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

Microwave photon detection is an open problem. Detectors based on quantum dots and nanobolometers are rapidly improving, but cannot yet resolve individual events. Those based on superconducting qubits can, but operate in cycles: detect, measure, reset. A photon arriving during the last two steps is missed. Here we demonstrate a detector based on a superconducting artificial molecule that resolves individual photons continuously. We achieve a continuous detection efficiency of 0.47 with a dark count rate of 1.3 kHz.