The second law of thermodynamics is a statement about the statistics of the entropy production, $\langle \mathrm{\Sigma }\rangle \ge 0$. For small systems, it is known that the entropy production is a random variable and negative values ($\mathrm{\Sigma }<0$) might be observed in some experiments. This situation is sometimes called apparent violation of the second law. In this sense, how often is the second law violated? For a given average $\langle \mathrm{\Sigma }\rangle$, we show that the strong detailed fluctuation theorem implies a lower tight bound for the apparent violations of the second law. As applications, we verify that the bound is satisfied for the entropy produced in the heat exchange problem between two reservoirs mediated by a bosonic mode in the weak-coupling approximation, a levitated nanoparticle, and a classical particle in a box.

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• Fluctuation theorems
• Nonequilibrium & irreversible thermodynamics
• Quantum thermodynamics
• Thermodynamics