Sub-THz photon counting receiver working at room temperature for polarization measurements of the cosmic microwave background radiation

In this paper, a general scheme for a sub-THz photon counting receiver working at room temperature is presented. Its working principle is based on nonlinear up-conversion of the sub-THz signal into the optical domain. For that, a lithium niobate (LiNbO3) WGM resonator is used in a double-resonant configuration. This system has theoretically shown capabilities towards photon counting. The concept is evaluated and is shown to be suitable for cosmic microwave background polarization measurements. Then, a practical configuration of several receivers is engineered for that purpose. The general receiver scheme is tested and experimental results are given.