  Glasses prepared by physical vapour deposition have been shown to be
remarkably more stable than those prepared by standard cooling protocols, with
properties that appear to be similar to systems aged for extremely long times.
When subjected to a rapid rise in temperature, ultrastable glasses anneal
towards the liquid in a qualitatively different manner than ordinary glasses,
with the seeming competition of different timescales and lengthscales. We
numerically reproduce the phenomenology of ultrastable glass annealing with a
kinetically constrained model, a three dimensional East model with soft
constraints, in a setting where the bulk is in an ultrastable configuration and
a free surface is permanently excited. Annealing towards the liquid state is
given by the competition between the ballistic propagation of a front from the
free surface and a much slower nucleation-like relaxation in the bulk. The
crossover between these mechanisms also explains the change in behaviour with
film thickness seen experimentally.
