Circuit cavity optomechanics approaching the ultrastrong coupling regime

Many of the recent advances in quantum control and measurement of cavity optomechanical systems were enabled by the ability to reach the strong coupling regime, where the parametrically-enhanced optomechanical coupling rate is larger than any decoherence rate in the system. Ideally, the coupling could be further increased to the ultrastrong regime, where it approaches or exceeds the mechanical resonance frequency. In this talk, I will present experimental progress on a new architecture for cavity optomechanics where a mechanically compliant vacuum-gap capacitor resonates with a 3-dimensional microwave cavity. Compared to the planar superconducting microwave resonators used in previous experiments, the cavity resonator provides a similar vacuum coupling rate but superior power handling capability, allowing us to achieve driven optomechanical coupling rates improved by an order of magnitude over previous results. This architecture for microwave cavity optomechanics could therefore enable a new generation of experiments in the ultra-strong coupling regime.