Quantitative characterization of superconducting qubits as particle detectors
Ionizing radiation is a major source of correlated errors in superconducting quantum processors, yet the mechanisms responsible for the qubit response to particle interactions remain only partially understood. In this work, we operate superconducting qubits as particle detectors while simultaneously monitoring the deposited energy with an independent semiconductor cryogenic sensor. This complementary measurement provides an absolute determination of the particle interaction rate in the chip, enabling the first direct measurement of the detection efficiency of superconducting qubits exposed to environmental radiation. We measure individual qubit detection efficiencies of about 30-40$\%$, increasing to approximately 50$\%$ when combining the response of two qubits. Under the conservative assumption that the efficiency loss is entirely determined by a finite detection threshold, we derive an upper limit on the effective energy threshold of the detector of $70\pm20~\mathrm{(stat)}\pm30~\mathrm{(syst)}~\mathrm{keV}$. Measurements with radioactive sources producing different deposited-energy spectra further show that the detection efficiency decreases for lower-energy interactions. We further investigate the origin of the qubit response by injecting controlled thermal pulses with a resistive heater. Although these pulses deposit energies comparable to those released by particle interactions, they do not reproduce the radiation-induced signatures, demonstrating that the qubit response to radiation cannot be explained by a transient increase in substrate temperature alone. Our results establish an experimental framework for quantitatively connecting particle energy deposition to radiation-induced responses in superconducting quantum circuits.