  Spectacular advances in heterodyne astronomy with both the Herschel Space
Observatory and Stratospheric Observatory for Far Infrared Astronomy (SOFIA)
have been largely due to breakthroughs in detector technology. In order to
exploit the full capacity of future THz telescope space missions (e.g. Origins
Space Telescope), new concepts of THz coherent receivers are needed, providing
larger bandwidths and imaging capabilities with multi-pixel focal plane
heterodyne arrays. Here we show that graphene, uniformly doped to the Dirac
point, enables highly sensitive and wideband coherent detection of THz signals.
With material resistance dominated by quantum localization, and thermal
relaxation governed by electron diffusion, proof-of-concept graphene bolometers
demonstrate a gain bandwidth of 8 GHz and a mixer noise temperature of 475 K,
limited by residual thermal background in our setup. An optimized device will
result in a mixer noise temperature as low as 36 K, with the gain bandwidth
exceeding 20 GHz, and a Local Oscillator power lower than 100 pW. In
conjunction with the emerging quantum-limited amplifiers at the intermediate
frequency, our approach promises quantum-limited sensing in the THz domain,
potentially surpassing superconducting technologies, particularly for large
heterodyne arrays.
