Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W27: Focus Session: Semiconductor Qubits- Optical Control, Donors, and Hybrid Systems |
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Sponsoring Units: GQI Chair: Jake Taylor, University of Maryland Room: C155 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W27.00001: Ultrafast optical entanglement control between two quantum dot spins Sam Carter, Danny Kim, Alex Greilich, Allan Bracker, Daniel Gammon A single electron spin in an InAs quantum dot is very attractive as a qubit since this system is potentially scalable and allows complete quantum control on an ultrafast timescale using optical pulses. While great progress has been achieved with single spin qubits, it is essential for quantum information applications to move toward entangled multi-qubit systems. Two-qubit systems have been studied in electrostatically-defined quantum dots, but their optical functionality remains unexplored. Here we demonstrate ultrafast optical control of two interacting qubits consisting of two electron spins in separate InAs dots. We initialize the system into a spin singlet state using a cw laser. We then manipulate the entangled state of the two spins with single qubit gates (acting only on one spin) by using pulses faster than the exchange interaction. This allows us to generate all four Bell states. Two-qubit gates are obtained either by the natural exchange precession or by using a longer laser pulse that induces a phase shift in the precession. The two-qubit exchange rate (30 GHz) here gives SWAP gate times of 16 ps, the fastest of any candidate for quantum information processing. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W27.00002: Complete ultrafast optical coherent control and spin echo of single InAs quantum dot spins Kristiaan De Greve, Peter McMahon, David Press, Thaddeus Ladd, Christian Schneider, Dirk Bisping, Martin Kamp, Lukas Worschech, Sven Hoefling, Alfred Forchel, Yoshihisa Yamamoto We report on recent progress on the complete ultrafast optical coherent control of individual InAs quantum dot spin qubits. We demonstrate Rabi-oscillations and Ramsey-fringes, and implement a spin echo to overcome time-averaged dephasing.\footnote{D. Press, K. De Greve, P. McMahon \textit{et al.}, Nat. Phot. \textbf{4}, 367 (2010)} We probe the hyperfine interaction of a single spin using optical pulse control. Interesting non-Markovian dynamics could be observed in the single electron spin free-induction decay, resulting from feedback between the strong electron spin Overhauser shift and spin dependent nuclear relaxation.\footnote{T. D. Ladd, D. Press, K. De Greve \textit{et al.}, Phys. Rev. Lett. 105, 107401 (2010)} [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W27.00003: Control of exciton relaxation channels in quantum dot molecules Kushal C. Wijesundara, Juan E. Rolon, Sergio E. Ulloa, Eric A. Stinaff, Allan Bracker, Dan Gammon We observe modulations in radiative lifetimes and intensities of the spatially indirect exciton as the InAs/GaAs coupled quantum dot system is tuned between molecular and atomic like states. With standard time-resolved single photon counting techniques the measured lifetimes were found to vary between 0.3 and 2.0 ns which resulted in modulations of the observed photoluminescence intensity of the indirect exciton. These modulations can be attributed to phonon mediated relaxations and carrier tunneling processes in good agreement with the modeled results. We clearly see the structure of the acoustic phonon distribution as shown in recent theoretical predictions. Tuning the relative energy levels in coupled quantum dots results in controllable modulation of exciton relaxation channels that may provide new directions in engineering decoherence in these systems. [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W27.00004: Charge dynamics and phonon induced oscillatory relaxation rates of indirect excitons in quantum dot molecules J.E. Rolon, K.C. Wijesundara, E.A. Stinaff, S.E. Ulloa Optoelectronic control of quantum dots is a thriving area of research with impact on fundamental physics and quantum information devices. Time-resolved photoluminescence experiments, carried out in charge tunable coupled quantum dots, have demonstrated non-monotonic behavior of neutral indirect exciton lifetimes over a wide range of applied electric fields [1]. We present a model for neutral indirect exciton lifetimes in electric field tunable quantum dot molecules. Our model includes field-dependent oscillatory phonon-induced relaxation rates [2], carrier tunneling rates, and carrier relaxation into nearby charged exciton states. To this end we have used a multi-excitonic Hamiltonian, and calculated the exciton population dynamics using a master equation with electric field dependent rates. We find that lifetime suppression is dominated by scattering with LA phonons at low fields, and that the maximum lifetime gives information on the effective dimensions of the molecule. In contrast, at high fields the lifetime suppression is dominated by the interplay of carrier population exchange with nearby charged excitons. This prompts for ways of controlling exciton lifetimes and possible decoherence in quantum dots. [1] K. C. Wijesundara et al., (unpublished), [2] J. I. Climente et al., Phys. Rev. B 74, 035313 (2006). [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W27.00005: Ultralong Coherence of Phosphorus Donors in High-Purity $^{28}$Si Silicon S.A. Lyon, A.M. Tyryshkin, S. Tojo, K.M. Itoh, J.J.L. Morton, T. Schenkel, M.L.W. Thewalt, H. Riemann, N.V. Abrosimov, P. Becker, H.