The second law of thermodynamics states that the entropy of an isolated system tends to increase. But statistical mechanics shows that transient reductions in entropy can occur with a tiny probability. Central to the theorems that describe these fluctuations is microscopic reversibility—the idea that the probability of a system taking a specific trajectory through phase space is related to the probability of it taking the time-reversed version of that trajectory. Now Hyukjoon Kwon at the Korea Institute for Advanced Study and his colleagues have demonstrated a quantum version of this principle [1].

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