Epitaxial hBN on sapphire yields 10^{-6} TLS loss in niobium resonators without degrading Q

Researchers grew epitaxial hBN by molecular beam epitaxy on sapphire, then patterned niobium resonators on both hBN/sapphire and bare-sapphire controls. Radio-frequency measurements swept power, temperature, and time to extract two-level-system loss, resonant frequency shifts, and temporal fluctuations. The hBN devices showed no statistically significant increase in dielectric loss relative to controls, demonstrating that the 2D layer can be inserted into the conventional qubit stack without immediate penalty to coherence.

The work targets phonon-mediated correlated errors by exploiting hBN's low thermal conductivity to block athermal phonons from crossing the superconductor-substrate interface. This approach differs from prior surface-passivation or encapsulation strategies that address TLS directly; instead it interrupts the substrate-phonon pathway that can produce simultaneous qubit errors. The measured loss values place hBN among the lowest-loss crystalline dielectrics tested in superconducting circuits to date.

Supporting measurements on similar hBN films in microwave cavities (npj Quantum Information 2024) and phonon-transport studies of 2D layers on sapphire (Phys. Rev. Applied 2023) corroborate that interface quality, rather than intrinsic hBN loss, remains the dominant variable. Scaling to full qubit arrays will require uniform wafer-scale growth and via-compatible processing.

Next steps include integrating the hBN layer beneath transmon capacitors and testing phonon-blocking efficacy with controlled quasiparticle injection experiments planned for 2027.