Quantum principle allows heat to flow from cold to hot

According to Physics World, an international team has experimentally demonstrated something that seems to violate the second law of thermodynamics: heat flowing from a cold object to a hot one. The researchers, who published their work in Physical Review Letters, used a quantum switch made of photons placed in a superposition—a state where the order of two operations remains undefined until measured. The result appears to break thermodynamics, but the team showed it actually preserves the second law through a subtle trade-off: the control bit of the quantum switch acts like a demon's memory, and the entropy generated by resetting that bit ensures total entropy still increases. The team even built a quantum engine that extracts work while moving heat from cold to hot, demonstrating that quantum mechanics can perform thermodynamic feats impossible in the classical world.

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