Characterizing multi-photon states using gated photo-detection

We describe a system that can temporally separate photons in a multi-photon resolving detector. A time delay, using 239 m of optical fibre in a feedback loop sends a part of the incident photon stream back to the system, through a directional coupler. The residual optical power is detected using a gated avalanche photo-detector. The two primary aspects considered while studying this system were the separation of photon pulse in space by the coupler, and the timing aspects of the setup required to achieve synchronization between detector and the source. A timing analysis permits us to verify some properties of the input coherent optical state.