, M.A. Eriksson Radio frequency quantum point contacts (RF{\_}QPC) are sensitive and fast electrometers. The capability to integrate such devices with semiconductor-based quantum dot systems makes them an attractive candidate for fast charge readout, and the increasing interest in spins in group-IV quantum dots motivates the development of such devices in silicon/silicon-germanium two-dimensional electron systems. We report the operation of an RF-QPC fabricated on a silicon/silicon-germanium heterostructure with an on-chip matching network. An on-chip spiral inductor and the capacitance from the bonding pads define the tank circuit. The inductance and capacitance parameters are optimized to achieve a resonant frequency of approximately one GHz. The operation of RF-QPC at milli-Kelvin temperature and the charge readout of a quantum dot system using an RF-QPC will be discussed. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H17.00005: Observation of tunnel rates of phosphorus dopants using silicon SETs H. Huebl, C.D. Nugroho, A. Morello, C. Escott, A.S. Dzurak, R.G. Clark, C. Yang, J.V. Donkelaar, A. Alves, D. Jamieson, M.A. Eriksson Charge centres, such as donors in semiconductors, have significant potential for quantum information processing. In silicon, which can be produced nuclear-spin free, phosphorus donors are a prime candidate for implementation of a qubit, due to their long spin coherence times. In this presentation we will discuss a hybrid structure, consisting of implanted phosphorus donors controlled by a gate potential in close vicinity to a gate-induced, MOS-based silicon single electron transistor (Si-SET). We study the dual functionality of the nearby Si-SET as a sensitive charge detector as well as a gate-induced electron reservoir. Experimentally, we observe shifts in the position of the Coulomb peaks of the Si-SET corresponding to $\sim$20\% of an electron charge. We attribute these shifts to charge transfers between the Si-SET island reservoir and the nearby phosphorus donors. Pulsed voltage spectroscopy on one of these charge transitions allows us to investigate the capture and emission times of a donor resulting in a capture rate of 3000 s-1 and an emission rate of 1000 s-1 corroborating expectations from device modelling. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H17.00006: Self-Correcting Dynamic Decoupling Pulse Sequences Alexei M. Tyryshkin, Stephen A. Lyon, Wenxian Zhang, Viatcheslav V. Dobrovitski Dynamic decoupling (DD) techniques employ a series of strong refocusing pulses to combat decoherence in quantum systems. However, each DD pulse is imperfect and thus introduces a small instrumental error; the error can accumulate rapidly upon applying many DD pulses, and this error can destroy the quantum state. We have examined several DD pulse sequences, including CPMG, XZXZ, XYXY and their concatenated variants, using electron spin resonance (ESR) of donors electron spins in silicon. While all these DD sequences performed comparably in cancelling the phase noise arising from magnetic field fluctuations, only one sequence (XYXY) demonstrated the ability to protect an arbitrary coherent state, including X, Y, and Z states in the rotating frame. The other sequences (CPMG and XZXZ) were able to store only one state (Y) while destroying other states (X and Z). The superior performance of XYXY arises from its internal ability to correct for pulse amplitude errors, the dominant error in these ESR experiments. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H17.00007: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H17.00008: Hyperfine tolerant triplet-singlet qubit in rotating double quantum dots David Drummond, Leonid P. Pryadko, Kirill Shtengel We examine the triplet-singlet state of pairs of GaAs single electron quantum dots, in which the singlet and m=0 triplet represent the logical 0 and 1. Each electron's spin feels a different magnetic field from the hyperfine coupling to the local nuclei of the GaAs, leading to a relative phase difference between the two electrons, and thus decoherence. Previous methods have delayed decoherence using complicated electrical pulse schemes or nuclear polarization with Overhauser fields, which can only be maintained for short times and require great energy. Instead we propose to eliminate hyperfine decoherence by repeatedly rotating the dots adiabatically, exposing the electrons to the same average magnetic field. In addition, we show that if these rotations are performed repeatedly in a symmetric fashion, the Dresselhaus and Rashba spin-orbit couplings are significantly suppressed. The construction of such a device might also be of interest to topological quantum computers, which depend on similar rotations. