Coherent vs incoherent filtering of optical signals: application to quantum communication

The recently demonstrated concept of a quantum pulse gate enables coherent filtering of an individual well-defined optical mode from a noisy signal. This contrasts with the standard technique of spectral filtering followed by temporal gating, which exhibits an inherent trade-off between the efficiency and the noise rejection level. Here we compare the performance of the two techniques in application to quantum key distribution using entangled photon pairs. It shown that coherent filtering may lead to a nearly 10 dB enhancement of the key rate.