  The concept of quantum phase transitions (QPT) plays a central role in the
description of condensed matter systems. In this contribution, we perform
high-quality wavefunction-based simulations to demonstrate the existence of a
quantum phase transition in a crucially relevant molecular system, namely
water, forming linear chains of rotating molecules. We determine various
critical exponents and reveal the water chain QPT to belong to the (1+1)
dimensional Ising universality class. Furthermore, the effect of breaking
symmetries is examined and it is shown that by breaking the inversion symmetry,
the ground state degeneracy of the ordered quantum phase is lifted to yield two
many-body states with opposite polarization. The possibility of forming
ferroelectric phases together with a thermal stability of the quantum critical
regime up to ~10 K makes the linear water chain a promising candidate as a
platform for quantum devices.
