Quantum Phase Transitions Go

U nderstanding matter—a collection of many interacting constituents, such as atoms and electrons—is a major endeavor in physics. While most studies in this field have focused on matter that is in thermal equilibrium with its environment, many phenomena occurring in nature involve matter out of equilibrium. The reason for this disparity may lie in the often-repeated observation that, to paraphrase Leo Tolstoy’s quote about happy and unhappy families, all forms of equilibrium matter are alike whereas every form of nonequilibrium matter seemingly behaves in its own way. It’s easy to take a model describing interacting atoms, run it on the computer, and study its nonequilibrium behavior. But it’s much harder to describe classes of nonequilibrium phenomena. Rainer Blatt from the Austrian Academy of Sciences and the University of Innsbruck, Austria, and colleagues [1] have now succeeded in observing one such class of phenomena—dynamical quantum phase transitions—in a quantum many-body spin system. These transitions are similar to those that take place when ice melts or water evaporates, but they are not driven by external factors such as temperature. Instead, they are triggered by internal changes in a nonequilibrium quantum system as it evolves in time.