Few-Photon Heteorodyne Spectroscopy

We perform a high resolution Fourier Transform Spectroscopy of optical sources in the few-photon regime based on the phenomenon of two-photon interference in a beam splitter. From the heterodyne interferogram between test and reference sources it is possible to obtain the spectrum of the test source relative to that of the reference. The method proves to be a useful asset for spectral characterization of faint optical sources below the range covered by classical heterodyne beating techniques.