  Radiation sensors based on the heating effect of the absorbed radiation are
typically relatively simple to operate and flexible in terms of the input
frequency. Consequently, they are widely applied, for example, in gas
detection, security, THz imaging, astrophysical observations, and medical
applications. A new spectrum of important applications is currently emerging
from quantum technology and especially from electrical circuits behaving
quantum mechanically. This circuit quantum electrodynamics (cQED) has given
rise to unprecedented single-photon detectors and a quantum computer supreme to
the classical supercomputers in a certain task. Thermal sensors are appealing
in enhancing these devices since they are not plagued by quantum noise and are
smaller, simpler, and consume about six orders of magnitude less power than the
commonly used traveling-wave parametric amplifiers. However, despite great
progress in the speed and noise levels of thermal sensors, no bolometer to date
has proven fast and sensitive enough to provide advantages in cQED. Here, we
experimentally demonstrate a bolometer surpassing this threshold with a noise
equivalent power of $30\, \rm{zW}/\sqrt{\rm{Hz}}$ on par with the current
record while providing two-orders of magnitude shorter thermal time constant of
500 ns. Importantly, both of these characteristic numbers have been measured
directly from the same device, which implies a faithful estimation of the
calorimetric energy resolution of a single 30-GHz photon. These improvements
stem from the utilization of a graphene monolayer as the active material with
extremely low specific heat. The minimum demonstrated time constant of 200 ns
falls greatly below the state-of-the-art dephasing times of roughly 100 {\mu}s
for superconducting qubits and meets the timescales of contemporary readout
schemes thus enabling the utilization of thermal detectors in cQED.
