Turbulence-free two-photon beats from incoherent thermal radiation in double-slit interferometer

It is well known that in a classic Young’s double-slit interferometer, when the separation of the double-slit, d, is greater than the spatial coherent length, l_c, of the field, no first-order interferences are observable from the spatially incoherent fields. We report a double-slit interferometer setup that is able to observe two-photon interference, a pair of randomly created and randomly paired photons interfering with the pair itself, from these incoherent fields, including two-photon beats. The light source of the interferometer emits incoherent fields in thermal state with two colors, one Green one Red, with difference between their frequencies ω_G − ω_R being much greater than the response frequency of the detectors. Certainly, no first-order interferences, including first-order optical beats, are observable when the observation plane of the double-slit interferometer is scanned with two point-like photon counting detectors, D₁ and D₂. However, in the joint measurement of D₁ and D₂, second-order interferences for ω_G and ω_R, as well as for ω_G − ω_R are all observable as functions of the relative distance between D₁ and D₂. More interestingly, all these two-photon interferences, including the two-photon beats, are turbulence-free, i.e., any fluctuations in the index of refraction resulting from turbulence in the optical paths of the interferometer do not affect the interference pattern. This observation is fundamentally interesting and practically useful.