A Cryogenic Muon Tagging System Based on Kinetic Inductance Detectors for Superconducting Quantum Processors

Ionizing radiation has emerged as a potential limiting factor for superconducting quantum processors, inducing quasiparticle bursts and correlated errors that challenge fault-tolerant operation. Atmospheric muons are particularly problematic due to their high energy and penetration power, making passive shielding ineffective. Therefore, monitoring the real-time muon flux is crucial to guide the development of alternative error-correction or protection strategies. We present the design, simulation, and first operation of a cryogenic muon-tagging system based on Kinetic Inductance Detectors (KIDs) for integration with superconducting quantum processors. The system consists of two KIDs arranged in a vertical stack and operated at 20 mK. Monte Carlo simulations based on Geant4 guided the prototype design and provided reference expectations for muon-tagging efficiency and accidental coincidences due to ambient γ-rays. We measured a muon-induced coincidence rate among the top and bottom detectors of 192 $pm$ 9 times 10^-3 events/s, in excellent agreement with the Monte Carlo prediction. The prototype achieves a muon-tagging efficiency of about 90% with negligible dead time. These results demonstrate the feasibility of operating a muon-tagging system at millikelvin temperatures and open the path toward its integration with multi-qubit chips to veto or correct muon-induced errors in real time.