Field correlation measurements of photon modes with sub-unity photon occupation per mode inside a Fabry-Perot cavity

The non-destructive nature of electro-optic sampling is exploited to study the first order coherence function of modes oscillating inside a cold Fabry Perot cavity with sub-unity photon occupation number per mode, where the vacuum field contribution is similar to the thermal contribution. For this purpose, a fast electro-optic correlation setup has been implemented inside a cryogenic environment to suppress the thermal background. The Fabry-Perot cavity is constituted by the detection crystal itself.