Apparatus for detecting pulse optical position modulated signals with high background noise by detection of quantum arrival rate at the detector are described. Pulse signals at the detection limit are characterized by the arrival of clusters of individual photons at an optical receiver. The receiver has embedded shot noise that interferes with the detection of the signal of interest. The apparatus distinguishes the signal of interest by a measurement of the rate of arrival of the photons of the signal of interest from the ambient shot noise rate of the receiver. The apparatus determines the optimal signal detection criteria and calculates the expected bit error rate of the decoded data.