Analysis of quasiparticle diffusion in microwave kinetic inductance detectors

Millimeter-wave quantum sensors can be used for the detection of dark matter axions and for narrowband spectroscopy of the cosmic microwave background. Here, we design, model, and fabricate Josephson junction-based slot and patch antennas that transduce photons to quasiparticles (QPs) with a predicted efficiency of >90%. The QPs generate a signal that can be read out using either a qubit or a superconducting coplanar waveguide resonator acting as a microwave kinetic inductance detector (MKID); here, we focus on the latter. The detailed response of the MKIDs will depend on the dynamics of the QPs, which have an energy-dependent diffusivity, scattering rate, and recombination rate. We simulate and fabricate a series of MKID devices that incorporate several geometries of slot and patch antennas. We expose these devices to broadband blackbody radiation as well as coherent radiation from a Josephson-based emitter, and we compare the measured data to the results of QP modelling. Our experiments allow us to refine models for the coupling of QPs to superconducting quantum circuits, including qubits.