Josephson-Threshold Calorimeter

We suggest a single-photon thermal detector based on the abrupt jump of the critical current of a temperature-biased tunnel Josephson junction formed by different superconductors, working in the dissipationless regime. The electrode with the lower critical temperature is used as a radiation-sensing element, so it is thermally floating and is connected to an antenna or absorber. The warming up resulting from the absorption of a photon can induce a drastic measurable enhancement of the critical current of the junction. We propose a detection scheme based on a threshold mechanism for single- or multiphoton detection. This Josephson-threshold detector indeed has calorimetric capabilities, being able to determine the energy of the incident photon. So, for the realistic setup that we discuss, our detector can work efficiently as a calorimeter for photons from the midinfrared, through the optical, into the ultraviolet, specifically, for photons with frequencies in the range 30 THz to 9×104THz. Over the whole range of detectable frequencies, we obtain a resolving power significantly larger than one. In order to reveal the signal, we suggest the fast measurement of the Josephson kinetic inductance. Indeed, the photon-induced change in the critical current affects the Josephson kinetic inductance of the junction, which can be read noninvasively through an LC tank circuit that is inductively coupled to the junction. Finally, this readout scheme shows remarkable multiplexing capabilities.