Enhancing the sensitivity of single microwave photon detection with bandwidth tunability

We report on the characteristics of a microwave photon counter device based on a superconducting transmon qubit. Its design is similar to [arXiv:2307.03614], with an additional bandwidth tuning circuit that allows optimizing the device efficiency and noise. Owing to this new feature and to improvements in device fabrication, a power sensitivity of $3\cdot {10}^{-23}\mathrm{W}\mathrm{/}\sqrt{\mathrm{H}\mathrm{z}}$ is reached. We confirm the high performance of the device by measuring single spin microwave fluorescence.