Our country's quantum technology research achieves new breakthroughs; quantum networks may become a reality

According to the University of Science and Technology of China, recently, researchers including Jian-Wei Pan and his colleagues at USTC have achieved a major breakthrough in scalable quantum network research. Wang Ye, Wan Yong, Zhang Qiang, Jian-Wei Pan, and others collaborated with the Jinan Quantum Technology Research Institute, the Shanghai Institute of Microsystem and Information Technology of the Chinese Academy of Sciences, the University of Hong Kong, Tsinghua University, and other institutions. They constructed the basic modules for scalable quantum repeaters, making long-distance quantum networks practically feasible. Meanwhile, Xiao-Hui Bao, Feihu Xu, Zhang Qiang, Jian-Wei Pan, and others collaborated with the Jinan Quantum Technology Research Institute, the National University of Singapore, the University of Waterloo in Canada, and others to realize long-distance high-fidelity entanglement between single-atom nodes, and based on this, broke the transmission distance of device-independent quantum key distribution (DI-QKD) beyond 100 kilometers, greatly advancing the practical application of this technology. These two achievements were published in the international authoritative academic journals Nature and Science on February 3 and 6 Beijing time, respectively.

These breakthroughs mark another milestone achievement for China in the field of quantum communication and quantum networks, following the “Mozi” quantum satellite. They indicate that fiber-optic quantum networks based on quantum entanglement are transitioning from theoretical concepts to practical possibilities, further strengthening China’s leading position in this field internationally.

The ultimate goal of quantum information science is to build efficient and secure quantum networks: utilizing quantum precision measurement for high-accuracy sensing of information, quantum communication for secure and efficient transmission, and quantum computing for exponential speed-up processing of information, thereby enabling a revolutionary leap in our understanding of the material world. The fundamental element for constructing quantum networks is long-distance deterministic quantum entanglement distribution. Based on quantum entanglement, not only can classical information be securely transmitted via quantum key distribution, but quantum teleportation also provides the only effective means for quantum information exchange between quantum computers and users.

[Figure 1. Schematic diagram of a quantum network]

The inherent loss in optical fibers causes the efficiency of quantum entanglement transmission to decay exponentially with distance, posing the greatest challenge to building scalable quantum networks. For example, after transmitting over 1000 kilometers of standard optical fiber, the optical signal will decay to about 10^-20 of its original strength (one trillionth of a trillionth), meaning that even if 10 billion entangled photon pairs are emitted per second, it would take an average of 300 years to receive just one entangled pair.

Quantum repeater schemes are an effective solution to fiber transmission loss: for example, in a 1000-kilometer fiber link, a repeater station can be set up every 100 kilometers to generate entanglement between neighboring stations, then use entanglement swapping to connect these segments and achieve effective entanglement distribution over long distances. Using this scheme, with the same emission rate, one hundred million entangled photon pairs can be received per second, increasing transmission efficiency by 10^18 times. Therefore, quantum repeaters have always been the most important research direction in fiber-based quantum networks.

[Figure 2. Schematic of quantum repeater principle. (1) Entanglement is established between neighboring repeater nodes (e.g., A and B, B and C) via photon interference. (2) Entanglement swapping at node B can establish entanglement between nodes A and C, and so on. (3) Multi-level entanglement swapping extends the entanglement distance step by step, ultimately establishing entanglement between the furthest nodes A and K.]

As early as 1998, Jian-Wei Pan and colleagues demonstrated quantum entanglement connection internationally. Since then, research teams both domestically and abroad have made a series of important progress. However, a major technical challenge that has remained unresolved for nearly 30 years is: the lifetime of entanglement is far shorter than the time needed to generate entanglement, making it impossible to deterministically produce entanglement between neighboring nodes within the entanglement’s lifetime, thus severely limiting the scalability of quantum repeaters.

To address this core challenge, the research team at USTC developed long-lifetime trapped ion quantum memories, high-efficiency ion-photon communication interfaces, and high-fidelity single-photon entanglement protocols, achieving long-lived quantum entanglement. The entanglement lifetime (550 milliseconds) significantly exceeds the time required to establish entanglement (450 milliseconds), successfully constructing the basic modules for scalable quantum repeaters and making long-distance quantum networks practically feasible.

[Figure 3. Schematic of basic module for scalable quantum repeater. (1) The experiment consists of long-lifetime trapped ion quantum memories, high-efficiency quantum frequency conversion modules, and high-contrast single-photon interference modules. (2) The entanglement generation rate is 2.226 Hz, with a waiting time of about 450 milliseconds. (3) The entanglement lifetime is approximately 550 milliseconds.]

A direct application of long-distance entanglement distribution is to realize quantum secure communication with the highest security level under real-world conditions. Traditional quantum secure communication schemes require precise calibration of device parameters to ensure security, which can be inconvenient in practical applications. The device-independent quantum key distribution (DI-QKD) scheme based on entanglement overcomes this limitation: even if the quantum devices are completely untrusted, as long as the communicating parties can establish sufficiently high-quality entanglement and verify a loophole-free violation of Bell inequalities, the security of key distribution can be strictly guaranteed without precise calibration of device parameters. As a result, DI-QKD has been hailed by Gilles Brassard, one of the founders of quantum cryptography and 2018 Wolf Prize laureate, as the “Holy Grail” sought by cryptographers for centuries.

However, experimental realization of DI-QKD faces extremely stringent technical challenges. Quantum entanglement between remote nodes must meet the following conditions: (1) extremely high detection efficiency to effectively close detection loopholes; (2) maintaining very high entanglement fidelity to ensure a significant violation of Bell inequalities. Due to fiber loss and system noise over long distances, most previous related experiments have been limited to short distances (usually meters to hundreds of meters), far from practical application needs.

Building on scalable quantum repeater technology, the USTC research team further successfully achieved high-fidelity long-distance entanglement between two rubidium atoms: on a fiber link up to 100 kilometers long, the entanglement fidelity between atomic nodes remained above 90%, significantly better than previous international results. Based on this, the team demonstrated device-independent quantum key distribution over metropolitan-scale fiber links: completing security analysis and rigorous proof over an 11-kilometer fiber link, with a transmission distance about 3000 times longer than previous best results; and demonstrated the feasibility of key generation over a 100-kilometer fiber link, surpassing the previous international best experiments by more than two orders of magnitude.

[Figure 4. Schematic of 100-kilometer DI-QKD experiment. Single atoms at both ends emit photons via Rydberg single-photon generation, which are transmitted through long-distance fiber to the middle node for interference. After detecting the heralding event, the atoms are projected into a long-distance entangled state, achieving entanglement distribution. Subsequently, the atoms are measured in random bases, and the measurement results are used for Bell inequality tests to verify security, followed by data post-processing to generate secure keys.]

These two research achievements have received support from the National Key R&D Program, the National Natural Science Foundation of China, the Chinese Academy of Sciences, and regional funding agencies including Anhui Province, Hefei City, Shandong Province, Jinan City, and the Hong Kong Research Grants Council.

(Source: Caixin)