Hydrogen crystals reduce dissipation in superconducting resonators

Abstract

We show that the internal quality factors of high-impedance superconducting resonators made of granular aluminum can be improved by coating them with micrometric films of solid parahydrogen molecular crystals. We attribute the average measured ≈8% reduction in dissipation to the absorption of stray terahertz radiation at the crystal-resonator interface and the subsequent dissipation of its energy in the form of phonons below the pair-breaking gap. Our results prove that contrary to expectations, replacing the vacuum dielectric atop a superconducting resonator can be beneficial, thanks to the added protection against Cooper pair-braking terahertz radiation. Moreover, at the level of internal quality factors in the 10⁵ range, the hydrogen crystal does not introduce additional losses, which is promising for embedding impurities to couple to superconducting thin-film devices in hybrid quantum architectures.