# Spin squeezing by one-photon-two-atom excitations processes in atomic ensembles

## TL;DR

This paper explores a collective atomic process where two atoms absorb one photon simultaneously, enabling efficient spin squeezing and entanglement in atomic ensembles, with potential applications in quantum sensing.

## Contribution

It introduces a novel collective process for spin squeezing using one-photon-two-atom excitations, leading to effective many-body spin interactions.

## Key findings

- Enables significant spin squeezing at the single-photon level
- Creates two-axis twisting-like spin interactions
- Applicable to on-chip quantum sensing architectures

## Abstract

It has been shown elsewhere that two spatially separated atoms can jointly absorb one photon, whose frequency is equal to the sum of the transition frequencies of the two atoms. We describe this process in the presence of an ensemble of many two-level atoms, and show that it can be used to generate spin squeezing and entanglement. This resonant collective process allows to create a sizeable squeezing already at the single-photon limit. It represents a novel way for generating manybody spin-spin interactions, yielding a two-axis twisting-like interaction among the spins, which is very effcient for the generation of spin squeezing. We perform explicit calculations for ensembles of magnetic molecules coupled to a superconducting coplanar cavities. This system represents an attractive on-chip architecture for the realization of improved sensing.