Observation of quantum beats in the resonance fluorescence of free excitons

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Abstract
The occurrence of quantum beats for an extended exciton state in a solid is demonstrated for the first time. They are observed as oscillations in the two-phonon resonance fluorescence decay of the indirect free exciton states in AgBr split in magnetic fields up to 1 T after excitation by picosecond laser pulses. The results are in agreement with calculations by a density-matrix formalism. From the beating frequencies the splitting of the exciton states can be obtained with high accuracy. Quantum dephasing times of several hundred picosecond are determined comparable to the energy relaxation time.