Ultrasensitive microwave nanobolometer*

We have fabricated microwave nanobolometers using normal-metal nanowires. A part of the wire acts as an absorber of the microwave radiation whereas the remaining parts act as a sensitive thermometer with proximity-induced superconductivity. We use electrothermal feedback enhance the coupling of the temperature to the electrical degrees of freedom in the bolometer allowing us to measure a change in the radio-frequency signal caused by the absorption of microwave radiation. At the regime of large positive feedback, two metastable electron temperature states arise enabling threshold detection of microwave pulses down to 200 8.4-GHz photons, i.e., 1 zeptojule of energy. The measured noise-equivalent power of our nanobolometer is 2*10^-20 W/Hz^1/2 at 1-ms timeconstant. Finally, I present results on the devices where the normal-metal is replaced by graphene for enhanced performance.