  Photon detection with quantum-level sensitivity is particularly challenging
in the terahertz regime (0.1-10 THz), which contains ~98% of all the photons
existing in the universe. Near-quantum-limited terahertz spectrometry has so
far only been possible through the use of cryogenically cooled superconducting
mixers as frequency downconverters. Here we introduce a spectrometry scheme
that uses plasmonic photomixing for frequency downconversion to offer
quantum-level sensitivities at room temperature for the first time. Frequency
downconversion is achieved by mixing terahertz radiation and a heterodyning
optical beam with a terahertz beat frequency in a plasmonics-enhanced
semiconductor active region. We demonstrate spectrometer sensitivities down to
3 times the quantum-limit at room temperature. With a versatile design capable
of broadband spectrometry, this plasmonic photomixer has broad applicability to
quantum optics, chemical sensing, biological studies, medical diagnosis, high
data-rate communication, as well as astronomy and atmospheric studies.
