Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session S33: Physical Implementations of Qubits |
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Sponsoring Units: GQI Chair: Alexander Korotkov, University of California, Riverside Room: Colorado Convention Center 403 |
Wednesday, March 7, 2007 2:30PM - 2:42PM |
S33.00001: Integrated Optics Approach to State Manipulation and Detection in Ion Trap Quantum Computation. Jungsang Kim, Changsoon Kim Ions trapped in RF Paul trap represent strong candidate physical system for realizing quantum information processor, evidenced by recent experimental demonstration in long coherence times and high fidelity quantum logic gate operations. The next level of progress requires a much more integrated approach to increase the number of physical qubits in the system, which will enable realization of a complete error-protected qubit. The nature of this challenge is highly technological, and advanced integration technologies can be used to dramatically increase the density of qubits handled in the system. In this paper, we discuss two classes of integrated optics technologies that can be utilized to enable the higher density manipulation of ion qubits. The first is the strategies for using optical micro-electromechanical systems (MEMS) technologies for creating miniaturized optical systems to increase the density of laser beams used to manipulate the ion states. The second is the use of micro-optical components and high efficiency detectors to realize scalable detection of ion states. We will present the estimated optical performance as well as preliminary updates on experimental progress. [Preview Abstract] |
Wednesday, March 7, 2007 2:42PM - 2:54PM |
S33.00002: Progress towards a multiplexed, semiconductor ion trap for quantum computation David Leibrandt, Robert Clark, Jaroslaw Labaziewicz, Kenneth Brown, Bernard Yurke, Richart Slusher, Isaac Chuang Moving forward from current few-qubit ion trap quantum information experiments to large-scale systems with thousands or more qubits will require multiplexed ion traps scalable to large ion density. Suitable designs have at least two problems relative to the three-dimensional, millimeter scale RF Paul traps used in most ion trap experiments: low trap depth and high heating rates. The standard loading method, electron bombardment of an atomic vapor, becomes inefficient at trap depths below about 1 eV because only the low energy tail of the ion energy distribution is captured and because nearby dielectric surfaces are charged by the electrons. We present alternative loading strategies including an experimental demonstration of loading a printed circuit board surface electrode trap using laser ablation of a metal alloy target which works below 0.5 eV. For $^{88}$Sr$^{+}$ in a particular design of multiplexed ion trap lithographically fabricated on a semiconductor substrate we predict heating rates to be of the order of $10^{3}$ s$^{-1}$ using the results of current experiments and the $d^{-4}$ scaling consistent with patch potentials. We expect a fundamentally limited heating rate of 9 s$^{-1}$ due to resistive thermal fluctuations for this trap. [Preview Abstract] |
Wednesday, March 7, 2007 2:54PM - 3:06PM |
S33.00003: High-speed linear optics quantum computing via active feed-forward Robert Prevedel, Philip Walther, Felix Tiefenbacher, Pascal Boehi, Rainer Kaltenbaek, Thomas Jennewein, Anton Zeilinger Quantum computers promise to be more efficient and powerful than their classical counterparts. In the one-way quantum computer model, a sequence of measurements processes qubits, which are initially prepared in a highly entangled cluster state. The key advantage of this scheme over the standard network approach of quantum computing is that inherent, randomly induced measurement errors can classically be fed-forward and corrected by adapting the basis of subsequent measurements. Active feed-forward is therefore crucial to achieve deterministic quantum computing once a cluster state is prepared. We have experimentally realized such a deterministic one-way quantum computation scheme by employing up to three active-switching Electro-Optical Modulators (EOM) in a four-qubit cluster state encoded into the polarization state of four photons. Using these switches we demonstrate deterministic one- and two-qubit gate operations as well as Grover's quantum search algorithm. A major advantage of optical quantum computation is the very short time for one computational step achievable by using these ultra-fast switches. With present technology this feed-forward step can be performed in less than 150 nanoseconds. [Preview Abstract] |
Wednesday, March 7, 2007 3:06PM - 3:18PM |
S33.00004: High fidelity universal quantum gates using non-adiabatic rapid passage Frank Gaitan, Ran Li Simulation results are presented which suggest that a class of non-adiabatic rapid passage sweeps known from NMR should be able to implement one-qubit Hadamard, phase, and $\pi /8$ gates as well as the two-qubit controlled-phase gate. This set of gates is known to be universal for quantum computation. For each of the gates in this set, sweep parameter values are provided which simulations indicate yield: (i) one-qubit gates that operate with gate error probability $P_{e}< 10^{-4}$; and (ii) a controlled-phase gate for which $P_{e}<2.65\times 10^{-3}$. These sweeps are non-composite and generate controllable quantum interference effects which allow the gates to operate non-adiabatically while maintaining high fidelity. The simulations suggest that the gates produced by these sweeps show promise as possible elements of a fault-tolerant scheme for quantum computing. [Preview Abstract] |
