Measurement of a microwave cavity via hybridization with a remote Josephson circuit

Efforts to accelerate cavity-based axion searches beyond the quantum-limited rate are hindered by the incompatibility of Josephson junctions with the strong magnetic field required for axion-photon conversion. Signals must be transported between regions of high and low field strength, introducing noise and loss before amplification can occur. Here, we present remote readout of a microwave cavity where the first-stage amplification occurs in situ. We hybridize the cavity mode with a degenerate Josephson circuit mode, using a waveguide to mediate the coupling between them while providing the distance required for shielding. Using the Josephson circuit to induce a two-mode squeezing and frequency-conversion interaction, the state of the cavity mode is amplified in transmission to the measurement port. We demonstrate several dB of gain, and we describe straightforward improvements that can be made to the existing setup to increase the amplification. With an optimized setup, we expect this method to surpass the quantum-limited scan rate of modern haloscopes, providing close to a factor of 10 acceleration.