Quantum circuits and the superconductor-insulator transition in a disordered superconductor

Overview & Synopsis

Strongly disordered superconductors undergo a transition to insulation upon increasingdisorder, driven by the localization of preformed Cooper pairs. A key and implicit question in this transition is the fate of the superfluid density as the critical disorder approaches.By measuring the kinetic inductance of superconducting resonators ofamorphous indium oxide, we show that the transition to superconductivity is bosonicwith a critical temperature ruled by superconducting phase fluctuations. Upon reachingthe critical disorder terminating superconductivity, the superfluid stiffness remainssurprisingly finite at the transition to insulation. This indicates an unexpected first-ordernature of the disorder-driven quantum phase transition to insulator, opening anew paradigm for superfluid disordered systems of quantum particles.An important by-product of the interplay between localization and superconductivity is the obtention of large kinetic inductance materials, allowing the design of the sought-after superinductances which play a major role in recent quantum circuit developments.