  At present, nearly quantum-limited heterodyne receivers for Terahertz (THz)
frequencies require cooling to temperatures below 4K. We are working to build
semiconductor-based "tunable antenna-coupled intersubband terahertz" (TACIT)
heterodyne mixers for 1-5 THz. We predict that they can achieve single-sideband
noise temperatures of a few hundred K with intermediate frequency (IF)
bandwidth >10 GHz, while operating at lattice temperatures >20 K and requiring
local oscillator (LO) power ~ 1 $\mu$W.
