We describe a technique to measure photon pair joint spectra by detecting the time-correlation beat note when nondegenerate photon pairs interfere at a beam splitter. The technique implements a temporal analog of the Ghosh-Mandel effect with one photon counter and a time-resolved Hong-Ou-Mandel interference with two. It is well suited to characterize pairs of photons, each of which can interact with a single atomic species, as required to study recently predicted photon-photon interaction in subwavelength atomic arrays. With this technique, we characterize photon pairs from cavity-enhanced parametric down-conversion with a bandwidth $\approx 5\mathrm{MHz}$ and frequency separation of $\sim 200\mathrm{MHz}$ near the $D_1$ line of atomic Rb.

• Received 11 January 2021
• Accepted 10 June 2021

DOI:https://doi.org/10.1103/PhysRevLett.127.043601