Patent ID: 8131160

Claim:
A polarization direction synchronization detecting circuit that receives a light signal modulated in two polarization directions according to signals and a light signal of a communication signal including a synchronization signal, the polarization direction synchronization detecting circuit comprising: a switch inputted with a modulation signal including the synchronization signal extracted from a received communication signal as a control signal, inputted with a detection signal detected by a photodetector that receives a polarization modulation signal adjusted to a polarization direction of a quantum cipher signal, and selectively switching and outputting one of the detection signal of the photodetector and an inverted signal of the detection signal according to the control signal; and a low-pass filter outputting the output signal of the switch, whereby a plus level signal is outputted when a relative angle difference Δθ between polarization axes on a transmission side and a reception side is larger than a predetermined polarization angle set in advance, a 0 level signal is outputted when the relative angle difference is equal to the predetermined polarization angle, and a minus level signal is outputted when the relative angle difference is in a range of the predetermined polarization angle from 0 degree.