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Post-selection free time-bin entanglement on a thin-film lithium niobate photonic chip

A. C. de C. Bernardi, Matteo Bacchi, Marco Clementi, Sara Congia, Francesco Garrisi, A. Martellosio, Marco Passoni, A. Wrobel, Federico Andrea Sabattoli, Mattéo Galli, Daniele Bajoni

Published Jan 1, 2025• 0 citations
Publisher

Abstract

We experimentally demonstrate a thin-film lithium niobate photonic chip to measure time-bin entanglement without the temporal post-selection loophole. We show a violation of the Bell inequality by more than 30 standard deviations.
DOI
10.1364/cleo_fs.2025.ff118_5

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•Jan 1, 2026•0 citations
Time-bin entanglement is a widely adopted approach for quantum communication because of its robustness against propagation over optical fibers networks. However, certain projective measurements in this encoding scheme necessitate post-selection, introducing a loophole in Bell inequality tests and requiring precise temporal resolution. We present an integrated photonic circuit that certifies time-bin entanglement while eliminating the need of post-selection through a high-speed optical switching mechanism. Implemented using thin-film lithium niobate technology, this device operates at 5 GHz switching rate, enabling high secure key rates. Our results demonstrate a Bell inequality violation exceeding 27 standard deviations, achieved without requiring time resolution at the scale of the time-bin separation.
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.
The distribution of entangled photons between remote parties is fundamental for quantum communication protocols such as quantum teleportation and quantum key distribution (QKD). Among the various encoding schemes, time-bin entanglement is particularly promising as it ensures reliable long-distance communication for both optical fibre and free-space channels. However, the practical utility of time bin entanglement is constrained by the post-selection loophole (PSL), which allows for the possibility of local hidden variable models [1]. Temporal post-selection also opens the door to attack schemes and demands high temporal resolution. Initial efforts to close the PSL employed hyperentangled states combining polarization and time-bin degrees of freedom [2]. Later, Vedovato et al. used active switching to remove the need for post-selection [3], and an integrated “hug” scheme was used to demonstrate PSL-free certification through a phase-stabilized optical link [4].
Time-bin encoded quantum states of light are crucial for quantum technology applications. The integration of manipulation functionalities into chip-scale devices is essential for deploying scalable, high-performance, and cost-effective quantum networks. Here we develop a fully integrated, high-throughput quantum receiver based on the thin-film lithium niobate (TFLN) platform, capable of high-speed electro-optic manipulation of time-bin encoded quantum states. The device's novel architecture enables active switching of time-bin quantum states with an electro-optic bandwidth exceeding 30 Ghz, while supporting real-time arbitrary projective measurements with a bandwidth of over 1 GHz. We showcase its versatility and performance through several applications, including the certification of entanglement with Bell's inequality violation by 38 standard deviations and with >95% visibility. We then apply it to a fiber-based quantum communication scenario, where we experimentally demonstrate an entanglement-based quantum key distribution (QKD) protocol, achieving stable finite-size secure key rates exceeding 25 kbit/s over 12 hours of continuous operation. By leveraging a high-speed active switching scheme, the system overcomes the need for temporal post-selection, eliminating a fundamental loophole that compromises the security of time-bin entanglement-based QKD protocols and relaxes the temporal resolution requirements of single-photon detectors. Moreover, it enables active selection of the projection basis, increasing the flexibility for communication parties. This approach establishes a versatile and scalable architecture for time-bin encoded quantum communication, enabling practical protocols on industry-grade photonic technology.