Always-on, highly efficient microwave photon detector based on a superconducting artificial molecule

Efficient detection of single microwave photons is a key capability for emerging quantum technologies. Yet, it remains far less developed than its optical domain counterpart. Realizing detectors that simultaneously achieve high efficiency, low dark counts, and continuous operation has proved challenging. Existing detectors operate cyclically, forcing a trade-off between efficiency and duty cycle. Here, we demonstrate a continuously operated microwave single-photon detector based on a superconducting artificial molecule. In our scheme, an incoming photon is captured by a bright state of the molecule and then transferred to a long-lived dark state via a driven-dissipative process. Photon "clicks" are revealed as quantum jumps in the continuously monitored dark state. We observe a cyclic detection efficiency of 0.73, and a continuous detection efficiency of 0.47 over a 5 MHz instantaneous bandwidth, with a 1 μs temporal resolution and a 15 μs dead time. By overcoming the trade-off between efficiency and duty cycle, this approach establishes continuous microwave photon detection for quantum sensing, quantum thermodynamics, and fundamental physics.