Counting Classical Nodes in Quantum Networks.

Area of Science:

• Quantum Information Science
• Quantum Communication Networks
• Quantum Entanglement

Background:

• Quantum networks utilize entangled graph states for distributed quantum information processing.
• Real-world quantum networks are susceptible to noise, leading to a transition from quantum to classical nodes.

Purpose of the Study:

• To introduce a figure of merit for quantifying classical nodes in quantum networks with arbitrary graph states.
• To develop a method for identifying and quantifying classical defects in quantum networks.

Main Methods:

• Exploiting Einstein-Podolsky-Rosen (EPR) steerability to reveal network properties.
• Experimentally demonstrating photonic quantum networks with varying numbers of quantum and classical nodes (up to 6 quantum, 18 classical).
• Utilizing spontaneous parametric down-conversion entanglement sources.

Main Results:

• A novel method for quantifying classical defects in quantum networks was successfully introduced.
• The proposed method was shown to be faithful in assessing multiphoton quantum networks.
• Experimental validation was performed on photonic quantum networks.

Conclusions:

• The developed figure of merit provides a reliable way to identify classical defects in quantum networks.
• This work offers a new approach for characterizing generic quantum networks and nonclassical correlations in graph states.
• The findings are significant for the advancement of robust distributed quantum information processing.