Charged fluctuators as a limit to the microscopic and macroscopic coherence of superconductors

By analyzing experiments on thin-film resonators of NbSi and TiN, we shed light on a novel decoherence mechanism at work in disordered superconductors. This decoherence is caused by charged Two Level Systems (TLS) which couple to the conduction electrons in the BCS ground state, inducing fluctuations of the kinetic inductance. Standard theories of mesoscopic disordered conductors are used to describe this effect, linking electronic (microscopic) decoherence and electromagnetic (macroscopic) decoherence in superconductors. This model is compared to the so called Generalised Tunelling Model (GTM), used to describe the impact of fluctuating TLS on the dielectric properties of the resonators. Given the omnipresence of charged TLS in solid-state systems, these decoherence mechanisms affects all experiments involving disordered superconductors, and even more so for devices with smaller cross-sections through the new mechanism presented here. In particular, we show it easily explains the poor coherence observed in quantum phase slip experiments and may contribute to lowering the quality factors in disordered superconductor resonators.