We suggest an all-optical scheme for the storage, retrieval and processing of a single-photon wave packet through its off-resonant Raman interaction with a series of coherent control beams. These control beams, each with distinct carrier frequency, are distributed along the way of single-photon propagation, thus effectively forming a gradient absorption structure which can be controlled in various ways to achieve different single-photon processing functionalities. Such a controllable frequency comb is a hybrid of Raman, gradient echo memory (GEM) and atomic frequency comb (AFC) methods, therefore demonstrates many of their advantages all together in one.
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