Reduced Flux-Trapping in Low-Tc MKIDs

Microwave Kinetic Inductance Detectors (MKIDs) are a well-established superconducting technology used in the measurement of photon and phonon signals. In Kinetic Inductance Phonon-Mediated (KIPM) detectors, MKID sensitivity to phonons is utilized to characterize phonon-producing interactions such as those expected to occur between some dark-matter models and standard-model targets. In order to achieve a low detection threshold and probe sub-GeV dark matter using such detectors, MKIDs with small superconducting band gap (and corresponding low transition temperature) are required. However, MKIDs with low Tc have historically shown relatively poor resonator quality factors due to high sensitivity to photon backgrounds and susceptibility to ambient magnetic flux trapped during the superconducting transition. Loss due to trapped flux increases with frequency and becomes a substantial problem for phonon-sensitive MKIDs at frequencies comparable to qubit readout resonators. We present the results of our work to improve quality factors in low-Tc KIPM detectors using a combination of light shielding, magnetic shielding, magnetic field nulling, and the inclusion of flux-trap minimizing features in the MKID design.