New multiplexing scheme accelerates long-distance quantum communication

Quantum networks, systems consisting of multiple connected nodes or devices that can transmit quantum information to one another, have the potential to advance future communications. These networks typically leverage entanglement, a quantum phenomenon that prompts two or more distant particles to become highly correlated, so that measuring one instantly affects the state of the other.

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To ensure that distant particles have become entangled and can transmit quantum states, some quantum scientists try to realize so-called heralded entanglement. This entails confirmation, from a detectable signal, that entanglement between nodes has been established.

Researchers at Tsinghua University and Hefei National Laboratory recently introduced a promising strategy to accelerate the generation of heralded entanglement between multiple ions (i.e., atoms with an electrical charge). Their proposed approach, outlined in a paper published in Physical Review Letters, relies on a so-called multiplexing scheme, a technique to send multiple signals through the same communication channel.

"Our central insight is that the multiplexing scheme employed in this experiment—which executes concurrent excitation attempts analogous to classical protocols—is essential for accelerating heralded entanglement generation between remote matter qubits, a fundamental building block of quantum networks and repeaters," Yunfei Pu, co-senior author of the paper, told Phys.org. "Multiplexing effectively overcomes the round-trip communication latency that severely limits entanglement generation rates."

The team's efforts to devise new multiplexing schemes

Pu and his colleagues have been developing multiplexing schemes for years, as they believe they will be indispensable for realizing quantum repeaters that can transmit information across distances of more than 1,000 km (620 miles). Their recent study is their latest effort toward this goal.

"Over the past decade, our group extensively investigated multiplexing techniques for neutral-atom and trapped-ion quantum networks, utilizing approaches such as two-dimensional AOD addressing and ion shuttling," said Pu.

"Our previous work successfully demonstrated the acceleration of atom-photon entanglement via multiplexing, but multiplexing-enhanced atom-atom entanglement had not yet been realized. In this work, we complete the protocol by demonstrating atom-atom entanglement and achieve highly favorable results, particularly in terms of entanglement fidelity."

To demonstrate the potential of their scheme for accelerating atom-atom entanglement, the researchers first built a quantum network consisting of two trapped ions connected by 1.2 km (0.75 miles) of optical fiber. For their experiment, they used ⁴⁰Ca⁺ ions, positively charged calcium atoms that produce or interact with near-infrared light (i.e., photons with a wavelength of 866 nm).

"Although this transition offers a favorable near-infrared (NIR) wavelength, it suffers from a low branching ratio of approximately 6% (probability of emitting the desired photons), leading to significant inefficiency in entanglement attempts," explained Pu.

"To address this, we previously designed a 'multiple excitation' scheme aimed at boosting the effective branching ratio toward unity. However, the viability of this scheme for atom-atom entanglement remained unverified."

The demonstration of multimode-enhanced atom-atom entanglement

The researchers successfully showed that their scheme can be applied to the generation of atom-atom entanglement. Using this scheme, they realized multimode-enhanced atom-atom heralded entanglement between two trapped-ion nodes forming a quantum network.

"In each node, we employ a multiple-excitation scheme to generate ion-photon entanglement randomly distributed across 10 temporal modes per round," said Pu. "Photons from both nodes are transmitted via 600 m of optical fiber and interfered at a central station for a photonic Bell-state measurement (BSM). A specific detector coincidence pattern heralds the successful generation of remote ion-ion entanglement."

To ensure that every successful entanglement event produced the same entangled state, the researchers used a real-time phase-compensation approach. To characterize the fidelity of the entangled state, they used a technique called quantum state tomography.

"By harnessing 10 temporal modes, we achieve a 4.59-fold increase in the remote ion-ion entanglement rate," explained Pu.

"While multimode-enhanced spin-spin entanglement has previously been realized only in solid-state systems, this work represents the first demonstration in a laser-cooled atomic system. Furthermore, the achieved ion-ion entanglement fidelity of 95.9% sets a new record for heralded matter-matter entanglement over distances beyond the laboratory scale."

Notably, this was the first demonstration of multimode-enhanced remote spin-spin entanglement in a laser-cooled atomic system. Laser-cooled atomic systems are established platforms for quantum computing and have already demonstrated high-fidelity quantum logic gates.

"Such capabilities are prerequisites for constructing quantum networks and repeaters capable of performing essential distributed operations, including entanglement swapping, purification, and quantum teleportation," said Pu. "The achieved ion-ion entanglement fidelity of 95.9% establishes a new record for heralded spin-spin entanglement over distances extending beyond the laboratory scale."

Toward the realization of a fast quantum internet

This recent paper introduced a promising multiplexing scheme, a strategy to stabilize the phase of generated entangled states and a method to utilize optical transitions with low branching ratios.

In the future, their newly introduced approaches could be tested in other experiments with trapped-ion quantum networks to further validate their potential for accelerating ion-ion entanglement generation.

"While certain optical transitions are at wavelengths favorable for long-distance fiber transmission, their utility is often limited by low branching ratios that reduce entanglement-generation efficiency," said Pu. "Typically, overcoming this limitation requires high-finesse optical cavities, which introduce significant experimental complexity. Using this method, we demonstrate for the first time that efficiency can be improved in a free-space configuration."

Pu and their colleagues predict their scheme will also be applicable to other atomic quantum networks that rely on telecom-wavelength transitions but are limited by low branching ratios, such as the 1,650 nm transition in ytterbium ions. Meanwhile, the researchers are planning more studies aimed at developing new practical quantum repeaters and realizing long-distance quantum communication.

"Technically, we aim to further enhance the performance of trapped-ion quantum networks through various strategies, such as single-photon heralding and other multiplexing techniques," added Pu.

"A key threshold for scaling quantum networks and repeaters is for the remote entanglement-generation rate to exceed the node decoherence rate. Achieving this is particularly challenging over long distances.

"Therefore, to reach this scalability threshold, we plan to improve entanglement generation efficiency by adopting a more efficient single-photon scheme (as opposed to the two-photon scheme used here) and incorporating multiplexing methods, such as ion shuttling or AOD addressing."

Written for you by our author Ingrid Fadelli, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly).

More information: Z.-B. Cui et al, Temporally Multimode Ion-Ion Entanglement over 1.2 Kilometer Fibers, Physical Review Letters (2026). DOI: 10.1103/9h14-sc8t. On arXiv: arxiv.org/abs/2510.20392

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