Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P17: Semiconducting Qubits I |
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Sponsoring Units: GQI Chair: Christie Simmons, University of Wisconsin Room: 318 |
Wednesday, March 18, 2009 8:00AM - 8:12AM |
P17.00001: Practical elimination of leakage in superconducting qubits by pulse shaping Felix Motzoi, Jay Gambetta, Patrick Rebentrost, Frank Wilhelm In trying to develop high-fidelity control of superconducting and optical lattice devices, many techniques have been borrowed from the NMR literature, such as shaped, computer-generated, and composite pulses. However, unwanted coupling to higher energy levels cause simple spin state control to fail. We have shown that we can effectively remove any coupling to the third level in specifically phase and transmon qubits by adding a second control proportional to the derivative of the first along a rotation axis perpendicular to that of the first control. The 2-control strategy gives implementations with basic pulse-shaping as small as 4 pixels. Using realistic values of decoherence(5 $\mu$s), we find errors as small as $10^{-4}$ for such a 4ns pulse, about an order of magnitude better than using one quadrature. An easy to implement analytic formula can also be applied that handily improves on any existing single-control analog or pixelated pulse. These results demonstrate that experimental calibration and decoherence effects are the limiting factors in achieving high-fidelity quantum gates, and the focus of attention should be on these rather than on increasing the anharmonicity. [Preview Abstract] |
Wednesday, March 18, 2009 8:12AM - 8:24AM |
P17.00002: Quantum Sensing in the Presence of Realistic Attenuation Yaakov Weinstein, Gerald Gilbert Quantum entangled states can be used to beat the standard quantum limit on the variance of a measurement and to beat the Rayleigh limit on resolution. These phenomena are known as supersensitivity and superresolution, respectively. We demonstrate that photonic implementation of either supersensitivity or superresolution will not be successful in the presence of realistic attenuating atmospheres. This is true even when superresolution is attempted with unentangled photons. [Preview Abstract] |
Wednesday, March 18, 2009 8:24AM - 8:36AM |
P17.00003: Free-Time and Fixed End-Point Optimal Control Theory in Quantum Mechanics: Application to Entanglement Generation Kenji Mishima, Koichi Yamashita We have constructed \textit{free-time} and fixed end-point optimal control theory for quantum systems and applied it to entanglement generation between rotational modes of two polar molecules coupled by dipole-dipole interaction. The motivation of the present work is to solve optimal control problems more flexibly by extending the popular \textit{fixed-time} and fixed end-point optimal control theory for quantum systems to \textit{free-time} and fixed end-point optimal control theory. Our theory can not only achieve high transition probabilities but also determine exact temporal duration of the laser pulses. As a demonstration, our theory is applied to entanglement generation in rotational modes of NaCl-NaBr polar molecular systems that are sensitive to the strength of entangling interactions. Using the tailored laser pulses, we discuss the fidelity of entanglement distillation and quantum teleportation. Our method will significantly be useful for the quantum control of non-local interaction such as entangling interaction, and other time-sensitive general quantum dynamics, chemical reactions. [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 8:48AM |
P17.00004: Quantum multiobservable control Raj Chakrabarti, Rebing Wu, Herschel Rabitz We present deterministic algorithms for the simultaneous control of an arbitrary number of quantum observables. Unlike optimal control approaches based on cost function optimization, quantum multiobservable tracking control (MOTC) is capable of tracking predetermined homotopic trajectories to target expectation values in the space of multiobservables. The convergence of these algorithms is facilitated by the favorable critical topology of quantum control landscapes. Fundamental properties of quantum multiobservable control landscapes that underlie the efficiency of MOTC, including the multiobservable controllability Gramian, are introduced. The effects of multiple control objectives on the structure and complexity of optimal fields are examined. With minor modifications, the techniques described herein can be applied to general quantum multiobjective control problems. [Preview Abstract] |
Wednesday, March 18, 2009 8:48AM - 9:00AM |
P17.00005: Quantum speed limit and optimal control Tommaso Caneva, Michael Murphy, Tommaso Calarco, Rosario Fazio, Simone Montangero, Vittorio Giovannetti, Giuseppe E. Santoro The Heisenberg uncertainty principle, $\Delta E\Delta t\geq \hbar$, implies that a system cannot pass through distinguishable, i.e. orthogonal, states within arbitrarily short time. In the case of a time-independent Hamiltonian, the presence of this ultimate bound has been well established and summarized in the concept of a maximum allowed velocity, called \emph{quantum speed limit} (QSL). On other hand for a time-dependent Hamiltonian the problem started to be addressed only very recently and is still open. Optimal control theory offers a valuable tool to explore this issue: we test its performance in two paradigmatic cases, Landau-Zener model and transfer of information along a chain of coupled spins, and show that the results are compatible with the ultimate limits enabled by quantum mechanics. [Preview Abstract] |
