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arXiv:quant-ph/0411021 · Physical Review B

Theory of multiwave mixing and decoherence control in a qubit array system

Masahide Sasaki, Atsushi Hasegawa, Junko Ishi‐Hayase, Yasuyoshi Mitsumori, F. Minami

Published Apr 18, 2005• 16 citations
arXivHTMLPDF

Abstract

We develop a theory to analyze the decoherence effect in a charged qubit array system with photon-echo signals in the multiwave mixing configuration. We present how the decoherence-suppression effect by the bang-bang control with the $\ensuremath{\pi}$ pulses can be demonstrated in the laboratory by using a bulk ensemble of exciton qubits and optical pulses with a pulse area that is even smaller than $\ensuremath{\pi}$. Analysis is made on the time-integated multiwave mixing signals diffracted into certain phase-matching directions from a bulk ensemble. Depending on the pulse-interval conditions, the crossover from the decoherence-acceleration regime to the decoherence-suppression regime, which is a peculiar feature of the coherent interaction between a qubit and the reservoir bosons, may be observed in the time-integated multiwave mixing signals in the realistic case, including the inhomogeneous broadening effect. Our analysis will be successfully applied to the precise estimation of the reservoir parameters from the experimental data of the direction-resolved signal intensities obtained in the multiwave mixing technique.

DOI
10.1103/physrevb.71.165314

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We develop a theory to analyze the decoherence effect in a charged qubit array system with photon-echo signals in the multiwave mixing configuration. We present how the decoherence-suppression effect by the bang-bang control with the \ensuremathπ\ensuremath{\pi}\ensuremathπ pulses can be demonstrated in the laboratory by using a bulk ensemble of exciton qubits and optical pulses with a pulse area that is even smaller than \ensuremathπ\ensuremath{\pi}\ensuremathπ. Analysis is made on the time-integated multiwave mixing signals diffracted into certain phase-matching directions from a bulk ensemble. Depending on the pulse-interval conditions, the crossover from the decoherence-acceleration regime to the decoherence-suppression regime, which is a peculiar feature of the coherent interaction between a qubit and the reservoir bosons, may be observed in the time-integated multiwave mixing signals in the realistic case, including the inhomogeneous broadening effect. Our analysis will be successfully applied to the precise estimation of the reservoir parameters from the experimental data of the direction-resolved signal intensities obtained in the multiwave mixing technique.
A useful method for reducing the decoherence of a quantum bit (qubit) is proposed, which is based on the photon-echo phenomenon in coherent optical transient. The decoherence of the qubit is caused by the dephasing coupling with a bosonic thermal reservoir. In the method, successive coherent π pulses are applied with very short pulse widths to reduce the decoherence. It is found that the decoherence of the qubit disappears at a certain time which can be controlled by appropriately choosing the pulse separation time and the number π pulses.
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We present a detailed general theory of multiwave mixing effects in a nonlinear medium produced by two incident coherent laser beams. Calculations are performed for localized as well as diffusive types of nonlinearities. The theory accounts for all the relevant parameters such as laser pulse widths, medium response times, nonlinearities, interaction length, intensities, beam ratio, phase modulation effects, losses, and side diffractions. The interrelationship between two- and multiwave mixing effects and the evolution of transient to stationary behavior is explicitly worked out. In particular, the gain experienced by a weak incident probe beam via these mixing effects is quantitatively analyzed as a function of the aforementioned parameters.
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