A measurement of the signal‐to‐noise ratio and minimum detectable power for heterodyne detection of scattered radiation at 10.6 &mgr;m has been made. Using photoconductive Ge:Cu as the detector, the observed minimum detectable power, at a frequency of 70 kHz and in a bandwidth of 270 kHz, was 3.5 × 10−14W. This corresponds to a minimum detectable power of 1.3 × 10−19W in a 1‐Hz bandwidth, which is within a factor of 10 of the theoretically perfect photon counter.