Nonlinear kinetic inductance sensor

The concept of nonlinear kinetic inductance sensor (NKIS) of electromagnetic radiation is proposed. The idea is based on divergency of kinetic inductance $L_k \sim dq/dI$ ($\hbar q$ is a momentum of superconducting electrons, $I$ is a supercurrent) of hybrid superconductor/normal metal (SN) bridge at current $I^*<I_{dep}$ ($I_{dep}$ is a depairing current of the hybrid) and temperature $T^*$ much smaller than critical temperature $T_c$. It makes possible to have large change of phase difference $\delta \phi$ along SN bridge in current biased regime at $I\simeq I^*$ even for small electron temperature increase. Appearance of $\delta \phi$ is accompanied by the change of the current and magnetic flux through the coupled superconducting ring which could be measured with help of superconducting quantum interference device (SQUID). In some respect proposed sensor may be considered as a superconducting counterpart of transition edge sensor (TES) those work is based on large derivative $dR/dT$ ($R$ is a resistance) near $T_c$. Because at $I \simeq I^*$ SN bridge is in gapless regime there is no low boundary for frequency of detected e.m. radiation. Our calculations show that such a sensor can operate in single photon regime and detect single photons with frequency $\nu \gtrsim$ 10 GHz. We argue that the nontrivial dependence $I(q)$ of SN bridge could be also used in detectors of continuous e.m. radiation, current and magnetic field sensors.