Fast Tunable High-Q-Factor Superconducting Microwave Resonators

We present fast tunable superconducting microwave resonators fabricated from planar NbN on a sapphire substrate. The 3 lambda/4 wavelength resonators are tuning fork shaped and tuned by passing a dc current that controls the kinetic inductance of the tuning fork prongs. The lambda/4 section from the open end operates as an integrated impedance converter that creates a nearly perfect short for microwave currents at the dc terminal coupling points, thus preventing microwave energy leakage through the dc lines. We measure an internal quality factor Q(int) > 10(5) over the entire tuning range. We demonstrate a tuning range of greater than 3% and tuning response times as short as 20 ns for the maximum achievable detuning. Because of the quasifractal design, the resonators are resilient to magnetic fields of up to 0.5 T