# Multi-mode and long-lived quantum correlations between photons and spins in a crystal

## TL;DR

This paper demonstrates multi-mode quantum correlations between photons and spins in a rare-earth crystal, advancing quantum network capabilities by enabling long-lived, multi-mode entanglement in solid-state systems.

## Contribution

It introduces a novel multimode DLCZ approach in a rare-earth crystal, achieving quantum correlations between a photon and a spin excitation across 12 temporal modes with millisecond storage.

## Key findings

- Quantum correlations verified by violating Cauchy-Schwarz inequality
- Achieved 12 temporal modes of photon-spin entanglement
- Demonstrated 1 ms spin excitation storage time

## Abstract

The realization of quantum networks and quantum repeaters remains an outstanding challenge in quantum communication. These rely on entanglement of remote matter systems, which in turn requires creation of quantum correlations between a single photon and a matter system. A practical way to establish such correlations is via spontaneous Raman scattering in atomic ensembles, known as the DLCZ scheme. However, time multiplexing is inherently difficult using this method, which leads to low communication rates even in theory. Moreover, it is desirable to find solid-state ensembles where such matter-photon correlations could be generated. Here we demonstrate quantum correlations between a single photon and a spin excitation in up to 12 temporal modes, in a $^{151}$Eu$^{3+}$ doped Y$_2$SiO$_5$ crystal, using a novel DLCZ approach that is inherently multimode. After a storage time of 1 ms, the spin excitation is converted into a second photon. The quantum correlation of the generated photon pair is verified by violating a Cauchy - Schwarz inequality. Our results show that solid-state rare-earth crystals could be used to generate remote multi-mode entanglement, an important resource for future quantum networks.