The quantum Mpemba effects

The Mpemba effect, in which a hotter system can equilibrate faster than a cooler one, has long been a subject of fascination in classical physics. In the past few years, notable theoretical and experimental progress has been made in understanding its occurrence in both classical and quantum systems. In this Perspective, we provide a concise overview of recent work and open questions on the Mpemba effect in quantum systems, with a focus on both open and isolated dynamics, which give rise to distinct manifestations of this anomalous non-equilibrium phenomenon. We discuss key theoretical frameworks, highlight experimental observations and explore the fundamental mechanisms that give rise to anomalous relaxation behaviours. Particular attention is given to the role of quantum fluctuations, integrability and symmetry in shaping equilibration pathways. In recent years, notable theoretical and experimental progress has been made in understanding both the classical and quantum versions of the Mpemba effect, in which a hotter system freezes faster than a cooler one. This Perspective discusses this phenomenon in open and isolated quantum systems.