Long-distance distribution of genuine energy-time entanglement

TL;DR

This paper demonstrates a secure, loophole-free Bell violation using energy-time entanglement over 1 km of optical fibers, advancing long-distance quantum communication.

Contribution

It introduces a novel experimental design and stabilization method to distribute genuine energy-time entanglement securely over long distances.

Findings

First loophole-free Bell violation with energy-time entanglement over 1 km fibers

Use of a new experimental setup avoiding geometrical loopholes

Active stabilization of fiber-based interferometer

Abstract

Any practical realization of entanglement-based quantum communication must be intrinsically secure and able to span long distances avoiding the need of a straight line between the communicating parties. The violation of Bell's inequality offers a method for the certification of quantum links without knowing the inner workings of the devices. Energy-time entanglement quantum communication satisfies all these requirements. However, currently there is a fundamental obstacle with the standard configuration adopted: an intrinsic geometrical loophole that can be exploited to break the security of the communication, in addition to other loopholes. Here we show the first experimental Bell violation with energy-time entanglement distributed over 1 km of optical fibers that is free of this geometrical loophole. This is achieved by adopting a new experimental design, and by using an actively stabilized fiber-based long interferometer. Our results represent an important step towards long-distance secure quantum communication in optical fibers.