Sensitivity of terahertz photonic receivers

We theoretically discuss sensitivity limitations of a THz receiver which is based on up-conversion of the THz radiation into optical domain using high quality factor crystalline whispering gallery mode resonators. We show that the sensitivity of the receiver operating in the nonlinear regime approaches the sensitivity of an ideal THz photon counter. Thermal noise of the counter can be substantially reduced because of transparency of the nonlinear material in the THz frequency range. We also show that the density of power fluctuations of the receiver operating in the linear regime is given by the THz shot noise.