@Nature Communications: Reversion of heat flow using quantum correlations

Irreversibility is a longstanding puzzle in physics. While microscopic laws of motion are invariant under time reversal, all macroscopic phenomena have a preferred direction in time. Heat, for instance, spontaneously flows from hot to cold. Eddington has called this asymmetry the arrow of time. At the phenomenological level, the second law of thermodynamics allows one to predict which processes are possible in nature: only those with non-negative mean entropy production do occur. A microscopic resolution of the apparent paradox of irreversibility was put forward by Boltzmann, when he noted that initial conditions break the time-reversal symmetry of the otherwise reversible dynamics. In addition to breaking time reversal, initial conditions also determine the arrow’s direction. The observation of the average positivity of the entropy production in nature is often explained by the low entropy value of the initial state. This opens the possibility to control or even reverse the arrow of time depending on the initial conditions.

In standard thermodynamics, systems are assumed to be uncorrelated before thermal contact. As a result, according to the second law heat will flow from the hot to the cold body. However, it has been theoretically suggested that for quantum correlated local thermal states, heat might flow from the cold to the hot system, thus effectively reversing the arrow’s direction.

We report the experimental demonstration of the reversal of heat flow for two initially quantum correlated qubits (two spin-1/2 systems) prepared in local thermal states at different effective temperatures employing Nuclear Magnetic Resonance (NMR) techniques.

Read more: Nature Communications 10, 2456 (2019) | pdf |

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