1. An apparatus comprising: a voltage source operative to bias a single-photon avalanche diode with a gating signal that is characterized by a fundamental frequency; and a first filter that is dimensioned and arranged to provide an output signal based on an input signal from the avalanche photodiode, the first filter having a filter response characterized by (1) a low-pass filter response having a rejection frequency that is less than twice the fundamental frequency and (2) a notch filter response centered at a center frequency, wherein the notch filter response attenuates a first frequency range that includes the fundamental frequency.