Frequency tunable single microwave photodetector based on irreversible qubit-photon coupling*

Single photon detection is a key resource for sensing at the quantum limit and is the enabling technology for measurement-based quantum computing, however microwave photons have energies 5 orders of magnitude lower than optical ones and are therefore ineffective at triggering measurable phenomena at macroscopic scales.
We report the observation of a new type of interaction between a two level system and a microwave resonator. These two quantum systems do not interact coherently but share a common dissipative mechanism to a cold bath : the qubit irreversibly switches to its excited state if and only if a photon enters the resonator. This highly correlated dissipation mechanism is used to detect itinerant photons impinging on a frequency tunable resonator. The scheme does not require any prior knowledge of photon waveform or arrival time, and dominant decoherence mechanisms do not trigger spurious events. We demonstrate a detection efficiency of 65%, a record low dark count rate of 1/ms over a frequency tuning range of 200 MHz and the capability to operate the detector in cyclic mode with
a 50% duty cycle on a 10 µs detection sequence, making it a practical tool for quantum sensing and measurement-based computing in microwave domain.