05.70.+o Quantum information & quantum control

Fast and robust quantum computation with ionic Wigner crystals

Date: 
2011-04-15
Author(s): 

J. D. Baltrusch, A. Negretti, J. M. Taylor, T. Calarco

Reference: 

Phys. Rev. A 83, 042319 (2011).

We present a detailed analysis of the modulated-carrier quantum phase gate implemented with Wigner crystals of ions confined in Penning traps. We elaborate on a recent scheme, proposed by two of the authors, to engineer two-body interactions between ions in such crystals. We analyze for the first time the situation in which the cyclotron (w_c) and the crystal rotation (w_r) frequencies do not fulfill the condition w_c=2w_r.

Control and tomography of a three level superconducting artificial atom

Date: 
2010-11-24
Author(s): 

R. Bianchetti, S. Filipp, M. Baur, J. M. Fink, C. Lang, L. Steffen, M. Boissonneault, A. Blais, and A. Wallraff

Reference: 

Phys. Rev. Lett. 105, 223601 (2010)

A number of superconducting qubits, such as the transmon or the phase qubit, have an energy level structure with small anharmonicity. This allows for convenient access of higher excited states with similar frequencies. However, special care has to be taken to avoid unwanted higher-level populations when using short control pulses. Here we demonstrate the preparation of arbitrary three level superposition states using optimal control techniques in a transmon.

Optimal Control of Open Quantum Systems: Cooperative Effects of Driving and Dissipation

Date: 
2011-09-21
Author(s): 

R. Schmidt, A. Negretti, J. Ankerhold, T. Calarco, J. T. Stockburger

Reference: 

Phys. Rev. Lett. 107, 130404 (2011)

We investigate the optimal control of open quantum systems, in particular, the mutual influence of driving and dissipation. A stochastic approach to open-system control is developed, using a generalized version of Krotov’s iterative algorithm, with no need for Markovian or rotating-wave approximations. The application to a harmonic degree of freedom reveals cooperative effects of driving and dissipation that a standard Markovian treatment cannot capture.

Trapped ions as quantum bits: Essential numerical tools (Colloquium)

Date: 
2010-09-14
Author(s): 

K. Singer, U. Poschinger, M. Murphy, P. Ivanov, F. Ziesel, T. Calarco, F. Schmidt-Kaler

Reference: 

Rev. Mod. Phys. 82, 2609 (2010)

Trapped laser-cooled atoms and ions are quantum systems which can be experimentally controlled with an as yet unmatched degree of precision. Due to the control of the motion and the internal degrees of freedom, these quantum systems can be adequately described by a well-known Hamiltonian. In this colloquium, powerful numerical tools for the optimization of the external control of the motional and internal states of trapped neutral atoms, explicitly applied to the case of trapped laser-cooled ions in a segmented ion-trap are presented.

Communication at the quantum speed limit along a spin chain

Date: 
2010-08-13
Author(s): 

M. Murphy, S. Montangero, V. Giovannetti, T. Calarco

Reference: 

Phys. Rev. A 82, 022318 (2010).

Spin chains have long been considered as candidates for quantum channels to facilitate quantum communication. We consider the transfer of a single excitation along a spin-1/2 chain governed by Heisenberg-type interactions. We build on the work of Balachandran and Gong [V. Balachandran and J. Gong, Phys. Rev. A 77, 012303 (2008)] and show that by applying optimal control to an external parabolic magnetic field, one can drastically increase the propagation rate by two orders of magnitude.

Correcting errors in a quantum gate with pushed ions via optimal control

Date: 
2010-07-30
Author(s): 

U. V. Poulsen, S. Sklarz, D. Tannor, T. Calarco

Reference: 

Phys. Rev. A 82, 012339 (2010).

We analyze in detail the so-called pushing gate for trapped ions, introducing a time-dependent harmonic approximation for the external motion. We show how to extract the average fidelity for the gate from the resulting semiclassical simulations. We characterize and quantify precisely all types of errors coming from the quantum dynamics and reveal that slight nonlinearities in the ion-pushing force can have a dramatic effect on the adiabaticity of gate operation.

Errors in quantum optimal control and strategy for the search of easily implementable control pulses

Date: 
2011-07-25
Author(s): 

A. Negretti, R. Fazio, T. Calarco

Reference: 

J. Phys. B: At. Mol. Opt. Phys. 44, 154012 (2011)

We introduce a new approach to assess the error of control problems we aim to optimize. The method offers a strategy to define new control pulses that are not necessarily optimal but still able to yield an error not larger than some fixed a priori threshold, and therefore provide control pulses that might be more amenable for an experimental implementation. The formalism is applied to an exactly solvable model and to the Landau–Zener model, whose optimal control problem is solvable only numerically.

Atom chip for BEC interferometry

Date: 
2010-02-11
Reference: 

A. E. Hinds et al.
J. Phys. B: At. Mol. Opt. Phys. 43 (2010) 051003

We have fabricated and tested an atom chip that operates as a matter wave interferometer. In this communication we describe the fabrication of the chip by ion-beam milling of gold evaporated onto a silicon substrate. We present data on the quality of the wires, on the current density that can be reached in the wires and on the smoothness of the magnetic traps that are formed. We demonstrate the operation of the interferometer, showing that we can coherently split and recombine a Bose–Einstein condensate with good phase stability.

Optical control of the refractive index of a single atom

Date: 
2010-04-29
Reference: 

Tobias Kampschulte, Wolfgang Alt, Stefan Brakhane, Martin Eckstein, René Reimann, Artur Widera, Dieter Meschede
Phys. Rev. Lett. 105, 153603 (2010)

The optical properties of an atomic medium can be changed dramatically by the coherent interaction with a near-resonant control light field: An optically dense medium can be rendered transparent and group velocities can be strongly reduced. So far the demonstration of this electromagnetically induced transparency (EIT) has relied on macroscopic ensembles of atoms probed by relatively intense light fields. Here we demonstrate the most elementary case, where the medium is formed by a single atom inside an optical cavity, probed by single photons.

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