Southwest Quantum Information and Technology

Fourteenth Annual Meeting, February 16-19, 2012
Albuquerque, New Mexico

Full Program | Thursday | Friday | Saturday | Sunday | All Sessions

SESSION 11a: Breakout A - Quantum Control and Measurement - Alvarado "A"
Session Chair:
4:15pm-4:45pm	Constantin Brif, Sandia National Laboratories
	Improving robustness of quantum gates to control noise
	Abstract. External controls are necessary to enact quantum logic operations, and the inevitable control noise will result in gate errors in a realistic quantum circuit. We investigate the robustness of quantum gates to random noise in an optimal control field, by utilizing properties of the quantum control landscape that relates the physical objective (in the present case, the quantum gate fidelity) to the applied controls. An approximate result obtained for the statistical expectation value of the gate fidelity in the weak noise regime is shown to be in excellent agreement with direct Monte Carlo sampling over noise process realizations for fidelity values relevant for practical quantum information processing. Using this approximate result, we demonstrate that maximizing the robustness to additive/multiplicative white noise is equivalent to minimizing the total control time/fluence. Also, a genetic optimization algorithm is used to identify controls with improved robustness to colored noise.
4:45pm-5:15pm	Thaddeus Ladd, HRL Laboratories, LLC
	Coherent Control of Si-based Qubits
	Abstract. Electrically defined silicon-based qubits are expected to show improved quantum memory characteristics in comparison to GaAs-based devices due to reduced hyperfine interactions with nuclear spins. Silicon-based qubit devices have proved more challenging to build than their GaAs-based counterparts, but recently several groups have reported substantial progress in single-qubit initialization, measurement, and coherent operation. I will present the recent observation of coherent oscillations in a spin singlet-triplet device built in a Si/SiGe heterostructure, and a measurement confirming that the dephasing time T2* is nearly two orders of magnitude longer than in comparable GaAs devices due to reduced hyperfine effects. Although complete SU(2) control is not yet demonstrated, fully controllable qubits may be enabled using exchange-only operations in Decoherence Free Subsystems (DFS). I will discuss some new control optimizations of the DFS system. Sponsored by the United States Department of Defense. Approved for public release, distribution unlimited.
5:15pm-5:45pm	Robert Cook, Center for Quantum Information and Control (CQuIC) and Department of Physics and Astronomy, University of New Mexico
	Single shot quantum state estimation via continuous measurement in a strong back-action regime
	Abstract. Quantum state reconstruction is a fundamental task in quantum information science. The standard approach employs many projective measurements on a series of identically prepared systems in order to collect sufficient statistics of an informationally complete set of observables. An alternative procedure is to reconstruct quantum state by performing weak continuous measurement collectively on an ensemble, while simultaneously applying time varying controls[1,2]. For known dynamics, the measurement history determines the initial state. In current implementations the shot noise of the probe dominates over projection noise so that measurement-induced backaction is negligible. We generalize this to the regime where quantum backaction can play a significant role, even for small numbers of particles. Using the framework of quantum filtering theory, we model the reconstruction of the state of a qubit through collective spin measurement via the Faraday interaction and magnetic field controls, and develop a maximum-likelihood estimate. [1] A. Silberfarb and I. H. Deutsch, Phys. Rev. Lett. 95, 030402 (2005). [2] C.A. Riofrio et. al., J. Phys. B: At. Mol. Opt. Phys. 44, 154007 (2011)
5:45pm-6:15pm	Olivier Landon-Cardinal, Université de Sherbrooke
	Learning in a variational class of states : efficient tomography method for Matrix Product and Multi Scale Entangled states
	Abstract. Quantum state tomography is essential to benchmark quantum devices but standard techniques fundamentally require a number of experiments and a post-processing effort that scales exponentially with the number of particles. However, by taking advantage of efficient representations of quantum states, such as matrix product states (MPS) or multi-scale entanglement renormalisation ansatz (MERA), we can do exponentially better. Indeed, since those variational classes of states are described by a few parameters, identifying the state amounts to learning those parameters. We describe a method for reconstructing a wide range of states from a small number of efficiently-implementable measurements and fast post-processing, namely all states well-approximated by MPS or MERA states. These variational classes are known to faithfully approximate ground states of local Hamiltonians in 1 dimension. Examples of interest include GHZ, W and cluster states. Our method prevents the build-up of errors from both numerical and experimental imperfections and contains a built-in certification procedure. These ideas extend to learning continuous-time quantum dynamics that are described by local Hamiltonians or Lindbladians. This covers work presented in da Silva, Landon-Cardinal and Poulin, PRL 107, 210404 (2011). It complements the Monte Carlo certification scheme presented in the same paper and independently derived by Flammia and Liu in PRL 106, 230501 (2011). Learning complements the work on certification and is of independent interest.