Sensitively searching for microwave dark photons with atomic ensembles

Dark photon is one of the promising candidates of light dark matter and could be detected by using its interaction with standard model particles via kinetic mixings. Here, we propose a feasible approach to detect the dark photons by nondestructively probing these mixing-induced quantum state transitions of atomic ensembles. Compared with the scheme by probing the mixing-induced quantum excitation of single-atom detector, the achievable detection sensitivity can be enhanced theoretically by a factor of $\sqrt{N}$ for the ensemble containing $N$ atoms. Specifically, we show that the dark photons, in both centimeter- and millimeter-wave bands, could be detected by using the artificial atomic ensemble detector, generated by surface-state electrons on liquid Helium. It is estimated that, with the detectable transition probability of $10^{-4}$, the experimental surface-state electrons (with $N = 10^8$ trapped electrons) might provide a feasible approach to search for the dark photons in $18.61-26.88$ $\mu$eV and $496.28-827.13$ $\mu$eV ranges, within about two months. The confidence level can exceed 95\% for the achievable sensitivities being $10^{-14} \sim 10^{-13}$ and $10^{-12} \sim 10^{-11}$, respectively. In principle, the proposal could also be generalized to the other atomic ensemble detectors for the detection of dark photons in different frequency bands.