To appreciate some of the most interesting quantum phenomena associated with electrons in a material, imagine a flock of birds. Although each individual bird has its own flight pattern, the flock as a whole moves in sync and avoids any collisions. Such collective motion is ubiquitous in nature and is behind a central concept in condensed-matter physics: emergence. It describes how assemblies of objects behave in ways fundamentally different from their individual constituents. Examples include social networks and superconducting materials, whose collective properties come from pairs of electrons. As Nobel laureate Philip Anderson explains in his 1972 article “More is different,” understanding that collective behavior is a fundamentally different task than understanding isolated components.1
Most theories developed to understand collective behaviors in materials hold only in the presence of equilibrium conditions, such as when energy is conserved. But releasing a physical system from that equilibrium constraint—for example, by introducing...