We demonstrate a single-photon detector operating in the microwave domain, based on photoassisted quasiparticle tunneling events that poison a superconducting island. The detection relies on continuously monitoring the island’s charge parity using microwave reflectometry. This scheme achieves 10% detection efficiency with sub 50 ns time resolution and short dead time ($\sim 1\mathrm{\mu s}$), for microwave photons at 10 GHz. The detector features three junctions connected to a superconducting island, which together carry out photoelectric conversion and charge readout. The enhanced light-matter coupling, crucial to photon-to-quasiparticle conversion, is provided by a granular aluminum-based high-impedance microwave resonator. The time-resolved detection of itinerant microwave photon opens up new perspectives in quantum sensing, microwave quantum optics, and mesoscopic physics.

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• Photoinduced effect
• Quantum circuits
• Quantum engineering
• Quantum transport
• Superconducting RF
• Superconducting devices
• Transport techniques