  When classical systems fail to explore their entire configurational space,
intriguing macroscopic phenomena like aging and glass formation may emerge.
Also closed quanto-mechanical systems may stop wandering freely around the
whole Hilbert space, even if they are initially prepared into a macroscopically
large combination of eigenstates. Here, we report numerical evidences that the
dynamics of strongly interacting lattice bosons driven sufficiently far from
equilibrium can be trapped into extremely long-lived inhomogeneous metastable
states. The slowing down of incoherent density excitations above a threshold
energy, much reminiscent of a dynamical arrest on the verge of a glass
transition, is identified as the key feature of this phenomenon. We argue that
the resulting long-lived inhomogeneities are responsible for the lack of
thermalization observed in large systems. Such a rich phenomenology could be
experimentally uncovered upon probing the out-of-equilibrium dynamics of
conveniently prepared quantum states of trapped cold atoms which we hereby
suggest.
