Entangled sensor-networks for dark-matter axion searches*

The hypothetical axion particle (of unknown mass) is a leading candidate for dark matter (DM). Many experiments search for axions with microwave cavities, where an axion may convert into a cavity photon, leading to a feeble excess in the output power of the cavity. Recent work has demonstrated that injecting squeezed vacuum into the cavity can substantially accelerate the axion search. Here, we go a step further and provide a theoretical framework to leverage the benefits of quantum squeezing in a network setting consisting of many sensor-cavities. We explore performance advantage from a local entanglement sensor network, which enjoys both coherence between the axion signals and entanglement between the sensors. Our analysis will be pertinent to next-generation DM-axion searches wishing to leverage a sensor-network and quantum resources in an optimal way. Finally, we assess the possibility of using a more exotic quantum state, the Gottesman-Kitaev-Preskill (GKP) state. Despite a constant-factor improvement in the scan-time relative to a single-mode squeezed-state in the ideal case, the advantage of employing a GKP state disappears when a practical measurement scheme is considered.