  Single photon detection is a key resource for sensing at the quantum limit
and the enabling technology for measurement based quantum computing. Photon
detection at optical frequencies relies on irreversible photo-assisted
ionization of various natural materials. However, microwave photons have
energies 5 orders of magnitude lower than optical photons, and are therefore
ineffective at triggering measurable phenomena at macroscopic scales. Here, we
report the observation of a new type of interaction between a single two level
system (qubit) and a microwave resonator. These two quantum systems do not
interact coherently, instead, they 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. We have used this highly correlated dissipation
mechanism to detect itinerant photons impinging on the resonator. This scheme
does not require any prior knowledge of the photon waveform nor its arrival
time, and dominant decoherence mechanisms do not trigger spurious detection
events (dark counts). We demonstrate a detection efficiency of 58% and a record
low dark count rate of 1.4 per ms. This work establishes engineered non-linear
dissipation as a key-enabling resource for a new class of low-noise non-linear
microwave detectors.
