  Hidden states of matter with novel and unusual properties may be created if a
system out of equilibrium can be induced to follow a trajectory to a state
which is inaccessible or does not even exist under normal equilibrium
conditions. Here we report on the discovery of a hidden (H) topologically
protected electronic state in a layered dichalcogenide 1T-TaS2 crystal reached
as a result of a quench caused by a single 35 fs laser pulse. The properties of
the H state are markedly different from any other state of the system: it
exhibits a large drop of electrical resistance, strongly modified single
particle and collective mode spectra and a marked change of optical
reflectivity. Particularly important and unusual, the H state is stable for an
arbitrarily long time until a laser pulse, electrical current or thermal erase
procedure is applied, causing it to revert to the thermodynamic ground state.
Major observed events can be reproduced by a kinetic model describing the
conversion of photo excited electrons and holes into an electronically ordered
crystal, thus converting a Mott insulator to a conducting H state. Its
long-time stability follows from the topological protection of the number of
periods in the electronic crystal.
