# Coherent photon manipulation in interacting atomic ensembles

## TL;DR

This paper introduces a novel, largely coherent method for strong photon interactions in atomic ensembles, enabling tunable photon reflection with minimal losses, advancing quantum optics applications.

## Contribution

It proposes a new interaction mechanism based on polariton conversion, reducing dissipation compared to traditional Rydberg blockade methods.

## Key findings

- Enables single-photon to mirror conversion with tunable phase.
- Reduces photon loss in nonlinear optical interactions.
- Potential for high-fidelity quantum optical devices.

## Abstract

Coupling photons to Rydberg excitations in a cold atomic gas yields unprecedentedly large optical nonlinearities at the level of individual light quanta, where the formation of nearby dark-state polaritons is blocked by the strong interactions between Rydberg atoms. This blockade mechanism, however, realizes an inherently dissipative nonlinearity, which limits the performance of practical applications. In this work, we propose a new approach to strong photon interactions via a largely coherent mechanism at drastically suppressed photon losses. Rather than a polariton blockade, it is based on an interaction induced conversion between distinct types of dark-state polaritons with different propagation characteristics. We outline a specific implementation of this approach and show that it permits to turn a single photon into an effective mirror with a robust and continuously tuneable reflection phase. We describe potential applications, including a detailed discussion of achievable operational fidelities.