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arXiv:2505.04598 · Physical Review Applied

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

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

Published May 12, 2026• 1 citation
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Abstract

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.

DOI
10.1103/1sjz-tw85

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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.
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 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.
The development of quantum internet demands on-chip quantum processor nodes and interconnection between the nodes. Path-encoded photonic qubits are suitable for on-chip quantum information processors, while time-bin encoded ones are good at long-distance communication. It is necessary to develop an on-chip converter between the two encodings to satisfy the needs of the quantum internet. In this work, a quantum photonic circuit is proposed to convert time-bin-encoded photonic qubits to path-encoded ones via a thin-film lithium niobate high-speed optical switch and low-loss matched optical delay lines. The performance of the encoding converter is demonstrated by the experiment of time-bin to path encoding conversion on the fabricated sample chip. The converted path qubits have an average fidelity higher than 97%. The potential of the encoding converter on applications in quantum networks is demonstrated by the experiments of entanglement distribution and quantum key distribution. The results show that the on-chip encoding converter can serve as a foundational component in the future quantum internet, bridging the gap between quantum information transmission and on-chip processing based on photons.
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].
Entanglement gives rise to correlations between distant quantum systems that cannot be explained by local realistic theories. Bell inequality violations provide a direct way to reveal these correlations and certify nonlocality, especially when the relevant experimental loopholes are closed. Time-bin encoding, in which quantum information is encoded into well-defined temporal modes, is a commonly used platform for distributing photonic entanglement in optical fibers. Yet loophole-free Bell tests with time-bin entanglement have received comparatively little attention, owing in part to the postselection loophole introduced by conventional interferometric measurements. Here, we demonstrate a fiber-based platform for Bell tests with time-bin entanglement that simultaneously closes the locality, freedom-of-choice, and postselection loopholes. We observe a CHSH violation of S=2.583±0.002S=2.583 \pm 0.002S=2.583±0.002, exceeding the local-realistic bound by over 265 standard deviations. Notably, this rigorous certification of nonlocality is achieved at a separation distance of 49.0±0.749.0 \pm 0.749.0±0.7 m, substantially shorter than previous photonic Bell tests addressing comparable space-time constraints. Beyond its foundational significance, our results demonstrate time-bin entanglement as a viable route towards practical device-independent quantum communication and a future quantum internet.