Coherent control of single-photon absorption and reemission in a two-level atomic ensemble.

We demonstrate coherent control of single-photon absorption and reemission in a two-level cold atomic ensemble. This is achieved by interfering the incident single-photon wave packet with the emission (or scattering) wave. For a photon with an exponential growth waveform with a time constant equal to the excited-state lifetime, we observe that the single-photon emission probability during the absorption can be suppressed due to the perfect destructive interference. After the incident photon waveform is switched off, the absorbed photon is then reemitted to the same spatial mode as that of the incident photon with an efficiency of 20%. For a photon with an exponential decay waveform with the same time constant, both the absorption and reemission occur within the waveform duration. Our experimental results suggest that the absorption and emission of a single photon in a two-level atomic ensemble may possibly be manipulated by shaping its waveform in the time domain.