  This article reviews the state of rapidly emerging terahertz hot-electron
nanobolometers (nano-HEB), which are currently among of the most sensitive
radiation power detectors at submillimeter wavelengths. With the achieved noise
equivalent power close to 10^{-19} W/Hz^{1/2} and potentially capable of
approaching NEP ~ 10^{-20} W/Hz^{1/2}, nano-HEBs are very important for future
space astrophysics platforms with ultralow submillimeter radiation background.
The ability of these sensors to detect single low-energy photons opens
interesting possibilities for quantum calorimetry in the mid-infrared and even
in the farinfrared parts of the electromagnetic spectrum. We discuss the
competition in the field of ultrasensitive detectors, the physics and
technology of nano-HEBs, recent experimental results, and perspectives for
future development.
