Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V29: Quantum Control and Measurement |
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Sponsoring Units: GQI DAMOP Chair: Daniel Lidar, University of Southern California Room: C148 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V29.00001: Robust high-fidelity universal quantum gates Ran Li, Frank Gaitan We show how a robust high-fidelity universal set of quantum gates can be produced using a single form of non-adiabatic rapid passage whose parameters are optimized to enhance gate fidelity and robustness. All gates in the universal set are found to: (i) operate with fidelities in the range 0.999 --- 0.99999, and (ii) use control parameters requiring no more than 14-bit precision. Such precision is within the reach of commercially available arbitrary waveform generators, suggesting the feasibility of an experimental study of this approach to high-fidelity quantum control. [Preview Abstract] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V29.00002: Exact, Floquet-based, Single Qubit Control Andrew Sornborger, Emily Pritchett Single-qubit gate design using oscillatory controls is related to the Rabi problem of rotating a spin. In the classical solution one drives the spin with an oscillatory electromagnetic field orthogonal to a background field. Here, we introduce a new, general method for constructing continuous, oscillatory quantum controls based on Floquet's theorem. We then derive a family of exact, analytical solutions to the generalized Rabi problem of completely controlling a single-qubit in a fixed background field. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V29.00003: Protection of quantum systems by nested Uhrig dynamical decoupling Zhen-Yu Wang, Ren-Bao Liu Based on a theorem we establish on dynamical decoupling of time-dependent systems, we present a scheme of nested Uhrig dynamical decoupling (NUDD) to protect multi-qubit systems in generic quantum baths to arbitrary decoupling orders. This scheme uses only single-qubit operations. Higher order decoupling is achieved at the cost of a polynomial increase in pulse number. For general multi-level systems, this scheme protects a set of unitary Hermitian system operators which mutually either commute or anti-commute, and hence all operators in the Lie algebra generated from this set of operators, generating an effective symmetry group for the system up to a given order of precision. We also show how to implement NUDD with pulses of finite amplitude, up to an error in the second order of the pulse durations. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V29.00004: Quantum noise of an electromagnetically controlled two level system Ching-Kit Chan, L.J. Sham A coherent control of a spin is limited by both the decoherence due to coupling with the environment and noise coming from the quantized control. A quantum noise study is particularly important in fault tolerant quantum computation where a very high fidelity is demanded. Here, we present a time evolution study of a two level system interacting with a laser pulse and the electromagnetic vacuum based on the multimode Jaynes- Cummings model. We develop a diagrammatic formalism in which one can easily identify the coherent Rabi oscillation of the TLS and its relaxation from corresponding diagrams. In the small time limit ($t\ll T_1$), where the noise level is small but still an issue to fault tolerant quantum computing, this method gives a quantitative evaluation of the quantum noise of the TLS under an optical control with an arbitrary pulse shape. Furthermore, this approach can be naturally extended from the Markovian to the non-Markovian regime, resulting in dynamics different from that obtained in the optical Bloch analysis. All these calculations are done without any stochastic assumption. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V29.00005: Optimal control of an ensemble of atoms in an optical lattice Botan Khani, Seth Merkel, Jay Gambetta, Felix Motzoi, Frank K. Wilhelm Controlling quantum systems in a manner that is robust to experimental errors and inhomogeneities is vital for practical realization of quantum gates. We demonstrate numerically the control of motional degrees of freedom of an ensemble of neutral atoms in an optical lattice of shallow trapping potential. Taking into account the range of quasi-momenta across different Brillouin zones results in an ensemble whose members effectively have inhomogeneous control fields as well as spectrally distinct control Hamiltonians. We present a modified optimal control technique that yields high fidelity control pulses, irrespective of quasi-momentum, with average fidelities above 90\%. The resultant controls show a broadband spectrum with gate times in the order of several Rabi oscillations to optimize gates with up to 75\% dispersion in the energies from the band structure. [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V29.00006: High Fidelity State Transfer Over an Unmodulated Linear $XY$ Spin Chain C. Allen Bishop, Yong-Cheng Ou, Zhao-Ming Wang, Mark Byrd We provide a class of initial encodings that can be sent with a high fidelity over an unmodulated, linear, $XY$ spin chain. As an example, an average fidelity of 96\% can be obtained using an 11-spin encoding to transmit a state over a chain containing 10,000 spins. An analysis of the magnetic-field dependence is given, and conditions for field optimization are provided. