Solution to the Fokker-Planck equation for slowly driven Brownian motion: Emergent geometry and a formula for the corresponding thermodynamic metric

TL;DR

This paper develops a rigorous perturbative solution to the Fokker-Planck equation for driven Brownian motion, deriving an exact thermodynamic metric and showing its role in optimal control protocols with geometric insights.

Contribution

It introduces a mathematically rigorous perturbation theory for the Fokker-Planck equation, leading to an exact formula for the thermodynamic metric in control parameter space.

Findings

Derived an exact formula for the thermodynamic metric.

Optimal protocols minimize the length defined by this metric.

The geometric structure emerges from the perturbative expansion.

Abstract

Considerable progress has recently been made with geometrical approaches to understanding and controlling small out-of-equilibrium systems, but a mathematically rigorous foundation for these methods has been lacking. Towards this end, we develop a perturbative solution to the Fokker-Planck equation for one-dimensional driven Brownian motion in the overdamped limit enabled by the spectral properties of the corresponding single-particle Schr\"odinger operator. The perturbation theory is in powers of the inverse characteristic timescale of variation of the fastest varying control parameter, measured in units of the system timescale, which is set by the smallest eigenvalue of the corresponding Schr\"odinger operator. It applies to any Brownian system for which the Schr\"odinger operator has a confining potential. We use the theory to rigorously derive an exact formula for a Riemannian "thermodynamic" metric in the space of control parameters of the system. We show that up to second-order terms in the perturbation theory, optimal dissipation-minimizing driving protocols minimize the length defined by this metric. We also show that a previously proposed metric is calculable from our exact formula with corrections that are exponentially suppressed in a characteristic length scale. We illustrate our formula using the two-dimensional example of a harmonic oscillator with time-dependent spring constant in a time-dependent electric field. Lastly, we demonstrate that the Riemannian geometric structure of the optimal control problem is emergent; it derives from the form of the perturbative expansion for the probability density and persists to all orders of the expansion.