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Frontiers in Optics

Postselection-Loophole-Free Time-Bin Entanglement Bell Test

Francesco Vedovato, Costantino Agnesi, Marco Tomasin, Marco Avesani, Jan-Åke Larsson, Giuseppe Vallone, Paolo Villoresi

Published Jan 1, 2019• 0 citations
Publisher

Abstract

Time-bin entanglement is an important resource for quantum information protocols, rendered unusable by the postselection-loophole. We present the first time-bin entanglement Bell test free of this loophole, enabling its use in future implementations.
DOI
10.1364/fio.2019.jtu4a.130

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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 us to obtain a postselection-loophole-free Bell violation of more than 9 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.
Entanglement is a unique resource for quantum information processing, which allows for device-independent (DI) quantum communication protocols, such as Quantum Key Distribution and Quantum Random Number Generation. Loophole-free violation of Bell inequalities is a necessary condition for DI protocols. While timebin entanglement is promising for long-distance distribution over fiber optics, its most common implementation suffers from a post-selection loophole (PSL) which invalidates Bell non-locality tests. We present an implementation of a postselection-free “genuine” time-bin entanglement scheme working with 1550 nm biphotons with an estimated Bell violation of S = 2.56.
Time-bin entanglement is a commonly used form of entanglement for quantum communication protocols over fiber networks, due to the natural resilience of this encoding scheme to thermal phase fluctuations and to birefringence in optical fibers. Projective measurements on some bases in the time-bin encoding need, however, postselection in the time domain of the measured events, introducing a loophole in Bell tests and requiring high temporal resolution. In this work, we demonstrate chip-integrated receivers for time-bin entanglement certification, including a high-speed optical switch to remove the temporal postselection loophole. The receivers are realized using thin-film lithium niobate and operate at a switching frequency of 5 GHz. We demonstrate a Bell inequality violation by more than 24 standard deviations without the need for time resolution at the time-bin separation level.