-J. Pohl We report on electron spin coherence measurements for phosphorus donors in high purity, highly-enriched $^{28}$Si, with residual $^{29}$Si of less than 50 ppm. At this low $^{29}$Si density, spectral diffusion processes by nuclear spin flip-flops are suppressed, and therefore other relaxation processes become prominent. By examining a series of $^{28}$Si crystals with a donor concentration of 1$\times $10$^{14}$ to 3$\times $10$^{15}$/cm$^{3}$, we identified three decoherence mechanisms, all related to dipole interactions between donors: (1) instantaneous diffusion, caused by flips of donor spins induced by the applied microwave pulses; (2) spectral diffusion caused by T$_{1}$-induced flips of neighboring donors; (3) spectral diffusion caused by donor spin flip-flops. We demonstrate how all three mechanisms can be suppressed, leading to measured coherence times extrapolating to T$_{2}\sim $10 sec. The work was funded by DOE and LPS. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W27.00006: Electrical Control of the high spin system Mn2+ in ZnO Richard George, John Morton, Arzhang Ardavan, James Edwards We examine the high spin impurity Mn$^{2+}$ in single crystal ZnO (S=5/2, I=5/2), and report a strong linear coupling (K = 52.3 rad/V/m) of the manganese electrical and magnetic moments that preserves quantum coherence. We combine pulsed EPR and electric field techniques to manipulate the Mn states and study electron spin lifetimes, finding $T_{2e}$ and $T_{1e}$ times of 0.8ms and 100ms at 2K in the natural material. We investigate the `forbidden' transitions that become allowed in the low symmetry environment and use these to manipulate the nuclear spin state on a sub-microsecond timescale that is inaccessible via ENDOR and RF techniques. Finally, we explore the existence of subspaces that are robust against strain-induced decoherence and the application of this material as an entanglement-based field sensor. [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W27.00007: Electron spin coherence and electron nuclear double resonance of Bi donors in natural Si John Morton, Stephanie Simmons, Richard George, Wayne Witzel, H. Riemann, Nikolai Abrosimov, N. Notzel, Mike Thewalt We have shown that the electron spin coherence times of Si:Bi donors in natural silicon are limited by the same mechanism of spectral diffusion as seen in Si:P, though the smaller Bohr radius of the Bi donor leads to $\sim30\%$ longer T$_2$ times (up to 0.8~ms). We have mapped out the 36 ENDOR transitions observable at X-band arising from the $I=9/2$ nuclear spin of $^{209}$Bi, going up to 1.3~GHz. We also demonstrate the transfer of electron spin coherence to and from the $^{209}$Bi nuclear spin with a fidelity of $\sim63\%$. Using pulsed ESR at W-band (100~GHz), we observe optically-induced dynamic nuclear polarisation, consistent with the mechanism of exciton capture proposed in by T. Sekiguchi \emph{et al.}. Finally, we explore the zero-field splitting of 7.5~GHz in this system, within the context of coupling to superconducting resonators. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W27.00008: Neutral donors interacting with a two-dimensional electron gas measured by electrically detected magnetic resonance up to 94GHz C.C. Lo, J. Bokor, V. Lang, R.E. George, J.J.L. Morton, A.M. Tyryshkin, S.A. Lyon, T. Schenkel Electrically detected magnetic resonance of a silicon field-effect transistor with channel-implanted donors is measured in a W-band ($94\:$GHz, $3.36\:$T) resonant microwave cavity. It is found that the two-dimensional electron gas (2DEG) resonance signal intensity increases by two orders of magnitude compared with conventional low-field X-band ($9.7\:$GHz, $0.35\:$T) measurements. On the other hand, the neutral donor resonance signals increase by over one order of magnitude. We interpret the results in terms of direct spin-dependent scattering and a polarization transfer from the donors to the 2DEG spin system. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W27.00009: Electrical Manipulation of Spin Qubits in Li-doped Si Andre Petukhov, Luke Pendo, Erin Handberg, Vadim Smelyanskiy We propose a complete quantum computing scheme based on Li donors in Si under external biaxial stress. The qubits are encoded on the ground state Zeeman doublets and coupled via long-range spin-spin interaction mediated by acoustic phonons. This interaction is unique for Li donors in Si due to their inverted electronic structure. Our scheme takes advantage of the fact that the energy level spacing in $1s$ Li-donor manifold is comparable with the magnitude of the spin-orbit interaction. As a result the Li spin qubits can be placed 100 nm apart and manipulated by a combination of external electric field and microwave field impulses. We present a specially-designed sequence of the electric field impulses which allows for a typical time of a two-qubit gate $\sim$~1~$\mu$s and a quality factor $\sim 10^{-6}$. These estimates are derived from detailed microscopic calculations of the quadratic Stark effect and electron-phonon decoherence times. [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W27.00010: Entanglement in a Solid State Spin Ensemble Stephanie Simmons, Richard Brown, Helge Riemann, Nikolai Abrosimov, Peter Becker, Hans-Joachim Pohl, Mike Thewalt, Kohei Itoh, John Morton Entanglement is a both a fascinating phenomenon and a critical ingredient in most emerging quantum technologies. Spin ensembles manipulated using magnetic resonance have demonstrated the most advanced quantum algorithms to date, however these studies contain no entanglement and hence constitute classical simulations of quantum algorithms. Here we report the on-demand generation of entanglement between an ensemble of electron and nuclear spins in isotopically engineered phosphorus-doped silicon. High field/low temperature electron spin resonance (3.4~T, 2.9~K) was used in conjunction with a hyperpolarisation sequence to reduce the spin entropy to a level sufficient to form an inseparable state. The generated entanglement was confirmed by measuring the state's density matrix which displayed a fidelity of 98\% compared to the ideal state at this field and temperature. The entanglement operation was performed simultaneously, with high fidelity, to $10^{10}$ spin pairs, and represents an essential requirement of a silicon- based quantum information processor. