Using a low-pass filter to implement source monitoring in quantum key distribution

Quantum key distribution (QKD) has been proven to be unconditionally secure in theory. The light source of QKD is assumed to be trusted in the standard security analysis. However, this assumption may be deviated in a real system, which will cause `untrusted source´ problem. To solve the problem, several source monitoring schemes have been proposed, in which passive schemes are more practical than active schemes. In this paper, we first review of these passive schemes, and then propose a new demonstration of the passive scheme with two-threshold detection using a low-pass filter, which can be well implemented in practical.