Non-Equilibrium Superconductivity in Kinetic Inductance Detectors for THz Photon Sensing

TL;DR

This paper models the non-equilibrium quasiparticle and phonon energy spectra in Kinetic Inductance Detectors operating at very low temperatures, incorporating both low and high frequency signals to understand detection efficiency.

Contribution

It provides a detailed theoretical calculation of non-equilibrium energy spectra in KIDs, including the effects of high frequency signals, within the coupled kinetic equations framework.

Findings

Quasiparticle distributions can be driven far from equilibrium at low temperatures.

Inclusion of a high frequency source affects detection efficiency.

Detailed spectra calculations inform detector performance understanding.

Abstract

Low temperature Kinetic Inductance Detectors (KIDs) are attractive candidates for producing quantumsensitive, arrayable sensors for astrophysical and other precision measurement applications. The readout uses a low frequency probe signal with quanta of energy well-below the threshold for pair-breaking in the superconductor. We have calculated the detailed non-equilibrium quasiparticle and phonon energy spectra generated by the probe signal of the KID when operating well-below its superconducting transition temperature Tc within the framework of the coupled kinetic equations described by Chang and Scalapino.[1] At the lowest bath temperature studied Tb/Tc = 0.1 the quasiparticle distributions can be driven far from equilibrium. In addition to the low frequency probe signal we have incorporated a high frequency (~ 1 THz) source signal well-above the pair-breaking threshold of the superconductor. Calculations of source signal detection efficiency are discussed