Wednesday, March 18, 2009 9:00AM - 9:12AM |
P17.00006: Performance Gains for Superconducting Qubits by Means of Optimal Control Theory Robert Roloff, Walter Poetz Superconducting circuits are promising candidates for the successful implementation of qubit--arrays and qubit--gates within solid--state systems. However, despite recent progress within coherent control of charge, phase and flux qubits, considerable improvement in gate fidelities is needed to build large--scale quantum information processing devices. We present an optimal control scheme based on process tomography, capable of taking into account relaxation, dephasing and unwanted state--leakage within the qubit (array). We apply this theory to explore the performance limits of Josephson charge qubits within current experimental means. Environmental effects, as well as state--leakage, are modeled microscopically, using a full quantum mechanical description and taking into account 1/f and Ohmic fluctuations based on experimental noise spectra. Within time--optimal control theory, we show that under typical conditions gate fidelities of $F=1-10^{-3}$ should be possible for a Josephson charge qubit. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:24AM |
P17.00007: Towards non-adiabatic control of a superconducting qubit Jonas Bylander$^1$, Mark S. Rudner$^1$, Andrey V. Shytov$^2$, Sergio O. Valenzuela$^1$, David M. Berns$^1$, Karl K. Berggren$^1$, Leonid S. Levitov$^1$, William D. Oliver$^1$ Transitions in a qubit driven through an energy-level avoided crossing can be controlled by carefully engineering the driving protocol. With the driving rate chosen to optimize the coupling strength, an arbitrary rotation of a qubit's quantum state on the Bloch sphere can be performed. This regime, if realized experimentally, may lead to fast quantum-logic gates with times of operation much shorter than those achieved by using Rabi transition-based protocols. We have performed an experiment with a superconducting persistent-current qubit in the non- adiabatic regime, driven by a large-amplitude radio-frequency field. By applying a waveform consisting of two harmonic components generated by a digital source, we demonstrate a mapping between the amplitude and phase of the harmonics produced at the source and those received by the device. This mapping allows us to image the actual waveform at the device and accurately produce the desired time dependence. Our method constitutes a step towards non-adiabatic control with arbitrary waveforms. [Preview Abstract] |
Wednesday, March 18, 2009 9:24AM - 9:36AM |
P17.00008: Universal quantum control of two electron spin qubits via dynamic nuclear polarization Hendrik Bluhm, Sandra Foletti, Diana Mahalu, Vladimir Umansky, Amir Yacoby Encoding a single logical qubit in the collective spin states of two electrons in a double quantum dot can provide sub-nanosecond electrically controlled gates that are fast enough to refocus dephasing due to slow fluctuations of the hyperfine field from the nuclei of the host material [1]. In this work, we experimentally demonstrate full quantum control of a GaAs two electron logical spin qubit. One fast electrical control axis resulting from coherently exchanging the two electrons has already been demonstrated [2]. We achieve coherent evolution around a second axis caused by a difference in the nuclear hyperfine fields felt by the two electrons. This field difference is obtained by dynamically polarizing the Ga and As nuclei by transferring spin from the electrons to the nuclei. It can reach up to several hundred mT and can be maintained in a steady state. We demonstrate rotations around this axis with a programmable frequency that can exceed 1 GHz. Using quantum state tomography enabled by both control axes, we characterize the evolution of the qubit state around a fixed but tunable combined axis. Our results establish full electrical quantum control at the single qubit level with gate times of a few nanoseconds. [1] Taylor et al., Nature Physics \bf{1}, 177 (2005). [2] Petta et al. Science \bf{309}, 2180 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 9:36AM - 9:48AM |
P17.00009: Control of exchange coupling in Si double quantum dots Dimitrie Culcer, L. Cywinski, Qiuzi Li, Sankar Das Sarma We determine the exchange coupling in a Si double quantum dot in the Heitler-London approximation. Qubit manipulation in bulk Si is hindered by the sixfold valley degeneracy of conduction band electrons which causes the exchange interaction between qubits to oscillate as a function of their separation. We demonstrate that in quantum dots these oscillations are suppressed by quantum confinement. We determine the dependence of the exchange coupling on the barrier potential between the dots and examine the role of charge fluctuations. Our results suggest that together with long Si spin lifetimes Si quantum dots could lead to improved control of spin qubits. Within the Heitler-London approximation the work presented is completely general and the results are valid for any ground state. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:00AM |