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W27.00011: Optimized Electron-spin-cavity coupling in a double quantum dot Xuedong Hu, Yu-xi Liu, Franco Nori We search for the optimal regime to couple an electron spin in a semiconductor double quantum dot to a superconducting stripline resonator via the electrically driven spin resonance technique. In particular, we calculate the spin relaxation rate in the regime when spin-photon coupling is strong, so that we can identify system parameters that allow the electron spin to reach the strong coupling limit. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W27.00012: Near Field Photon Emission and Revival in Quantum Dot Qubits S. Tafur, M.N. Leuenberger Modeling the spontaneous emission of photons coupled to the electronic states of quantum dots is important for understanding quantum interactions and entanglement in condensed matter as applied to proposed solid-state quantum computers, quantum networks, single photon emitters, and single photon detectors. A quantum dot initially in an excited state can be experimentally observed to decay to its ground state and the observed homodyne tomography of the emitted photon can yield information about the qubit state of the emitter. Though the characteristic lifetime of photon emission is traditionally modeled via the Weisskopf-Wigner approximation, we seek to model the fully quantized spontaneous emission, including near field effects, of a photon from the excited state of a quantum dot beyond the Markovian limit. We further investigate subsequent interactions between the emitted photon and adjacent quantum dots in an effort to describe multipartite entanglement. We propose the use of discretized central-difference approximations of space and time partial derivatives, similar to finite-difference time domain models, to describe single photon states via single photon operators. Additionally, within the future scope of this model, we seek results in the Purcell and Rabi regimes for spontaneous emission events from quantum dots embedded in micro-cavities. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W27.00013: A Theoretical Model of Single Photon Source at Room Temperature Ahmed Elhalawany, Michael Leuenberger In this work we present a theoretical model for an electrically injected single photon source at room temperature. The source is made of three regions. The region containing the source is n-doped, the middle region is an intrinsic semiconductor heterostructure. The region containing the drain is p-doped. The configuration of the intrinsic region is designed to trap a single pair of electron and hole; this is due to Pauli Exclusion Principle and Coulomb blockage. This is achieved by applying a reverse voltage to neutralize the intrinsic electric field between the n- and p-doped regions. Based on the calculated tunneling time of the electron/hole, the reverse voltage will be switched o. For the kinetics at the room temperature operation is calculated by means of the Master equations. For this we use an effective Hamiltonian in the tight-binding approximation. The results show that a single electron and a single hole are trapped simultaneously for an adequate period of time until they recombine. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W27.00014: Phonons and solid-state qubits for quantum technology \"{O}.O. Soykal, Rusko Ruskov, Charles Tahan Phonons in the context of quantum information processing are traditionally negatives. They induce relaxation or decoherence of or between qubit states. Learning to control phonons for positive purposes, both as supporting technology for quantum information processing, and for other quantum devices is of great possible interest. Already, acoustic waves are used as a supporting technology in microelectronics and optoelectronics (e.g. their slow speed can be useful in certain contexts). Here we consider some methods for making phonons useful and describe the physics of such systems in several potential solid-state systems including silicon. Our results may also be of interest to the optomechanics community. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W27.00015: Towards electrons floating over diamond M.P. Ray, J.W. Baldwin, M.K. Zalalutdinov, J.L. Shaw, J.E. Butler, B.B. Pate, T.I. Feygelson The opportunities for development of a 2D electron system of image potential surface electrons over negative electron affinity diamond are examined. Image potential surface electron states, located spatially outside the solid, are well established on a variety of surfaces (metals, semiconductors and dielectrics). In particular, laterally confined electrons above liquid helium have been demonstrated and proposed for advanced computing applications [1,2]. Unlike the surface of liquid helium, the electron affinity of the diamond surface can be varied [3], providing the ability to lithographically pattern surface electron `pools' and `wires'. We present candidate structures for lateral charge control that make use of buried and surface features patterned in and on diamond. Electronic properties and spectroscopy of electrons over diamond in our fabricated structures are discussed. \\[4pt] [1] S. A. Lyon, Phys. Rev. A \textbf{74}, 052338 (2006).\\[0pt][2] P. M. Platzman and M. I. Dykman, Science \textbf{284}, 1967(1999).\\[0pt][3] J. Ristein, Surf. Sci. \textbf{600}, 3677 (2006). [Preview Abstract] |
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