Scalable architecture for dark photon searches: Superconducting-qubit proof of principle

The dark photon is a well-motivated candidate of dark matter due to its potential to open the window of new physics beyond the Standard Model. A fundamental mass-range-sensitivity dilemma is always haunting the dark photon searching experiments: The resonant haloscopes have excellent sensitivity but are narrowband, and vice versa for the non-resonant ones. A scalable architecture integrating numerous resonant haloscopes will be a desirable solution to this dilemma. However, even the concept of scalable searching remains rarely explored, due to the size limitation of conventional haloscopes imposed by the dark photon wavelength. Here we propose and demonstrate a novel architecture using superconducting qubits as sub-wavelength haloscope units. By virtue of the scalability of superconducting qubits, it is possible to integrate multiple qubits with different frequencies on a chip-scale device. Furthermore, the frequencies of the qubits can be tuned to extend the searching mass range. Thus, our architectures allow for searching for dark photons in a broad mass range with high sensitivity. As a proof-of-principle experiment, we designed and fabricated a three-qubit chip and successfully demonstrated a scalable dark-photon searching. Our work established constraints on dark photons in the mass range of 15.632 $\mu$eV$\sim$15.638 $\mu$eV, 15.838 $\mu$eV$\sim$15.845 $\mu$eV, and 16.463 $\mu$eV$\sim$16.468 $\mu$eV, simultaneously, and the constraints are much more stringent than the cosmology constraints. Our work can be scaled up in the future to boost the scrutiny of new physics and extended to search for more dark matter candidates, including dark photons, axions and axion-like particles.