Zeptojoule calorimetry

The measurement of energy is a fundamental tool used in quantum technology and computing. Some of the most sensitive energy detectors—bolometers and calorimeters—are thermal, meaning that they operate by absorbing incoming energy, converting it into heat and reading out the resulting temperature change electrically using a thermometer. Recently, superconductor–normal-conductor–superconductor radiation sensors with metallic and graphene absorbers haven been predicted to be capable of full-width at half-maximum energy resolutions of 0.75 zJ and 0.05 zJ, respectively. However, these estimates are only mathematically extracted from steady-state noise and responsivity measurements. Here we show that a metallic superconductor–normal-conductor–superconductor sensor can be used for zeptojoule calorimetry. With the approach, we measure the energy of 1-μs-long 8.4-GHz microwave pulses with a full-width at half-maximum energy resolution finer than 0.95 ± 0.02 zJ (=5.9 ± 0.12 meV) corresponding to 170 photons at 8.4 GHz. The technique provides a potential path to real-time calorimetric detection of single photons in the 10-GHz range.