Thermodynamic formula for the cumulant generating function of time-averaged current

TL;DR

This paper derives a thermodynamic formula for the cumulant generating function of time-averaged current in non-equilibrium Brownian systems, linking it to a variational principle and extending linear response theory.

Contribution

It introduces a new thermodynamic formula for the cumulant generating function, connecting it to a variational principle and extending fluctuation-response relations beyond linear response.

Findings

The first derivative of the cumulant generating function equals the current expectation in a modified system.

The formula generalizes the fluctuation-dissipation relation beyond linear response.

Application to a driven Brownian particle demonstrates the formula's practical utility.

Abstract

The cumulant generating function of time-averaged current is studied from an operational viewpoint. Specifically, for interacting Brownian particles under non-equilibrium conditions, we show that the first derivative of the cumulant generating function is equal to the expectation value of the current in a modified system with an extra force added, where the modified system is characterized by a variational principle. The formula reminds us of Einstein's fluctuation theory in equilibrium statistical mechanics. Furthermore, since the formula leads to the fluctuation-dissipation relation when the linear response regime is focused on, it is regarded as an extension of the linear response theory to that valid beyond the linear response regime. The formula is also related to previously known theories such as the Donsker-Varadhan theory, the additivity principle, and the least dissipation principle, but it is not derived from them. Examples of its application are presented for a driven Brownian particle on a ring subject to a periodic potential.