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V29.00007: Geometric Phase Gates via Adiabatic Control Using Electron Spin Resonance Hua Wu, Erik Gauger, Richard George, John Morton, Mikko M\"ott\"onen High fidelity operations are essential elements of quantum information processing. In contrast with the dynamic pulses that are routinely used in electron spin resonance for spin qubit manipulation, geometric phase gates achieved via adiabatic control are less sensitive to certain kinds of noise and field inhomogeneities. Here, we employ theoretical and numerical tools to show that these geometric operations can be realized in electron spin systems with greater fidelities than composite dynamic pulses for large inhomogeneities in the microwave field. We further show that the adiabatic geometric phase is robust against fast fluctuations in the static magnetic field. Finally, we investigate adiabatic geometric phase operations experimentally, showing that we are able to apply such robust phase gates to the electron spin on the microseconds timescale. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V29.00008: Nanoscale control of individual proximal NV spins via a scanning magnetic field-gradient Michael Grinolds, Patrick Maletinsky, Sungkun Hong, Mikhail Lukin, Ronald Walsworth, Amir Yacoby Nanoscale ensembles of nitrogen-vacancy (NV) spins have been proposed for implementing quantum information protocols as well as performing sensitive nanoscale magnetometry. However, it has proven experimentally difficult to control individual NV spins without affecting the state of other, proximal spins, as spins are read-out optically and are often collectively driven by applied radio-frequency fields. We demonstrate that single-spin control in NV-spin ensembles can be achieved via a scanning magnetic field-gradient, which locally splits the electron spin resonances of proximal NVs. With this method, we achieve 9 nm spatial resolutions in imaging, characterization, and simultaneous manipulation of individual NVs, roughly two orders of magnitude better than the optical diffraction limit. We discuss applications of this individual control such as generating entangled spin-states and performing sensitive magnetometry. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V29.00009: Nested Uhrig Dynamical Decoupling with Non-uniform Error Suppression Gregory Quiroz, Daniel Lidar Here the performance of Nested Uhrig Dynamical Decoupling (NUDD) for qubit systems is analyzed when error suppression is non-uniform. The error suppression provided by NUDD is controlled by the sequence order of each nested sequence. The properties of the error suppression are characterized with respect to varying sequence order to verify the expected error suppression scaling of UDD, order $N+1$ error suppression with respect to the total time of evolution for an $N$th order sequence. The system operators present in the system-environment evolution are isolated and used to quantify the order of error suppression associated with each system error operator. Using this as a measurement, error suppression is examined with respect to the strength of system-enviroment interaction, as well as the pure bath strength. In the case of single-qubit NUDD, known as Quadratic Dynamical Decoupling (QDD), the results show that the error suppression provided by the inner sequence scales exactly with that of UDD, while the outer sequence dynamics leads to error suppression greater than or equal to that expected from UDD. These results can be extended to multi-qubit systems where the error suppression scaling for the inner sequence applied to each qubit follows that of UDD and the outer sequence applied to each qubit gives an error suppression greater than or equal to $N+1$. