  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 tunnelling events, which are known as
quantum phase slips (QPS). Here we report strong evidence for individual
quantum tunnelling events undergone by the superconducting order-parameter
field in homogeneous nanowires.We accomplish this via measurements of the
distribution of switching currents-the high-bias currents at which
superconductivity gives way to resistive behaviour-whose width exhibits a
rather counter-intuitive, monotonic increase with decreasing temperature. We
outline a stochastic model of phase slip kinetics which relates the basic phase
slip rates to switching rates. 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
tunnelling events of the order-parameter field, i.e., QPS. Importantly,
measurements on several wires show that quantum fluctuations tend to dominate
over thermal fluctuations at larger temperatures in wires having larger
critical currents. This fact provides strong supports 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.
