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.