Quantum fluctuations can promote or inhibit glass formation

Glasses are dynamically arrested states of matter that do not exhibit the long-range periodic structure of crystals(1-4). Here we develop new insights from theory and simulation into the impact of quantum fluctuations on glass formation. As intuition may suggest, we observe that large quantum fluctuations serve to inhibit glass formation as tunnelling and zero-point energy allow particles to traverse barriers facilitating movement. However, as the classical limit is approached a regime is observed in which quantum effects slow down relaxation making the quantum system more glassy than the classical system. This dynamical 'reentrance' occurs in the absence of obvious structural changes and has no counterpart in the phenomenology of classical glass-forming systems.