Quantum sensing through the resonant transduction of pair-breaking photons to quasiparticles

The structure of a typical superconducting qubit is the aperture dual of a wire loop antenna, with a resonance between 100 GHz and 1 THz. This results in the resonant absorption of pair-breaking radiation, generating non-equilibrium quasiparticles. These quasiparticles inhibit qubit performance and are an important source of initialization errors in state-of-the-art qubit devices. Here, we employ this physics to realize next-generation quantum sensors. We have fabricated devices with antenna structures incorporating Josephson junctions optimized to efficiently absorb mm-wave radiation. These are embedded in a Nb groundplane and are connected to weakly charge-sensitive transmon qubits through an Al channel. When radiation is absorbed by the antenna, quasiparticles are generated and diffuse through the Al channel to the qubit, switching the charge parity of the qubit, which can be read out using a Ramsey-based pulse sequence. We utilize a voltage biased Josephson junction as an emitter of mm-wave radiation and characterize our sensors with measurements of the qubit excitation rate and parity switching rate as a function of the radiation frequency from 100 GHz to 1 THz. We describe our work to optimize these sensors to reduce dark count rates. These devices could have an important application in the search for dark-matter axions, in a mass range that is difficult to access with more conventional techniques.