P17.00010: Optimal experiment design for parameter estimation as applied to dipole- and exchange-coupled qubits Kevin Young, Mohan Sarovar, Birgitta Whaley, Robert Kosut We consider the problem of quantum parameter estimation with the constraint that all measurements and initial states are separable. Two qubits are presumed coupled through the dipole and exchange interactions. The resulting Hamiltonian generates a unitary evolution which, when combined with arbitrary single-qubit operations,contributes to a universal set of quantum gates. However, while the functional form of the Hamiltonian is known, a particular experimental realization depends on several free parameters - in this case, the position vector relating the two qubits and the magnitude of the exchange interaction. We use the Cramer-Rao bound on the variance of a point estimator to construct the optimal series of experiments to estimate these free parameters. Our method of transforming the constrained optimal estimation problem into a convex optimization is powerful and widely applicable to other systems. [Preview Abstract] |
Wednesday, March 18, 2009 10:00AM - 10:12AM |
P17.00011: Measurement of the nonadiabatically-induced coherent time evolution of a single-electron wavefunction in a surface acoustic wave dynamic quantum dot Adam Thorn, Masaya Kataoka, Michael Astley, Daniel Oi, Crispin Barnes, Chris Ford, Dave Anderson, Geb Jones, Ian Farrer, Dave Ritchie, Michael Pepper Observation of coherent single-electron dynamics is severely limited by experimental bandwidth. We present a method to overcome this using moving quantum dots defined by surface acoustic waves. Each dot holds a single electron, and travels through a static potential landscape. When the dot moves abruptly between regions of different confinement, the electron is excited into a superposition of states, and oscillates unitarily from side to side. These oscillations are measured almost non-invasively, by allowing a small amount of tunnelling out of the dot each time the wavefunction approaches a tunnel barrier. We have modelled this in detail by solving the single-particle time-dependent Schr\"odinger equation for a realistic potential, and find good agreement between the measurements and the simulations. [Preview Abstract] |
Wednesday, March 18, 2009 10:12AM - 10:24AM |
P17.00012: Weak measurement of a solid-state qubit revealed in low-frequency noise Alexander Korotkov Weak quantum measurement becomes a subject of experimental study with solid-state qubits. Partial collapse, quantum uncollapsing, and persistent Rabi oscillations have been already demonstrated with superconducting qubits by the UCSB and Saclay groups. Now we propose an experiment, in which the features of a weak quantum measurement are revealed in the low- frequency noise of the detector signal. (Here we mean a frequency much lower than the Rabi frequency, though sufficiently high to avoid 1/f noise.) The idea is to use two detectors measuring the same qubit, so that one detector collapses the qubit, while the other detector senses the result of the collapse. Then the cross-correlation of low-frequency noises in outputs of the two detectors carries information about the collapse process. The experiment can be realized with superconducting or semiconductor qubits. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 10:36AM |
P17.00013: Error Accounting in Electron Counting Experiments Michael Wulf, Alexander B. Zorin Electron counting experiments attempt to provide a current of a known number of electrons per unit time. We propose architectures utilizing a few readily available electron-pumps or turnstiles with error rates of 1 part in $10^4$ with common sensitive electrometers to achieve the desireable accuracy of 1 part in $10^8$. This is achieved not by counting electrons but by counting the errors of individual devices; these are less frequent and therefore readily recognized and then accounted for. We thereby ease the route towards quantum based standards of current and capacitance. [Preview Abstract] |
Wednesday, March 18, 2009 10:36AM - 10:48AM |
P17.00014: High-fidelity universal quantum gates through quantum interference Frank Gaitan, Ran Li Numerical simulation results are presented which suggest that a class of non-adiabatic rapid passage sweeps first realized experimentally in 1991, and which give rise to controllable quantum interference effects observed in 2003 using NMR, should be capable of implementing a universal set of quantum gates $\mathcal{G}$ that operate with high-fidelity. $\mathcal{G}$ consists of the Hadamard and NOT gates, together with variants of the phase, $\pi /8$, and controlled-phase gates. Sweep parameter values are provided which simulations indicate will produce the different gates in $\mathcal{G}$, and for each gate, yield an error probability $P_{e} < 10^{-4}$. These simulations suggest that the universal gate set produced by these rapid passage sweeps show promise as possible elements of a fault-tolerant scheme for quantum computing. We discuss current challenges facing experimental implementation of this approach to universal quantum computing. [Preview Abstract] |
Wednesday, March 18, 2009 10:48AM - 11:00AM |
P17.00015: Dynamics of Quantum Control for Bosons in Optical Lattices Analabha Roy, Linda Reichl We investigate the possibility of quantum control in an ultracold atom Bose gas in an optical lattice by looking at numerical simulations of the dynamics of controlled excitations in these systems. These excitations are mediated by pulsed signals that cause Stimulated Raman Adiabatic passage (STIRAP) from the ground state to excited states. The transition to chaos affects the quantum dynamics of such systems as has been demonstrated for single-particle and mesoscopic-systems in optical potentials. We determine the influence of Bose statistics on this dynamics, as well as the effects of controlling quantum phase transitions in this manner for interacting cold atom systems. [Preview Abstract] |
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