Flux-Tunable Cavity for Dark Matter Detection

Developing a dark matter detector with wide mass tunability is an immensely desirable property, yet, it is challenging due to maintaining strong sensitivity. Resonant cavities for dark matter detection have traditionally employed mechanical tuning, moving parts around to change electromagnetic boundary conditions. However, these cavities have proven challenging to operate in sub-Kelvin cryogenic environments due to differential thermal contraction, low heat capacities, and low thermal conductivities. Instead, we develop an electronically tunable cavity architecture by coupling a superconducting 3D microwave cavity with a dc flux tunable superconducting quantum interference device. With a flux delivery system engineered to maintain high coherence in the cavity, we perform a hidden-photon dark matter search below the quantum-limited threshold. A microwave photon counting technique is employed through repeated quantum nondemolition measurements using a transmon qubit. With this device, we perform a hidden-photon search and constrain the kinetic mixing angle to 𝜖 <8.2 ×10⁻¹⁵ in a tunable band from 5.672 to 5.694 GHz. By coupling multimode tunable cavities to the transmon, wider hidden-photon searching ranges are possible.