Integrating planar circuits with superconducting 3D microwave cavities using tunable low-loss couplers

"We design and test a low-loss interface between superconducting three-dimensional microwave cavities and two-dimensional circuits, where the coupling rate is highly tunable. This interface seamlessly integrates a loop antenna and a Josephson junction-based coupling element. We demonstrate that the loss added by connecting this interface to the cavity is 1.28 kHz, corresponding to an inverse quality factor of 1 / ( 4.5 × 10 6 ).