Funded by the National Natural Science Foundation of China (Nos: 11774331, 11774335, 11504362, 11821404, 11654002), for the first time the research team of LI Chuanfeng and ZHOU Zongquan of the University of Science and Technology of China used solid-state quantum memories and external entangled photon sources to realize heralded quantum entanglement between two absorptive quantum memories and demonstrated multi-mode quantum repeater. The result is published in Nature as a cover article (Fig. 1) on June 2, 2021, titled Heralded entanglement distribution between two absorptive quantum memories. The link of the paper is https://www.nature.com/articles/s41586-021-03505-3.

Due to the loss of single photon transmission in optical fiber, the distance of quantum state transmission in optical fiber is limited to the order of hundreds of kilometers. In order to establish a national and even global quantum network, a quantum repeater scheme is needed. The basic idea is to decompose the task of long-distance entanglement transmission into multiple short-distance elementary links, establish heralded entanglement between quantum memories on the elementary links, and then use entanglement swapping technology to extend the quantum entanglement to the target distance.

The existing elementary quantum repeater links in the world are all based on emissive quantum memory, and the entangled photons are emitted by the memory itself. This architecture is difficult to support deterministic photon emission and multi-mode multiplexing storage simultaneously, which fundamentally limits the rate of entanglement distribution. Theoretical research shows that a quantum repeater architecture based on absorptive quantum memory can solve this problem. This architecture separates the quantum memory and the quantum photon source, so it can be compatible with deterministic photon sources and multi-mode multiplexing at the same time. It is the theoretically optimal quantum repeater solution for communication speed at present.

The research team, led by Prof. LI Chuanfeng and Prof. ZHOU Zongquan from University of Science and Technology of China, focuses on the research of absorptive quantum memories based on rare-earth-ion-doped crystals. In the past ten years, they have continuously improved the performance of solid-state quantum memory to meet the technical requirements of quantum repeater, including storage fidelity up to 99.9%, the number of modes up to 100, and the storage time of coherent light up to 1 hour. In this experiment, the research team prepared two sets of entangled photon sources based on spontaneous parametric down-conversion technology, and prepared two sets of solid-state quantum memories based on their original sandwich-like structure. One photon of each pair of entangled photons was stored in the sandwich-like quantum memory, and the other photon was simultaneously transmitted to the intermediate site for Bell state measurement. A successful Bell state measurement completed a successful entanglement swapping operation, which established quantum entanglement between two solid-state quantum memories separated by 3.5 meters, even though the two memories have not had any direct interaction. In the demonstration of the elementary link of quantum repeater, the research team realized the multiplexing of 4 time modes, which increased the rate of entanglement distribution by 4 times, and the measured fidelity of entanglement reached 80.4%.

This work confirmed the feasibility of constructing quantum repeater based on absorptive quantum memory, and demonstrated the acceleration effect of multi-mode multiplexing in quantum repeater for the first time, laying a solid foundation for the construction of practical high-speed quantum networks.

Fig. 1. The cover of Nature magazine. The title of cover story is“Quantum connections”

Fig. 2. Schematic diagram of the principle of quantum rereater based on absorptive quantum memory