  We present a superconducting cavity-coupled double quantum dot (DQD)
photodiode that achieves a maximum photon-to-electron conversion efficiency of
25% in the microwave domain. With a higher-quality-factor cavity and improved
device design to prevent photon leakages through unwanted pathways, our device
measures microwave signals down to 100 aW power level and achieves sensitivity
to probe microwave signals with one photon at a time in the cavity. We analyze
the photodiode operation using Jaynes-Cummings input-output theory, identifying
the key improvements of stronger cavity-DQD coupling needed to achieve
near-unity photodetection efficiency. The results presented in this work
represent a crucial advancement toward near unity microwave photodetection
efficiency with single cavity-photon sensitivity for studies of photon
statistics in the microwave range and applications related to quantum
information processing.
