Measurement-Device-Independent Entanglement Quantification in a Fully Connected Time-Bin Quantum Network

TL;DR

This paper demonstrates a practical method for measurement-device-independent entanglement verification and quantification in a fully connected time-bin quantum network, enabling reliable quantum communication without trusting measurement devices.

Contribution

It introduces a novel MDI entanglement measurement technique using polarization encoding, applicable to scalable quantum networks with high fidelity over long distances.

Findings

Successfully verified and quantified entanglement in a 4-user network over 20 km fiber links.

Achieved high-fidelity entanglement distribution without active stabilization.

Demonstrated MDI measurements without ancillary photons or extra resources.

Abstract

Fully connected quantum networks enable scalable quantum communication, yet reliable entanglement characterization without trusting measurement devices remains challenging. Here we experimentally demonstrate measurement-device-independent (MDI) entanglement verification and quantification in a time-bin-encoded fully connected quantum network. Using a broadband periodically poled lithium niobate on insulator source combined with dense wavelength-division multiplexing, we distribute all six pairwise entangled links among four users over 20-km fiber channels, preserving high-fidelity entanglement without active stabilization of the long-distance fiber links. We show that conventional entanglement witnesses can fail under untrusted measurement conditions. By encoding trusted input states in the polarization degree of freedom of the same photons, we realize MDI measurements without ancillary photons or additional experimental resources. Both entanglement verification and quantification are obtained from the same measurement dataset. Our results establish a practical and scalable approach for reliable entanglement characterization in quantum networks.