# Fundamental limits on nonequilibrium sensing

## TL;DR

This paper explores how nonequilibrium sensors can surpass standard limits by using nonreciprocal interactions to boost signal-to-noise ratios.

## Contribution

The study identifies how nonreciprocal couplings enable arbitrarily large signal-to-noise ratios in nonequilibrium systems.

## Key findings

- Nonreciprocal interactions allow violations of the fluctuation-dissipation relation, improving sensor performance.
- Signal-to-noise ratios can be arbitrarily large at low frequencies with optimized parameters.
- Such enhancements occur without increasing overall dissipation.

## Abstract

The performance of equilibrium sensors is restricted by the laws of equilibrium thermodynamics. Here, we investigate the physical limits on nonequilibrium sensing in bipartite systems with both reciprocal and nonreciprocal couplings. We show that one of the subsystems, acting as a Maxwell demon, can significantly suppress the fluctuations of the other subsystem relative to its response to an external perturbation. The importance of nonreciprocal interactions for such negative violations of the fluctuation-dissipation relation to occur is identified. We further demonstrate that these violations can considerably improve the signal-to-noise ratio above its corresponding equilibrium value, allowing the subsystem to operate as an enhanced sensor. In addition, we find that the nonequilibrium signal-to-noise ratio of linear systems may be arbitrarily large at low frequencies after proper parameter optimization, even at a fixed overall amount of dissipation. These results indicate that highly accurate nonreciprocal sensors can be designed at a finite energetic cost.

It has been suggested that operating sensors far from thermodynamical equilibrium can enhance signal-to-noise ratio in a way which breaks the standard limits coming from stochastic fluctuations. Here, the authors show how this can in principle allow an arbitrarily large signal-to-noise ratio.