Microwave Single-Photon Detector based on a Superconducting Lambda system with Engineered Couplings

Efficient single-photon detection in the microwave regime remains an outstanding challenge, primarily due to low photon energies in this frequency range. Superconducting circuits, which operate at similar energy scales, offer a promising platform to address these challenges. One way to perform single photon detection in a superconducting circuit architecture is to deterministically convert propagating photons into qubit excitations ("latching"), which can be detected with conventional qubit readout. Here, we implement latching in a Lambda system created using a superconducting artificial molecule composed of two identical coupled transmons. The decay rates along the two legs are equal, latching the photon onto the long-lived dark state. We characterize the efficiency, dark count and bandwidth of the device using measurements of the dark state population while sending low power coherent pulses to the bright state. Finally, we perform real-time photodetection using continuous measurements of quantum jumps in the dark state. We envisage applications of our device in microwave quantum optics experiments, quantum communication and sensing.