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

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 π 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 π. 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.