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H17.00009: Multiple Nuclear Polarization States in a Double Quantum Dot Jeroen Danon In a double quantum dot under conditions of electron paramagnetic resonance we have observed multiple stable states of nuclear polarization and also switching between those states. The system exhibited strong hysteretic behavior over a large range of magnetic fields, indicating the dynamical buildup of effective nuclear magnetic fields up to 150 mT. We have explained these findings in the framework of an elaborated theoretical model. The results reported enable applications of this nuclear polarization effect, including manipulation and control of the nuclear fields and possible use of this for improving the electron spin coherence time. [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H17.00010: Electron spin dephasing by hyperfine-mediated interactions in a nuclear spin bath Lukasz Cywinski, Wayne M. Witzel, Sankar Das Sarma We investigate pure dephasing decoherence (free induction decay and spin echo) of a quantum dot spin qubit interacting with a nuclear spin bath. While for infinite magnetic field $B$ the only decoherence mechanism is spectral diffusion due to dipolar flip-flops of nuclear spins, with decreasing $B$ the hyperfine-mediated interactions between the nuclear spins become important. We give a theory [1] of decoherence due to these interactions which takes advantage of their long range nature. For a thermal uncorrelated bath we show that our theory is applicable down to $B\sim$10 mT, allowing for comparison with recent experiments on spin echo in GaAs quantum dots [2].\\ {[1]} L. Cywinski, W.M. Witzel, and S. Das Sarma, preprint arXiv:0809:0003 (2008).\\ {[2]} F.H.L. Koppens, K.C. Nowack, and L.M.K. Vandersypen, Phys. Rev. Lett. \textbf{100}, 236802 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H17.00011: Nuclear State Preparation via Landau-Zener-St\"uckelberg transitions in Double Quantum Dots Hugo Ribeiro, Guido Burkard We theoretically model a nuclear-state preparation scheme that increases the coherence time of a two-spin qubit in a double quantum dot. The two-electron system is tuned repeatedly across a singlet-triplet level-anticrossing with alternating slow and rapid sweeps of an external bias voltage. Using a Landau-Zener-St\"uckelberg model, we find that in addition to a small nuclear polarization that weakly affects the electron spin coherence, the slow sweeps are only partially adiabatic and lead to a weak nuclear spin measurement and a nuclear-state narrowing which prolongs the electron spin coherence. Based on our description of the weak measurement, we simulate a system with up to n=200 nuclear spins per dot and qualitatively explain recent experimental findings. Scaling in n indicates a stronger effect for larger n, also in qualitative agreement with experiments. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H17.00012: Electromagnetic fluctuations as a source of decoherence for double quantum dot charge-based qubits Diego Valente, Frank Wilhelm, Eduardo Mucciolo Solid-state quantum dots are strong candidates for the physical realization of qubits. They present the ubiquitous advantage of easier scalability, but also couple rather effectively to external degrees of freedom which lead to decoherence phenomena. One such source of decoherence lays in the electromagnetic fluctuations occurring in the circuits utilized for preparation and measurement of these qubits. Here we investigate these sources of decoherence in double quantum dot charge-based qubit systems. We use effective circuit models and estimates of time correlations of such fluctuations to calculate the energy ($T_1$) and phase ($T_2$) relaxation times introduced into the qubit system. We also present ideas on how to suppress some of the destructive effects of these fluctuations and increase the quality factor for quantum oscillations. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H17.00013: Probing the spin structure of $\nu=2$ quantum Hall fluid around an antidot Lee Bassett, Chris Ford, Nigel Cooper, Jonathan Griffiths, David Anderson, Ian Farrer, Geb Jones, Dave Ritchie We experimentally investigate spin and charge excitations in a small closed edge of integer quantum Hall fluid encircling a nano-scale potential island, or antidot (AD), in a two dimensional electron system. Using quantum point contacts to inject and detect spin-polarized currents via edge states, we have measured spin-resolved transport through single ADs at filling factor two. At relatively low magnetic fields ($\approx 1$T), tunneling between the AD states and higher Landau levels in the bulk produces pairs of Coulomb blockade peaks in conductance above the $2e^2/h$ plateau. These transmission resonances were thought to result from spin-polarized tunneling through individual single-particle AD states (of alternating spin), but our experiments show that, while spin is generally conserved during transport, the tunneling current is not spin-polarized. We interpret these results as signatures of interactions within the AD which result in a separation of the energy scales associated with spin and charge excitations. [Preview Abstract] |
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