  Photo-induced switching between collective quantum states of matter is a
fascinating rising field with exciting opportunities for novel technologies.
Presently very intensively studied examples in this regard are nanometer-thick
single crystals of the layered material 1T-TaS2 , where picosecond laser pulses
can trigger a fully reversible insulator-to-metal transition (IMT). This IMT is
believed to be connected to the switching between metastable collective quantum
states, but the microscopic nature of this so-called hidden quantum state
remained largely elusive up to now. Here we determine the latter by means of
state-of-the-art x-ray diffraction and show that the laser-driven IMT involves
a marked rearrangement of the charge and orbital order in the direction
perpendicular to the TaS2-layers. More specifically, we identify the collapse
of inter-layer molecular orbital dimers, which are a characteristic feature of
the insulating phase, as a key mechanism for the non-thermal IMT in 1T-TaS2,
which indeed involves a collective transition between two truly long-range
ordered electronic crystals.

Strongly Correlated Electrons