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We report an investigation to establish the physical mechanisms responsiblefor decoherence in the generation of photon pairs from atomic ensembles, viathe protocol of Duan et. al for long distance quantum communication [Nature(London) 414, 413 (2001)] and present the experimental techniques necessary toproperly control the process. We develop a theory to model in detail thedecoherence process in experiments with magneto-optical traps. Theinhomogeneous broadening of the ground state by the trap magnetic field isidentified as the principal mechanism for decoherence. In conjunction with ourtheoretical analysis, we report a series of measurements to characterize andcontrol the coherence time in our experimental setup. We use copropagatingstimulated Raman spectroscopy to access directly the ground state energydistribution of the ensemble. These spectroscopic measurements allow us toswitch off the trap magnetic field in a controlled way, optimizing therepetition rate for single-photon measurements. With the magnetic field off, wethen measure nonclassical correlations for pairs of photons generated by theensemble as a function of the storage time of the single collective atomicexcitation. We report coherence times longer than 10 microseconds,corresponding to an increase of two orders of magnitude compared to previousresults in cold ensembles. The coherence time is now two orders of magnitudelonger than the duration of the excitation pulses. The comparison between theseexperimental results and the theory shows good agreement. Finally, we employour theory to devise ways to improve the experiment by optical pumping tospecific initial states.Comment: 16 pages, 11 figures, submitted for publicatio

Control of decoherence in the generation of photon pairs from atomic ensembles