Fig. 1: Conceptual plot of the entanglement-enhanced microwave haloscopes for axion dark matter search.

A pair of entangled probes, including the signal (red beam) and the ancilla (blue beam), are generated at the bottom-left box. Note that the blue beam becomes inaccessible when entanglement assistance is forbidden. The signal probe is then shined on the input port of a microwave cavity, which is coupled with the axion (top-left beam) via a strong magnetic field. At this moment, the unknown parameter of axion is encoded on the signal probe, while the ancilla is shared with the receiver intact. The receiver applies a nonlinear processing to the returned probes jointly, e.g., a two-mode squeezer for the entanglement-enhanced case. Finally, the receiver collects the photon counts and estimate the unknown parameter of axion based on the readout. Background image credit: ©James Webb Space Telescope.