Photon-counting-based optical frequency metrology

Low power can be a concern for the calibration of frequency-stabilized lasers by traditional heterodyne beating at a photodiode. On the other hand, time-correlated photon counts in a Hong-Ou-Mandel interferometer are able to reveal the frequency difference between a pair of few-photon laser sources. This paper evaluates the photon-counting method as a metrological tool for optical frequency calibration traced to radiation standards. Measurement procedure and uncertainty budget are developed. The method's uncertainty is determined as 0.24 MHz from measurements with a pair of frequency-stabilized He-Ne lasers. The optical frequency traces to standard radiation with 2.9 MHz uncertainty, limited by stability of the sources used. Validation measurements using classical heterodyne technique agree within 0.12 MHz, thus establishing the photon-counting approach as a resource for frequency metrology of extremely faint laser sources.