Subcycle measurement of intensity correlations in the terahertz frequency range

The terahertz frequency range is lacking an experimental implementation of Hanbury Brown and Twiss photon correlation measurements with sub-cycle time resolution and high sensitivity. Such a technique would be needed, for example, to observe photon pairs predicted to be released in a nonadiabatic modulation of an ultrastrongly coupled light-matter system. In this paper, we propose a room-temperature measurement of photon correlations in the THz range based on electro-optic sampling. We apply this technique to a THz quantum cascade laser and measure below and above threshold first-and second-order degree of coherence with a subcycle temporal resolution of 146 fs. The sensitivity of the proposed measurement scheme is so far limited to similar to 1500 photons. This technique can, in principle, be extended for ultrahigh bandwidth single-photon sensitivity in a wide range of frequencies.