Optimization of the coupling of thin-film superconducting interferometers with a dielectric resonator

W. Richter, G. Thomas Albrecht

A. Larsen, Hans Dalsgaard Jensen, J. Mygind

W. Richter, G. Thomas Albrecht

C.J. Ballard, Rangga P. Budoyo, Kristen Voigt +3 more

Kristen Voigt, Jared Hertzberg, Jennifer E. Hoffman +10 more

We have coupled a tapered optical microfiber to a translatable thin-film lumped-element superconducting Al microwave resonator that is cooled to 15 mK. The thin-film resonator has a resonance frequency of 6.14 GHz, a quality factor of Q = 2.59 × 105, and is mounted inside a 3-D Al microwave cavity that provides a well-isolated electromagnetic environment. The microfiber is held fixed while the cavity is mounted on an x-z translation stage that allows the lumped-element resonator and the optical fiber to be brought close to each other (less than 1 mm). When 780-nm light is sent through the fiber, Rayleigh scattering in the fiber causes a weak position-dependent nonuniform illumination of the thin-film resonator, exciting quasiparticles in the superconducting film and thereby affecting its resonance frequency foand Q. We report on the response of the resonator to both the scattered optical power and the presence of the fiber's dielectric, as the thin-film resonator is moved in situ with respect to the fiber.

Ch. Kaiser, Sebastian T. Skacel, S. Wünsch +4 more

We have developed a reliable method for the direct measurement of dielectric losses in thin films using a special design of lumped element superconducting resonators. The method presented allows obtaining quantitative values for the losses in the film volume as well as the metal/dielectric interfaces. Numerous different resonator geometries were designed and measured in order to study the losses in dielectric thin films usually used for Josephson junction fabrication, such as Nb 2 O 5 , SiO and SiN x , at low temperatures and low gigahertz frequencies. The results show that for such amorphous materials, the bulk losses clearly exceed the interface losses. Furthermore, the frequency dependence of the losses in this working regime was studied for the first time. Our results are in good agreement with the universal law describing the frequency dependence of dielectric losses and suggest that the losses are due to many-body interacting dipoles, which is an important fact for the theoretical modelling of two-level fluctuators. We also investigated the losses in multi-layer films and find that the results are in good agreement with the theoretical expectations, which allows optimization of the dielectric multi-layers used in Josephson junction and qubit fabrication.