Under certain conditions, warm water can freeze faster than cold water. This phenomenon was named the Mpemba effect after Erasto Mpemba, a Tanzanian high schooler who described the effect in the 1960s. The phenomenon has sparked intense debates for more than two millennia and continues to do so. Similar processes, in which a system relaxes to equilibrium more quickly if it is initially further away from equilibrium, are being intensely explored in the microscopic world. Now three research teams provide distinct perspectives on quantum versions of Mpemba-like effects, emphasizing the impact of strong interparticle correlations, minuscule quantum fluctuations, and initial conditions on these relaxation processes. The teams’ findings advance quantum thermodynamics and have potential implications for technologies, ranging from information processors to engines, powered by quantum resources.
In top-down strategies, physicists use observations of macroscopic (classical) phenomena to infer fundamental microscopic (quantum) processes; in bottom-up strategies, they use studies of those fundamental processes to predict classical phenomena. Historically, studies of the Mpemba effect began with empirical observations and ad hoc assumptions about the microscopic world. Despite descriptions of the effect by Aristotle and Descartes, and modern attention from Mpemba, the phenomenon has not influenced the field of thermodynamics. The Mpemba effect is complex, lacks a precise definition, and has reproducibility issues. As a result, experimental observations and explanations have been debated for decades without consensus, making the effect often seem like just a curiosity.
To study the Mpemba effect and other relaxation processes, physicists define a figure of merit, such as temperature, for the system of interest and record it as a function of the duration over which the system interacts with an external environment. This relationship quantifies how quickly the system goes from an initial state to a steady state, determining whether the faster of two paths to equilibrium is the one that begins further away from equilibrium. A few years ago, researchers made great strides by observing that a colloidal system can cool exponentially faster if it starts at a higher temperature, in alignment with specific theoretical models. This work strengthened the top-down strategy, leading these scientists to conclude that the Mpemba effect is not merely a curiosity but an archetype for a wide range of anomalous relaxation phenomena...
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