Microwave Properties of Exfoliated Hexagonal Boron Nitride*

Interfacial and bulk dielectric losses in superconducting qubits are some of the key factors limiting device coherence. We investigate the microwave properties of thin flakes of hexagonal boron nitride (h-BN), which have the potential to serve as low-loss elements in superconducting transmon qubits. The flakes, which are exfoliated from high-quality single-crystals to thicknesses of a few tens of nanometers, have atomically flat interfaces and low defect densities. Our process allows them to be coated with superconducting layers on both sides, thus forming parallel-plate capacitors. By embedding these capacitors in superconducting microwave resonators, we can determine the dielectric constant and loss tangent of h-BN under conditions relevant for qubit operation. Apart from the promise of improved coherence, the parallel-plate geometry would enable qubits to be designed with drastically reduced footprints and cross-talk compared to planar devices.