Water in a glass cools, ice melts, and a kettle heats the stove: that's how the world works: heat always flows from hot to cold. This is the second law of thermodynamics, a law that has never been broken. But the quantum world knows how to surprise: now physicists have made heat flow in the opposite direction, not by breaking the law, but by exploiting the quirky properties of particles.
The scientists assembled a chain of several spins and sent 'collisions' along it — short interactions, like a relay race. By changing the strength and phase of coherence, they controlled the heat flow: sometimes reversing it, sometimes amplifying it. It's like adjusting a faucet, but instead of water, it's a flow of energy at the quantum level.
Such control could be useful in quantum refrigerators or heat engines the size of a molecule. Where ordinary laws of heat get in the way, quantum effects will open new paths. And then heat, like water in a fountain, will flow wherever the engineer's hand directs it.
In the quantum world, heat can flow from cold to hot if particles are in a state of quantum coherence: their wave properties are synchronized, like dancers in a ballet.