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V29.00010: Pulsed Quantum Optomechanics Michael R. Vanner, Igor Pikovski, Garrett D. Cole, Myungshik Kim, Caslav Brukner, Klemens Hammerer, Gerard J. Milburn, Markus Aspelmeyer By combining quantum optics with mechanical resonators an avenue is opened to extend investigations of quantum behavior into unprecendented mass regimes. The field resulting from this combination - ``cavity quantum optomechanics'' -- is receiving a surge of interest for its potential to contribute to quantum measurement and control, studies of decoherence and non-classical state preparation of macroscopic objects. However, quantum state preparation and especially quantum state reconstruction of mechanical oscillators is currently a significant challenge. We are pursuing a scheme that employs short optical pulses to realize quantum state tomography, squeezing via measurement and state purifcation of a mechanical resonator. The pulsed scheme has considerable resilience to initial thermal occupation, provides a promising means to explore the quantum nature of massive oscillators and can be applied to other systems such as trapped ions. Our theoretical proposal and experimental results will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V29.00011: Quantum measurement with Mach-Zehnder Interferometer Yunjin Choi, Justin Dressel, Andrew Jordan We use an electronic Mach-Zehnder Interferometer (MZI) as a measurement device. We perform a measurement on a system by coupling with MZI using a phase shift induced by Clulombic coupling. By reading current and noise cross-correlations, strange conditioned averages can be constructed using the contextual values technique. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V29.00012: Continuous phase amplification with a Sagnac interferometer Nathan Williams, David Starling, Ben Dixon, Andrew Jordan, John Howell We describe a weak value inspired phase amplification technique in a Sagnac interferometer. We monitor the relative phase between two paths of a slightly misaligned interferometer by measuring the average position of a split-Gaussian mode in the dark port. Although we monitor only the dark port, we show that the signal varies linearly with phase and that we can obtain similar sensitivity to balanced homodyne detection. We derive the source of the amplification both with classical wave optics and as an inverse weak value. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V29.00013: Conservation of Vacuum in an Interferometer Dominic Berry, Alexander Lvovsky Source efficiency and photon loss are major problems in optical metrology and quantum information. To understand how to address loss for these applications, it is vital to know how the loss behaves under linear optical (LO) processing including conditional measurements. We have developed a theory for the behavior of loss under LO processing, resolving many long-standing questions from previous work [1,2]. In particular, we have shown that, provided the efficiency of the sources is appropriately quantified, the efficiency of the state in any single mode cannot be increased beyond that of the highest-efficiency mode available at the input [1]. It is also not possible to increase efficiency in a catalytic way, using some high-efficiency modes to increase the efficiency of other modes [2]. The results provide a powerful unifying framework for quantifying efficiency by the incoherent vacuum contribution to optical states, even when entangled over multiple modes. The amount of vacuum is invariant under interferometers, and can only be increased by measurement. \\[4pt] [1] D.\ W.\ Berry and A.\ I.\ Lvovsky, Phys.\ Rev.Lett.\textbf{105}, 203601 (2010).\\[0pt] [2] D.\ W.\ Berry and A.\ I.\ Lvovsky, e-print:1010.6302 (2010). [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V29.00014: Dynamics of entanglement in two-dimensional spin system Qing Xu, Sabre Kais, Gehad Sadiek We consider the time evolution of entanglement in a finite two dimensional transverse Ising model. The model consists of a set of 7 localized spin-$\frac{1}{2}$ particles in a two dimensional triangular lattice coupled through exchange interaction $J$ in presence of an external time dependent magnetic field $h(t)$. The magnetic field is presented in various function forms. We find that the magnetic field with sudden change does not provide a way to control or tuning the entanglement. While for the smoothly changing field, when its the character frequency is small, entanglement tends to follow the change of external magnetic field; when it gets larger, entanglement gradually loses pace with the field. It is also shown that the mixing of even a few excited states by small thermal fluctuation is devastating to the entanglement of the ground state. [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V29.00015: Probing Majorana edge states with a qubit Chang-Yu Hou, Fabian Hassler, Johan Nilsson, Anton Akhmerov A pair of counter-propagating Majorana edge modes can be described by an Ising conformal field theory. These modes appear in a chiral p-wave superconductor or in some superconducting system belonging to the same universality class. We show how a superconducting flux qubit attached to a such system couples to the two chiral edge modes via the disorder field of the Ising model. Thus, measuring the back-action of the edge states on the qubit allows to probe the properties of Majorana edge modes. [Preview Abstract] |
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