  In high-sensitivity bolometers and calorimeters, the photon absorption often
occurs at a finite distance from the temperature sensor to accommodate antennas
or avoid the degradation of superconducting circuitry exposed to radiation. As
a result, thermal propagation from the input to the temperature readout can
critically affect detector performance. In this report we model the performance
of a graphene bolometer, accounting for electronic thermal diffusion and
dissipation via electron-phonon coupling at low temperatures in three regimes:
clean, supercollision, and resonant scattering. Our results affirm the
feasibility of a superconducting readout without Cooper-pair breaking by mid-
and near-infrared photons, and provide a recipe for designing graphene
absorbers for calorimetric single-photon detectors. We investigate the tradeoff
between the input-readout distance and detector efficiency, and predict an
intrinsic timing jitter of ~2.7 ps. Based on our result, we propose a
spatial-mode-resolving photon detector to increase communication bandwidth.
