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arXiv:2510.20392 · Physical Review Letters

Temporally Multimode Ion-Ion Entanglement over 1.2 Kilometer Fibers

Z.-B. Cui, Z.-Q. Wang, P.-Y. Liu, Y. Wang, P.-C. Lai, J.-X. Shi, Y.-D. Sun, Z.-C. Tian, H.-S. Sun, Y.-B. Liang, B. Qi, Y.-Y. Huang, Z.-C. Zhou, Yukai Wu, Yong Xu, Yunfei Pu, L-M Duan

Published Jun 9, 2026• 0 citations
arXivHTMLPDF

Abstract

Quantum networks and quantum repeaters represent the promising avenues for building large-scale quantum information systems, serving as foundational infrastructure for distributed quantum computing, long-distance quantum communication, and networked quantum sensing. A critical step in realizing a functional quantum network is the efficient and high-fidelity establishment of heralded entanglement between remote quantum nodes. A multimode entangling scheme offers a powerful strategy to accelerate remote entanglement distribution, particularly over long optical fibers. Here, we experimentally demonstrate multimode-enhanced heralded entanglement between two trapped-ion quantum network nodes. By harnessing ten temporal photonic modes, we achieve a 4.59-fold speedup in ion-ion entanglement generation and attain an entanglement fidelity of 95.9%±1.5% over 1.2 km of fiber. Employing a dual-type architecture, our system is readily scalable to multiple nodes, thereby establishing a key building block for future large-scale quantum networks.

