  Single-photon terahertz (THz) detection is one of the most demanding
technology for a variety of fields and could lead to many breakthroughs.
Although its significant progress has been made in the last two decades,
operating it at room temperature still remains a great challenge. Here, we
demonstrate, for the first time, the room temperature THz detector at
single-photon levels based on nonlinear wave mixing in thermal Rydberg atomic
vapor. The low-energy THz photons are coherently upconverted to the high-energy
optical photons via a nondegenerate Rydberg state involved six-wave-mixing
process, and therefore, the single-photon THz detection is achieved by a
conventional optical single-photon counting module. The noise equivalent power
of such a detector is reached to be 9.5*10^-19 W/Hz^1/2, which is more than
four orders of magnitude lower than the state-of-the-art room temperature THz
detectors. The optimum quantum efficiency of the whole wave-mixing process is
about 4.3% with 40.6 dB dynamic range, and the maximum conversion bandwidth is
172 MHz, which is all-optically controllable. The developed fast and
continuous-wave single-photon THz detector at room temperature operation has a
great potential to be portable and chip-scale, and could be revolutionary for a
wide range of applications in remote sensing, wireless communication,
biomedical diagnostics, and quantum optics.
