Image Number 7 for United States Patent #8131160.
A switch is to be inputted a modulation signal including the synchronization signal extracted from a received communication signal as a control signal, is to be inputted 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 switches and outputs one of the detection signal and an inverted signal of the detection signal according to the control signal. A low-pass filter outputs the output signal of the switch, whereby a plus level signal is outputted when a relative angle difference .DELTA..theta. between polarization axes on a transmission side and a reception side is larger than a predetermined polarization angle, a 0 level signal is outputted when .DELTA..theta. is equal to the predetermined polarization angle, and a minus level signal is outputted when .DELTA..theta. is in a range of the predetermined polarization angle from 0 degree.