Tunable superconducting microstrip resonators

We report on a simple yet versatile design for a tunable superconducting microstrip resonator. Niobium nitride is employed as the superconducting material and aluminum oxide, produced by atomic layer deposition, as the dielectric layer. We show that the high quality of the dielectric material allows to reach the internal quality factors in the order of Qi∼104 in the single photon regime. Qi rapidly increases with the number of photons in the resonator N and exceeds 105 for N∼10−50. A straightforward modification of the basic microstrip design allows to pass a current bias through the strip and to control its kinetic inductance. We achieve a frequency tuning δf=62 MHz around f0=2.4 GHz for a fundamental mode and δf=164 MHz for a third harmonic. This translates into a tuning parameter Qiδf/f0=150. The presented design can be incorporated into essentially any superconducting circuitry operating at temperatures below 2.5 K.