Wednesday, March 7, 2007 3:18PM - 3:30PM |
S33.00005: A general approach to two qubit gate construction for coupled-qubit models Emily Pritchett, Michael Geller We describe a practical approach for two-qubit gate construction and apply it to a general model of weakly coupled qubits. The procedure involves generating gates from a small set of primitive operations, then comparing their Makhlin invariants to those of the desired target gate. Several new CNOT implementations are found using this method. [Preview Abstract] |
Wednesday, March 7, 2007 3:30PM - 3:42PM |
S33.00006: Proposal for optical rotations of electron spin trapped in a quantum dot Sophia Economou, Lu Sham, Yanwen Wu, Duncan Steel, Thomas Reinecke The spin of an electron trapped in a semiconductor quantum dot and manipulated optically is an attractive qubit candidate, as it combines the merits of the solid state with those of laser technology. Optical rotation of the electron spin has not been experimentally demonstrated to date. In this work we propose a method for ultra fast U(1) spin rotation based on the analytical properties of the hyperbolic secant pulses. The method is tailored for systems with a dark state, such as the three-level system comprised of the two Zeeman split electronic sublevels and the trion state in GaAs quantum dots. For a system with a higher electronic g factor, such as self assembled InAs quantum dots, extra freedom arising from frequency selectivity allows us to combine these pulses with optically created dark states and design an arbitrary spin rotation through an exact solution of the three-level $\Lambda $ system. [Preview Abstract] |
Wednesday, March 7, 2007 3:42PM - 3:54PM |
S33.00007: Few Electron Quantum Dots in Si/SiGe Nakul Shaji, Christie Simmons, Levente Klein, Don Savage, Susan Coppersmith, Mark Friesen, Hua Qin, Robert Blick, Mark Eriksson Quantum information processing in silicon-based materials offers potential advantages like low spin orbit coupling and long spin coherence times. We report the fabrication and measurement of few electron quantum dots in strained Si/SiGe heterostructures. The quantum dots are formed by depleting the underlying two-dimensional electron gas using Schottky top gates. The design incorporates a capacitively coupled quantum point contact charge sensor to enable the read out of the number of electrons in the quantum dot. Low-noise measurement through the quantum dot reveals stable coulomb diamonds in the few electron regime. Interesting effects such as Kondo coupling of electron spins with the leads and Fano lineshapes for the coulomb peaks are observed in our measurements. We have investigated in detail the ground state and excited state transport spectroscopy through the quantum dots in the few electron limit at a base temperature of 20mK. In the presence of an external magnetic field (up to 4 Tesla) applied normal to the plane of electron transport we observe shifts in peak height and position enabling a discussion of the nature of these transport channels in our quantum dot. [Preview Abstract] |
Wednesday, March 7, 2007 3:54PM - 4:06PM |
S33.00008: Quantum gates between capacitively coupled double quantum dot two-spin qubits Guido Burkard, Dimitrije Stepanenko We study the two-qubit controlled-not gate operating on qubits encoded in the spin state of a pair of electrons in a double quantum dot. We assume that the electrons can tunnel between the two quantum dots encoding a single qubit, while tunneling between the quantum dots that belong to different qubits is forbidden. Therefore, the two qubits interact exclusively through the direct Coulomb repulsion of the electrons. We find that entangling two-qubit gates can be performed by the electrical biasing of quantum dots and/or tuning of the tunneling matrix elements between the quantum dots within the qubits. The entangling interaction can be controlled by tuning the bias through the resonance between the singly-occupied and doubly-occupied singlet ground states of a double quantum dot. [Preview Abstract] |
Wednesday, March 7, 2007 4:06PM - 4:18PM |
S33.00009: Single quantum dot nanowire LEDs Maarten van Kouwen, Ethan Minot, Freek Kelkensberg, Jorden van Dam, Leo Kouwenhoven, Valery Zwiller, Magnus Borgstr\"om, Olaf Wunnicke, Marcel Verheijen, Erik Bakkers Electrically-driven conversion of single electron spins into polarized photons will enable new experiments in the field of quantum information processing. We are developing nanowire light emitting diodes with the goal of combining single electron and single photon control in the same device. We will report on the reproducible fabrication of InP-InAsP nanowire LEDs in which electron-hole recombination is restricted to a quantum-dot sized InAsP section. We have investigated the operation of these nano-LEDs with a consistent series of experiments at room temperature and at 10 K, demonstrating the potential of this system for single photon applications. [Preview Abstract] |
Wednesday, March 7, 2007 4:18PM - 4:30PM |
