  Since the introduction of bolometers more than a century ago, they have been
applied in a broad spectrum of contexts ranging from security and the
construction industry to particle physics and astronomy. However, emerging
technologies and missions call for faster bolometers with lower noise. Here, we
demonstrate a nanobolometer that exhibits roughly an order of magnitude lower
noise equivalent power, $20\textrm{ zW}/\sqrt{\textrm{Hz}}$, than previously
reported for any bolometer. Importantly, it is more than an order of magnitude
faster than other low-noise bolometers, with a time constant of 30 $\mu$s at
$60\textrm{ zW}/\sqrt{\textrm{Hz}}$. These results suggest a calorimetric
energy resolution of $0.3\textrm{ zJ}=h\times 0.4$ THz with a time constant of
30 $\mu$s. Thus the introduced nanobolometer is a promising candidate for
future applications requiring extreme precision and speed such as those in
astronomy and terahertz photon counting.
