Postselection-loophole-free Bell violation with genuine time-bin entanglement

TL;DR

This paper demonstrates the first experimental violation of Bell's inequality using genuine time-bin entanglement free from the postselection loophole, enabling more secure quantum communication over long distances.

Contribution

Introduces a novel interferometer-based scheme with fast optical switches to achieve loophole-free Bell violation using time-bin entanglement.

Findings

Achieved a Bell violation exceeding nine standard deviations.

Scheme is compatible with fiber-based components and integrated photonics.

Paves the way for long-distance secure quantum communication.

Abstract

Entanglement is an invaluable resource for fundamental tests of physics and the implementation of quantum information protocols such as device-independent secure communications. In particular, time-bin entanglement is widely exploited to reach these purposes both in free-space and optical fiber propagation, due to the robustness and simplicity of its implementation. However, all existing realizations of time-bin entanglement suffer from an intrinsic postselection loophole, which undermines their usefulness. Here, we report the first experimental violation of Bell's inequality with "genuine" time-bin entanglement, free of the postselection loophole. We introduced a novel function of the interferometers at the two measurement stations, that operate as fast synchronized optical switches. This scheme allowed to obtain a postselection-loophole-free Bell violation of more than nine standard deviations. Since our scheme is fully implementable using standard fiber-based components and is compatible with modern integrated photonics, our results pave the way for the distribution of genuine time-bin entanglement over long distances.