# Realization of nonlinear optical nonreciprocity on a few-photon level based on atoms strongly coupled to an asymmetric cavity

## TL;DR

This paper demonstrates optical nonreciprocity at the few-photon level using atoms strongly coupled to an asymmetric cavity, enabling low-power optical diodes with tunable power thresholds.

## Contribution

It presents the first experimental realization of nonlinear optical nonreciprocity at the few-photon level leveraging strong atom-cavity coupling and cavity asymmetry.

## Key findings

- Nonreciprocal transmission achieved with extremely low power.
- Nonreciprocity can be tuned by adjusting the number of coupled atoms.
- Potential application as optical diodes in photonic chips.

## Abstract

Optical nonreciprocity is important in photonic information processing to route the optical signal or prevent the reverse flow of noise. By adopting the strong nonlinearity associated with a few atoms in a strongly coupled cavity QED system and an asymmetric cavity configuration, we experimentally demonstrate the nonreciprocal transmission between two counterpropagating light fields with extremely low power. This nonreciprocity can even occur on a few-photon level due to the high optical nonlinearity of the system. The working power can be flexibly tuned by changing the effective number of atoms strongly coupled to the cavity. The idea and result can be applied to optical chips as optical diodes by using fiber-based cavity QED systems. Our work opens up new perspectives for realizing optical nonreciprocity on a few-photon level based on the nonlinearities of atoms strongly coupled to an optical cavity.