# Time Entanglement between a Photon and a Spin Wave in a Multimode Solid-state Quantum Memory

## TL;DR

This paper demonstrates the generation of time-entangled photon-spin wave pairs in a solid-state quantum memory, showcasing high-quality entanglement suitable for quantum communication and repeater applications.

## Contribution

It presents the first direct generation of time-entangled photon and spin wave pairs in a rare earth ion doped ensemble with Bell inequality violation.

## Key findings

- Successful generation of time-bin entanglement in a solid-state system
- High entanglement quality enabling Bell inequality violation
- Implementation of atomic frequency comb technique for analysis

## Abstract

The generation and distribution of entanglement are key resources in quantum repeater schemes. Temporally multiplexed systems offer time-bin encoding of quantum information which provides robustness against decoherence in fibers, crucial in long distance communication. Here we demonstrate the direct generation of entanglement in time between a photon and a collective spin excitation in a rare earth ion doped ensemble. We analyze the entanglement by mapping the atomic excitation onto a photonic qubit and by using time-bin qubits analyzers implemented with another doped crystal using the atomic frequency comb technique. Our results provide a solid-state source of entangled photons with embedded quantum memory. Moreover, the quality of the entanglement is high enough to enable a violation of a Bell inequality by more than two standard deviations.