# Linear and non-linear coherent coupling in a Bell-Bloom magnetometer

## TL;DR

This paper reveals that linear and non-linear spin-exchange interactions in a Bell-Bloom magnetometer can generate and enhance atomic coherence, improving measurement capabilities in room-temperature alkali vapours.

## Contribution

It demonstrates both theoretically and experimentally that non-linear spin-exchange coupling can create new coherent modes and extend coherence lifetime in a low-density caesium vapour.

## Key findings

- Non-linear spin exchange acts like wave-mixing to generate coherence.
- Coupling increases the natural coherence lifetime.
- Effective at low densities and non-zero magnetic fields.

## Abstract

Spin-exchange collisions in hot vapours are generally regarded as a decoherence mechanism. In contrast, we show that linear and non-linear spin-exchange coupling can lead to the generation of atomic coherence in a Bell-Bloom magnetometer. In particular, we theoretically and experimentally demonstrate that non-linear spin exchange coupling, acting in an analogous way to a wave-mixing mechanism, can create new modes of coherent excitation which inherit the magnetic properties of the natural Larmor coherence. The generated coherences further couple via linear spin-exchange interaction, leading to an increase of the natural coherence lifetime of the system. Notably, the measurements are performed in a low-density caesium vapour and for non-zero magnetic field, outside the standard conditions for collisional coherence transfer. The strategies discussed are important for the development of spin-exchange coupling into a resource for an improved measurement platform based on room-temperature alkali-metal vapours.