Quantum sensors based on resonant transduction of pair-breaking photons to quasiparticles

Superconducting qubits are resonant absorbers of pair-breaking radiation, as the qubit structure is the aperture dual of a wire loop antenna. Typical Josephson junction parameters result in a junction impedance that is reasonably well matched to the fundamental antenna mode. Here we exploit this physics to realize a new class of quantum sensors for mm-wave radiation based on the resonant transduction of pair-breaking photons to quasiparticles. We use broadband thermal radiation and coherent mm-wave radiation derived from a Josephson photon source to characterize these detectors over the band from 100 GHz to 1 THz. We present experimental measurements of upward and downward transition rates and charge-parity rates as a function of radiator temperature or coherent drive frequency. This scheme could form the basis for next-generation detectors of dark-matter axions.