# Entanglement of two quantum memories via fibers over dozens of kilometres

## TL;DR

This paper demonstrates long-distance entanglement of atomic ensembles over dozens of kilometers using fiber-optic transmission and quantum frequency conversion, advancing quantum internet capabilities.

## Contribution

It achieves entanglement over 22 km and 50 km fibers, surpassing previous physical separation limits with novel fiber-based quantum communication techniques.

## Key findings

- Entanglement over 22 km field-deployed fibers.
- Entanglement over 50 km coiled fibers.
- Use of cavity enhancement and quantum frequency conversion.

## Abstract

Quantum internet will enable a number of revolutionary applications. It relies on entanglement of remote quantum memories over long distances. Despite enormous progresses so far, the maximal physical separation achieved between two nodes is 1.3 km, and challenges for long distance remain. Here we make a significant step forward by entangling two atomic ensembles in one lab via photon transmission through metropolitan-scale fibers. We use cavity enhancement to create bright atom-photon entanglement, and harness quantum frequency conversion to shift the atomic wavelength to telecom. We realize entanglement over 22 km field-deployed fibers via two-photon interference, and entanglement over 50 km coiled fibers via single-photon interference. Our experiment can be extended to physically separated nodes with similar distance as a functional segment for atomic quantum networks, thus paving the way towards establishing atomic entanglement over many nodes and over much longer distance.