  Single photon detectors are key for time-correlated photon counting
applications [1] and enable a host of emerging optical quantum information
technologies [2]. So far, the leading approach for continuous and efficient
single-photon detection in the optical domain has been based on semiconductor
photodiodes [3]. However, there is a paucity of efficient and continuous
single-photon detectors in the microwave regime, because photon energies are
four to five orders of magnitude lower therein and conventional photodiodes do
not have that sensitivity. Here we tackle this gap and demonstrate how
itinerant microwave photons can be efficiently and continuously converted to
electrical current in a high-quality, semiconducting nanowire double quantum
dot that is resonantly coupled to a cavity. In particular, in our detection
scheme, an absorbed photon gives rise to a single electron tunneling event
through the double dot, with a conversion efficiency reaching 6 %. Our results
pave the way for photodiodes with single-shot microwave photon detection, at
the theoretically predicted unit efficiency [4].
