  We combine parametric frequency upconversion with single-photon counting
technology to achieve detection sensitivity down to the terahertz (THz)
single-photon level. Our relatively simple detection scheme employs a
near-infrared ultrafast source, a GaP nonlinear crystal, optical filters, and a
single photon avalanche diode. This configuration is capable of detecting a
weak THz signal with an energy of 590 zJ contained within a single pulse.
Through averaging over 50k data points, the configuration can resolve a 0.5 zJ
pulse energy, corresponding to an average of 0.5 photon per pulse. The
corresponding noise-equivalent power and THz-to-NIR photon detection efficiency
are $4.1 \times 10^{-17} W/\sqrt{Hz}$ and 0.19%, respectively. To test our
scheme, we perform spectroscopy of water vapor between 1.0 and 3.7 THz and
obtain results in agreement with to those acquired with a standard
electro-optic sampling (EOS) method. Our technique provides a 0.2 THz spectral
resolution offering a fast alternative to EOS THz detection for monitoring
specific spectral components in THz spectroscopy, imaging and communications
applications.
