Postselection-Loophole-Free Bell Test Over an Installed Optical Fiber Network

TL;DR

This paper demonstrates a loophole-free Bell inequality violation over 3.7 km of deployed optical fiber using energy-time entangled photons, advancing secure quantum communication with existing infrastructure.

Contribution

First in-field Bell test over optical fiber free of the post-selection loophole, using energy-time entanglement over a real-world network.

Findings

Achieved Bell violation over 3.7 km fiber

First loophole-free Bell test in deployed fiber network

Supports secure quantum communication infrastructure

Abstract

Device-independent (DI) quantum communication will require a loophole-free violation of Bell inequalities. In typical scenarios where line-of-sight between the communicating parties is not available, it is convenient to use energy-time entangled photons due to intrinsic robustness while propagating over optical fibers. Here we show an energy-time Clauser-Horne-Shimony-Holt Bell inequality violation with two parties separated by 3.7 km over the deployed optical fiber network belonging to the University of Concepci\'on in Chile. Remarkably, this is the first Bell violation with spatially separated parties that is free of the post-selection loophole, which affected all previous in-field long-distance energy-time experiments. Our work takes a further step towards a fiber-based loophole-free Bell test, which is highly desired for secure quantum communication due to the widespread existing telecommunication infrastructure.