We present a method of spatially mapping microwave kinetic inductance detector (MKID) arrays, in a dark setup. MKIDs are superconducting natively multiplexed resonators which enable kilopixel arrays, such as for the proposed Probe far-Infrared Mission for Astrophysics (PRIMA). In such telescope applications one must map the spatial location of each MKID with their individual resonance frequencies. Traditional LED arrays or beam-mapping methods become increasingly difficult as pixel spacing decreases, e.g., 900 {\mu}m separated MKIDs in the spectrometer module of PRIMA. Our new mapping technique uses a cryogenic interferometer in reflection mode. As on-resonance signals reflect from an MKID, they accrue a phase proportional to the path-length, exactly corresponding to their physical distance on the feedline. Specifically, we use a superconducting transmission line that has nonlinear kinetic inductance. The slow-wave structure of this nonlinear device is designed to have a signal speed of 0.64% the speed of light, enabling a compact system. Current biasing this line allows for varying the wave speed and ensuring that the phase measured is periodic within a nulling interferometric mode. Using this setup, we measure a length ordering that reflects the bimodal MKID distribution of a 44 pixel array of MKIDs designed for PRIMA which contains the same spacing as the final kilopixel array design.
@article{arxiv.2604.12287,
title = {In Situ Interferometric Spatial Mapping Of A Microwave Kinetic Inductance Detector Array},
author = {Chris Albert and Ritoban Basu Thakur and Farzad Faramarzi and Byeong Ho Eom and Sumit Dahal and Andrew Bear and Reinier Janssen and Henry LeDuc and Thomas Stevenson and Peter Day},
journal= {arXiv preprint arXiv:2604.12287},
year = {2026}
}