Spectacular advances in heterodyne astronomy1,2 have been largely due to breakthroughs in detector technology3. To exploit the full capacity of future terahertz (̃300 GHz-5 THz) telescope space missions4, new concepts of terahertz coherent receivers are needed, providing larger bandwidths and imaging capabilities with multipixel focal plane heterodyne arrays5. Here we show that graphene uniformly doped to the Dirac point, with material resistance dominated by quantum localization and thermal relaxation governed by electron diffusion, enables highly sensitive and wideband coherent detection of signals from 90 to 700 GHz and, prospectively, across the entire terahertz range. We measure on proof-of-concept graphene bolometric mixers an electron diffusion-limited gain bandwidth of 8 GHz (corresponding to a Doppler shift of 480 km s-1 at 5 THz) and intrinsic mixer noise temperature of 475 K (which would be equivalent to 2 hf/kB at f = 5 THz, where h is Planck's constant, f is the frequency and kB is the Boltzmann constant), limited by the 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 of <100 pW. In conjunction with the emerging quantum-limited amplifiers at the intermediate frequency6,7, our approach promises quantum-limited sensing in the terahertz domain, potentially surpassing superconducting technologies, particularly for large heterodyne arrays.