# Counting Classical Nodes in Quantum Networks

## TL;DR

This paper introduces a new method to quantify classical nodes in quantum networks using EPR steerability, demonstrated experimentally with photonic networks up to 6 quantum and 18 classical nodes, aiding in understanding quantum network integrity.

## Contribution

It presents a novel figure of merit for classical nodes in quantum networks based on EPR steerability, with experimental validation in photonic systems.

## Key findings

- Successfully quantified classical nodes in quantum networks.
- Demonstrated the method with up to 6 quantum and 18 classical nodes.
- Provided a new way to identify nonclassical correlations in graph states.

## Abstract

Quantum networks illustrate the use of connected nodes of quantum systems as the backbone of distributed quantum information processing. When the network nodes are entangled in graph states, such a quantum platform is indispensable to almost all the existing distributed quantum tasks. Unfortunately, real networks unavoidably suffer from noise and technical restrictions, making nodes transit from quantum to classical at worst. Here, we introduce a figure of merit in terms of the number of classical nodes for quantum networks in arbitrary graph states. Such a network property is revealed by exploiting a novel Einstein-Podolsky-Rosen steerability. Experimentally, we demonstrate photonic quantum networks of $n_q$ quantum nodes and $n_c$ classical nodes with $n_q$ up to 6 and $n_c$ up to 18 using spontaneous parametric down-conversion entanglement sources. We show that the proposed method is faithful in quantifying the classical defects in prepared multiphoton quantum networks. Our results provide novel identification of generic quantum networks and nonclassical correlations in graph states.