The incoming signal (red, lower left) proceeds through a beam splitter to the photon detector, which has an attached time register (top right). The receiver sends the reference beam to the beam splitter to cancel the incoming pulse so that no light is detected. If even one photon is detected, it means that the receiver used an incorrect reference beam, which needs to be adjusted. The receiver uses exact times of photon detection to arrive at the right adjustment with fewer guesses. The combination of recorded detection times and the history of reference beam frequencies are used to find the frequency of the incoming signal.