Coherence Preserving Tunable Coupler For 3D Cavities

Superconducting 3D cavities remain the highest coherence objects in the circuit QED community. Control of these long lifetime cavity states requires a nonlinear element, typically a transmon circuit with dipole coupling to one or many cavity modes. Charge drives applied to these transmons have used to demonstrate both beam-splitter and two mode squeezing interactions between neighboring cavities via four wave mixing, but Purcell loss and dielectric loss from the transmon substrate limit the performance of these couplers. We propose a tunable coupler that preserves the coherence of the neighboring cavities by minimizing interactions with lossy degrees of freedom while maintaining a high on/off ratio in coupling strengths. We present a practical tunable coupler implementation using existing high-Q 3D cavity fabrication techniques and estimate the performance of parametric operations between two cavities connected to the coupler.