S33.00010: Spin qubits in graphene quantum dots Bjoern Trauzettel, Denis Bulaev, Daniel Loss, Guido Burkard We propose how to form spin qubits in graphene. A crucial requirement to achieve this goal is to find quantum dot states where the usual valley degeneracy in bulk graphene is lifted. We show that this problem can be avoided in quantum dots based on ribbons of graphene with semiconducting armchair boundaries. For such a setup, we find the energies and the exact wave functions of bound states, which are required for localized qubits. Additionally, we show that spin qubits in graphene can not only be coupled between nearest neighbor quantum dots via Heisenberg exchange interaction but also over long distances. This remarkable feature is a direct consequence of the quasi-relativistic spectrum of graphene. [Preview Abstract] |
Wednesday, March 7, 2007 4:30PM - 4:42PM |
S33.00011: Charge sensing in Si/SiGe quantum dots using single electron transistors Feng Pan, Tim Gilheart, Alexander Rimberg, Lisa McGuire, Christie Simmons, Mark Eriksson, Don Savage Silicon-based solid-state qubit schemes have obvious economic appeal as well as compelling physical motivations, such as a long spin-spin dephasing time. Proposed silicon qubit schemes include quantum dots coupled to fast readout devices, such as quantum point contacts or single electron transistors (SETs). Recently, Si/SiGe quantum dots defined by Schottky gates deposited on a Si/SiGe heterostructure containing a high mobility two-dimensional electron gas have been characterized. Here we report the integration of a SET with such a Si/SiGe quantum dot. Recent measurements, including transport and sensing of the dot charge with the SET, will be discussed. [1] Slinker, K. A. et al. New J. Phys. 7 246 (2005) [2] Klein, L. J. et al. J. Appl. Phys. 99, 23509 (2006) [3] Sakr, M. R. et al. Appl. Phys. Lett. 87, 223104 (2005) [4] Berer, T. et al. Appl. Phys. Lett. 88, 162112 (2006) [Preview Abstract] |
Wednesday, March 7, 2007 4:42PM - 4:54PM |
S33.00012: Feasibility of the controlled-NOT gate from certain model Hamiltonians Mark W. Coffey, Gabriel G. Colburn There has been much interest of late in characterizing two-qubit operations, optimizing the number of quantum logic gates in small circuits, and developing minimal universal bases of quantum gates. The controlled-NOT (CNOT) gate is widely used in quantum circuits and in current and proposed quantum computing technologies. We investigate the feasibility and minimal implementation of CNOT from specific model Hamiltonian operators that have appeared in the literature. We first address the question whether certain parameterized Hamiltonians can generate a CNOT up to single-qubit gates in a definite time. If so, we determine the time for this unitary evolution. We follow an algebraic approach that provides an analytic solution. Our method has direct relevance to two-qubit Hamiltonians currently being considered for spin-based and superconductivity-based systems for quantum computing as well as to other implementations. [Preview Abstract] |
Wednesday, March 7, 2007 4:54PM - 5:06PM |
S33.00013: Randomized Benchmarking of Quantum Gates E. Knill, D. Leibfried, R. Reichle, J. Britton, R. B. Blakestad, J. D. Jost, C. Langer, R. Ozeri, S. Seidelin, D. J. Wineland A key requirement for scalable quantum computing is that quantum gates can be implemented with sufficiently low error. One method for determining the error of a gate implementation is to perform process tomography. However, this is limited by errors in state preparation, measurement and one-qubit gates. It suffers from inefficient scaling with number of qubits and does not detect adverse error-compounding. An additional problem is that experimentally proving that error probabilities are below the desirable $0.0001$ is challenging. We describe a randomized benchmarking method that yields estimates of the computationally relevant errors without relying on accurate state preparation and measurement. It also verifies that error behavior is stable when used in long computations. We implemented randomized benchmarking on trapped atomic ion qubits, establishing a one-qubit error probability per $\pi$ pulse below $.01$. [Preview Abstract] |
Wednesday, March 7, 2007 5:06PM - 5:18PM |
S33.00014: Electrical spin measurements of diffused phosphorous donors in crystalline silicon Heather Seipel, Christoph Boehme With recent experimental demonstration of the electrical detection of electron spins of phosphorous donors as well as their hyperfine coupling to the $^{31}$P phosphorous nuclear spin [Stegner et al., Nature Physics, doi:10.1038/nphys465, (2006).], a potential mechanism for a $^{31}$P in crystalline silicon (c-Si) nuclear spin readout based on spin-dependent $^{31}$P -$\mathrm{P_b}$ recombination is available. To further investigate the properties of this mechanism, we present pulsed electrically detected magnetic resonance (pEDMR) measurements on diffusion doped silicon samples. For their preparation, c-Si (111) wafers are diffused with a profile whose concentration at the surface leads to a degenerately doped c-Si before it then drops off into the semiconducting region. Deep trenches are made with a plasma enhanced reactive ion etch where the choice of the trench depth determines the dopant concentration of the sample without changing any other sample preparation parameters. A study of the qualitative and quantitative nature of the observed pEDMR signals is presented for different etch depths. [Preview Abstract] |
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