# Experimental certification of millions of genuinely entangled atoms in a solid

## TL;DR

This paper reports the experimental certification of 16 million genuinely entangled atoms in a solid, demonstrating the robustness and scalability of multipartite entanglement in quantum memories for future quantum networks.

## Contribution

It introduces a new entanglement witness method applicable to large-scale solid-state systems, enabling certification of massive multipartite entanglement even with losses.

## Key findings

- Certified 16 million entangled atoms in a solid-state quantum memory.
- Developed a scalable entanglement witness based on collective emission.
- Demonstrated the robustness of multipartite entanglement at unprecedented scales.

## Abstract

Quantum theory predicts that entanglement can also persist in macroscopic physical systems, albeit difficulties to demonstrate it experimentally remain. Recently, significant progress has been achieved and genuine entanglement between up to 2900 atoms was reported. Here we demonstrate 16 million genuinely entangled atoms in a solid-state quantum memory prepared by the heralded absorption of a single photon. We develop an entanglement witness for quantifying the number of genuinely entangled particles based on the collective effect of directed emission combined with the nonclassical nature of the emitted light. The method is applicable to a wide range of physical systems and is effective even in situations with significant losses. Our results clarify the role of multipartite entanglement in ensemble-based quantum memories as a necessary prerequisite to achieve a high single-photon process fidelity crucial for future quantum networks. On a more fundamental level, our results reveal the robustness of certain classes of multipartite entangled states, contrary to, e.g., Schr\"odinger-cat states, and that the depth of entanglement can be experimentally certified at unprecedented scales.