Remote entanglement of a cavity with a Josephson circuit for axion search acceleration*

A quantum sensing technique involving squeezed vacuum states was recently used to circumvent the quantum limit in the search for microwave-frequency axion dark matter. This was made challenging by the incompatibility of superconducting Josephson circuits with the strong magnetic field used for axion-photon conversion. In that experiment, a cascaded, directional geometry was used to transport squeezed states to and from axion-sensitive regions of high magnetic field strength and to protect the cavity from amplifier backaction. This approach yielded a factor of 2 improvement over the quantum-limited search rate. Nevertheless, scanning the 1-10 GHz frequency band at benchmark coupling remains a dauntingly time- and resource-expensive task. Realizing additional scan rate enhancement from quantum sensing requires making better use of the nonlinearity of the Josephson circuit. Here, we present progress towards performing entanglement and frequency-conversion on a microwave cavity mode which is coupled to a Josephson three-wave-mixing element via a waveguide. The quantum-non-demolition nature of the resulting interaction enables us to circumvent the limits imposed by a cascaded geometry, unlocking the potential for further scan rate enhancement.