Characterization of two-level system noise for microwave kinetic inductance detector comprising niobium film on silicon substrate

A microwave kinetic inductance detector (MKID) is a cutting-edge superconducting detector. It comprises a resonator circuit constructed with a superconducting film on a dielectric substrate. To expand its field of application, it is important to establish a method to suppress the two-level system (TLS) noise that is caused by the electric fluctuations between the two energy states at the surface of the substrate. The time dependence of the fluctuation causes a frequency dependence. Similarly, the electric field power dependence of the fluctuation causes a readout power dependence. The electric field density can be decreased by expanding the strip width (S) and gap width from the ground plane (W) in the MKID circuit, allowing the suppression of TLS noise. However, the effectiveness of this method has not yet been confirmed for MKIDs made with niobium films on silicon substrates. In this study, we demonstrate its effectiveness for such MKIDs. We expanded the dimension of the circuit from (S, W) = (3.00{\mu}m,4.00{\mu}m) to (S, W) = (5.00{\mu}m, 23.7{\mu}m), and achieved an increased suppression of 5.5dB in TLS noise.