  Metastable phases present a promising route to expand the functionality of
complex materials. Of particular interest are light-induced metastable phases
that are inaccessible under equilibrium conditions, as they often host new,
emergent properties switchable on ultrafast timescales. However, the processes
governing the trajectories to such hidden phases remain largely unexplored.
Here, using time- and angle-resolved photoemission spectroscopy, we investigate
the ultrafast dynamics of the formation of a hidden quantum state in the
layered dichalcogenide 1T-TaS$_2$ upon photoexcitation. Our results reveal the
nonthermal character of the transition governed by a collective
charge-density-wave excitation. Utilizing a double-pulse excitation of the
structural mode, we show vibrational coherent control of the phase-transition
efficiency. Our demonstration of exceptional control, switching speed, and
stability of the hidden phase are key for device applications.
