Switching current distributions of superconducting nanowires: Evidence of quantum phase slip events

Phase slips are topological fluctuation events that carry the superconducting order-parameter field between distinct current carrying states. Owing to these phase slips low-dimensional superconductors acquire electrical resistance. In quasi-one-dimensional nanowires it is well known that at higher temperatures phase slips occur via the process of thermal barrier crossing by the order-parameter field. At low temperatures, the general expectation is that phase slips should proceed via quantum tunneling events, which are known as quantum phase slips (QPS). However, experimental observation of QPS is a subject of strong debate and no consensus has been reached so far about the conditions under which QPS occurs. In this study, strong evidence for individual quantum tunneling events undergone by the superconducting order-parameter field in homogeneous nanowires is reported. This is accomplished via measurements of the distribution of switching currents---the high-bias currents at which superconductivity gives way to resistive behavior---whose width exhibits a rather counter-intuitive, monotonic increase with decreasing temperature. A stochastic model of phase slip kinetics which relates the basic phase slip rates to switching rates is outlined. Comparison with this model indicates that the phase predominantly slips via thermal activation at high temperatures but at sufficiently low temperatures switching is caused by individual topological tunneling events of the order-parameter field, i.e., QPS. Importantly, measurements show that in nanowires having larger critical currents quantum fluctuations dominate thermal fluctuations up to higher temperatures. This fact provides strong support for the view that the anomalously high switching rates observed at low temperatures are indeed due to QPS, and not consequences of extraneous noise or hidden inhomogeneity of the wire. In view of the QPS that they exhibit, superconducting nanowires are important candidates for qubit implementations.