Frequency tunable microwave photon source and detector using a double transmon coupler

In quantum optics, photon detectors play a critical role in distributing entanglement. At microwave frequencies, photon detection can be achieved by making careful use of level structure. We demonstrate a novel superconducting circuit as a detector. This circuit uses a double-transmon coupler (DTC) to controllably couple the modes of a waveguide with a long-lived transmon. The 1.7 MHz bandwidth of the detector may be tuned over several hundred MHz, making this detector uniquely narrow band and tunable, by threading flux through the DTC’s superconducting quantum interference device (SQUID). Parametrically driving the DTC flux at a frequency resonant with a select pair of states shared by the long-lived transmon and the DTC can mediate efficient detection. Our demonstration connects two such devices: one device emits a Fock state microwave photon while the other detects the photon by flipping the state of the long-lived transmon. We separated the devices with over a meter of non-superconducting cable and a circulator.