DOI
10.1103/9h14-sc8t

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Quantum networks and quantum repeaters represent the promising avenues for building large-scale quantum information systems, serving as foundational infrastructure for distributed quantum computing, long-distance quantum communication, and networked quantum sensing. A critical step in realizing a functional quantum network is the efficient and high-fidelity establishment of heralded entanglement between remote quantum nodes. A multimode entangling scheme offers a powerful strategy to accelerate remote entanglement distribution, particularly over long optical fibers. Here, we experimentally demonstrate multimode-enhanced heralded entanglement between two trapped-ion quantum network nodes. By harnessing ten temporal photonic modes, we achieve a 4.59-fold speedup in ion-ion entanglement generation and attain an entanglement fidelity of 95.9%±1.5% over 1.2 km of fiber. Employing a dual-type architecture, our system is readily scalable to multiple nodes, thereby establishing a key building block for future large-scale quantum networks.
Quantum network and quantum repeater are promising ways to scale up a quantum information system. In a functional quantum network, it is required that the distribution rate of heralded remote entanglement should be higher than the decoherence rate of each local node. A promising scheme to accelerate the remote entanglement distribution is through multiplexing enhancement. In this work, we experimentally realize a multiplexed quantum network node based on a chain of 40Ca+ ions. We employ a hybrid multiplexing scheme in which maximally 44 time-bin modes are generated and sent through a long fiber to boost the entangling rate. Via this scheme, we can generate heralded ion-photon entanglement with a success rate of 4.28 s−1 over a 12 km fiber. In addition, a dual-type framework is utilized to protect quantum information from the destructive ion-photon entangling attempts and a memory coherence time of 366 ms is achieved, which has exceeded the entanglement generation time. Despite recent advances with trappedion-based platforms, achieving quantum networks with link efficiency greater than unity on metropolitan scales is still a challenge. Here, the authors demonstrate a multiplexed quantum network generating heralded entanglement at a rate faster than local decoherence.
Concatenation of quantum repeater nodes has been proposed as a means to distribute entanglement efficiently across future long-distance networks. Here, we demonstrate a trapped-ion repeater node that establishes photonic entanglement over a 17 km fiber traversing a city, moving beyond laboratory-only prototypes. The node comprises two atomic ions coupled to an optical cavity. First, a detailed characterization is performed of the long fiber channel and of ion-photon entanglement distributed across it. Next, a repeater-node protocol is implemented, achieving entanglement between a local photon and a remote photon with a Bell-state fidelity of 81.6−1.4+1.9%81.6^{+1.9}_{-1.4}\%81.6−1.4+1.9​%. These results demonstrate that trapped-ion repeater-node technology can operate over existing city-scale fiber infrastructure and represent a step toward distributed quantum repeater networks.
Envisioned large-scale quantum networks demand the ability to rapidly distribute and store entanglement between remote matter-based nodes using photons [1] . The minimum time required to distribute entanglement between two nodes is the time it takes light to travel between them, which over 100 km distance already 0.33 ms. The speed of light thus sets a fundamental limit on the achievable entanglement distribution rate: with a single quantum emitter in a node, one has to wait for the photon travel time before trying again with a new photon for any application that would require also classical communication (e.g. to realize a quantum repeater) [2] . Over 100 km of fiber, this limits the entanglement distribution attempt rate to 2 kHz and, given 1.5% transmission in the best telecom fibres, a success rate of 30 Hz. This is a general problem for entanglement distribution over long distances as far higher rates are critical for the practical realisation of quantum networks and their applications. To overcome this limit, several entanglement distribution attempts have to be done in parallel. This requires multiple modes of the node (memory qubits) to couple selectively to the multiple modes of the link.
Quantum networks, integrating quantum communication, quantum metrology, and distributed quantum computing, could provide secure and efficient information transfer, high-resolution sensing, and an exponential speed-up in information processing. Deterministic entanglement distribution over long distances is a prerequisite for scalable quantum networks, enabling the utilization of device-independent quantum key distribution (DI-QKD) and quantum teleportation to achieve secure and efficient information transfer. However, the exponential photon loss in optical fibres prohibits efficient and deterministic entanglement distribution. Quantum repeaters, incorporating entanglement swapping and entanglement purification with quantum memories, offer the most promising means to overcome this limitation in fibre-based quantum networks. Despite numerous pioneering efforts toward realizing quantum repeaters, a critical bottleneck remains, as remote memory-memory entanglement suffers from decoherence more rapidly than it can be established and purified over long distances. We overcome this by developing long-lived trapped-ion memories, an efficient telecom interface, and a high-visibility single-photon entanglement protocol. This allows us to establish and maintain memory-memory entanglement over a 10 km fibre within the average entanglement establishment time for the same distance. As a direct application, we demonstrate metropolitan-scale DI-QKD, distilling 1,917 secret keys out of 4.05*10^5 Bell pairs over 10 km. We further report a positive key rate over 101 km in the asymptotic limit, extending the achievable distance by more than two orders of magnitude. Our work provides a critical building block for quantum repeaters and marks an important step toward scalable quantum networks.
A three-qubit quantum network node based on trapped atomic ions is presented. The ability to establish entanglement between each of the qubits in the node and a separate photon that has travelled over a 101km-long optical fiber is demonstrated. By sending those photons through the fiber in close succession, a remote entanglement rate is achieved that is greater than when using only a single qubit in the node. Once extended to more qubits, this multimode approach can be a useful technique to boost entanglement distribution rates in future long-distance quantum networks of light and matter.
Trapped atomic ions constitute one of the leading physical platforms for building the quantum repeater nodes to realize large-scale quantum networks. In a long-distance trapped-ion quantum network, it is essential to have crosstalk-free dual-type qubits: one type, called the communication qubit, to establish entangling interface with telecom photons; and the other type, called the memory qubit, to store quantum information immune from photon scattering under entangling attempts. Here, we report the first experimental implementation of a telecom-compatible and crosstalk-free quantum network node based on two trapped 40^{40}40Ca+^{+}+ ions. The memory qubit is encoded on a long-lived metastable level to avoid crosstalk with the communication qubit encoded in another subspace of the same ion species, and a quantum wavelength conversion module is employed to generate ion-photon entanglement over a 12 12\,12km fiber in a heralded style. Our work therefore constitutes an important step towards the realization of quantum repeaters and long-distance quantum networks.