We performed an experiment to observe the storages of an input probe fieldand an idler field generated through an off-axis four-wave mixing (FWM) processvia a double-lambda configuration in a cold atomic ensemble. We analyzed theunderlying physics in detail and found that the retrieved idler field came fromtwo parts if there was no single-photon detuning for pump pulse: part 1 wasfrom the collective atomic spin (the input probe field, the coupling field andthe pump field combined to generate the idle field through FWM, then the idlerwas stored through electromagnetically induced transparency.); part 2 was fromthe generated new FWM process during the retrieval process (the retrieved probefield, the coupling field and the pump field combined to generate a new FWMsignal). If there was single-photon detuning for pump pulse, then the retrievedidler was mainly from part 2. The retrieved two fields exhibited dampedoscillations with the same oscillatory period when a homogeneous externalmagnetic field was applied, which was caused by the Larmor spin precession. Wealso experimentally realized the storage and retrieval of an image of lightusing FWM for the first time. In which, an image was added into the inputsignal. After the storage, the retrieved idler beams and input signal carriedthe same image. This image storage technique holds promises for application inimage processing, remote sensing and quantum communication.
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