Fluctuation theorems extended to quantum coherence
Fluctuation theorems pin down how entropy-decreasing fluctuations, though exponentially rare, are not strictly forbidden in small systems. A new photonic experiment extends one of these theorems into the quantum regime, where coherence complicates the very notion of a time-reversed process.
From Jarzynski and Crooks to quantum coherence
The fluctuation theorems of Jarzynski (1997) and Crooks (1999) relate the work done in a forward process to that done in its time-reverse, turning the second law from an inequality into a precise statistical equality. They make quantitative the idea that a small system can momentarily run "backwards", lowering its entropy, with a probability that falls off exponentially with the size of the fluctuation. Carrying these results into quantum mechanics is subtle, because quantum coherence has no classical counterpart and is sensitive to measurement.
A photonic test
Reporting in Science Advances, researchers used a photonic platform to validate a generalized Crooks-type quantum fluctuation theorem that holds even when coherence is present. The framework relates forward and time-reversed processes through generalized entropy-production components — which can even take complex values — and involves multiple time-reversal operations, reflecting the richer structure of quantum dynamics compared with the classical case.
Why it matters
Establishing fluctuation relations that survive in the presence of coherence sharpens our understanding of irreversibility and the arrow of time at the quantum scale. It also provides tools for quantum thermodynamics, where the cost and reversibility of operations on coherent states bear directly on emerging quantum technologies.
Source: Science Advances (2025); open version on arXiv:2506.00524.