Generation of spontaneous parametric down-converted photons in the sub-terahertz frequency range at 660 nm

There are two crucial steps towards terahertz quantum sensing. First, there is the challenge to deliver a source of entangled photons in the terahertz range. Second, measuring terahertz radiation using photons in a different spectral range has to be possible. As we generated entangled photons in the terahertz range by spontaneous parametric down conversion (SPDC), we reached the first milestone. The detected signals match both for down- and for up-conversion for forward as well as for backward terahertz radiation the calculated frequency angular spectrum both qualitatively and quantitatively [1]. By observing difference frequency generation of external terahertz radiation with VIS pump photons, we achieved both steps towards terahertz quantum sensing.