  Predicting practical rates of ion transport from atomistic descriptors
enables the rational design of materials, devices, and processes, which is
especially critical to developing low-carbon energy technologies such as
rechargeable batteries. The correlated mechanisms of ionic conduction,
variation of conductivity with timescale and confinement, and ambiguity in the
vibrational origin of translation, the attempt frequency, call for a direct
atomic probe of the most fundamental steps of ionic diffusion: ion hops.
However, such hops are rare-event large-amplitude translations, and are
challenging to excite and detect. Here we use single-cycle terahertz pumps to
impulsively trigger ionic hopping in battery solid electrolytes. This is
visualized by an induced transient birefringence enabling direct probing of
anisotropy in ionic hopping on the picosecond timescale. The relaxation of the
transient signal measures the decay of orientational memory, and the production
of entropy in diffusion. We extend experimental results using in silico
transient birefringence to identify attempt frequencies for ion hopping. Using
nonlinear optical methods, we probe ion transport at its fastest limit,
distinguish correlated conduction mechanisms from a true random walk at the
atomic scale, and demonstrate the connection between activated transport and
the thermodynamics of information.
