Chinese Researchers Construct a Multiplexed Quantum Repeater Based on Absorptive Quantum Memories

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Multiplexed Quantum Repeater

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An primary link of a quantum repeater based upon 2 absorptive QMs with the Sandwich-like structure. Credit: WANG Guoyan and MA Yanbing

Chinese scientists understood a primary link of a quantum repeater based upon absorptive quantum memories (QMs) and showed the multiplexed quantum repeater for the very first time. On June second, 2021, the work was released in Nature.

The essential job of a quantum network is to disperse quantum entanglement in between 2 remote areas. However, the transmission loss of fiber optics has actually restricted the range of entanglement circulation to around 100 km on the ground. Quantum repeaters can conquer this trouble by dividing long-distance transmission into numerous short-distance primary links. The entanglement of 2 end nodes of each link is produced first of all. Then the entanglement range is slowly broadened through entanglement switching in between each link.

Previously, a primary link of a quantum repeater has actually been understood in cold atomic ensembles and single quantum systems. These presentations are all based upon emissive QMs, in which the knotted photons are given off from QMs. Quantum repeaters built by emissive QMs have easy structures, however bad compatibilities. It is of excellent obstacle to support deterministic entanglement sources and multiplexed operations all at once, which are 2 crucial innovations to boost the entanglement circulation rate. Quantum repeaters based upon absorptive QMs can conquer such constraints due to the fact that they separate the quantum memories and the knotted photon sources.

The research study group, led by Prof. LI Chuanfeng and Prof. ZHOU Zongquan from University of Science and Technology of China (USTC), concentrates on the research study of absorptive QMs based upon rare-earth-ion-doped crystals. For this sort of QMs, the entanglement source can be flexibly picked, consisting of deterministic entanglement sources, while staying the ability of multiplexed operations, and for that reason must be more effective for quantum repeater applications. In this work, they utilized external knotted photon-pair sources (EPPSs) based upon spontaneous parametric down-conversion and attained declared entanglement circulation in between 2 absorptive QMs for the very first time.

They constructed a primary relate to an intermediate station and 2 nodes at the ends. Each node includes an absorptive QM with a bandwidth of 1GHz and a bandwidth-matched EPPS. In each node, one knotted photon of each photon set was kept in the “Sandwich-like” QM while the other was transferred to the middle station for joint Bell-state measurement (BSM). An effective entanglement switching operation was declared by the effective click of BSM. The entanglement in between 2 QMs 3.5 meters apart was developed with a fidelity of around 80.4%, although there weren’t any direct interactions in between 2 remote QMs. Four temporal modes were used in this presentation of a primary link of a quantum repeater, speeding up the entanglement circulation rate by 4 times.

Prof. ZHOU Zongquan stated: “The use of absorptive quantum memory is expected to achieve high efficiency quantum repeater and quantum network in the future, and further promote the communication between ‘Cowherd and Weaver Girl’ in the quantum world.”

This work offers a possible roadmap for the advancement of useful quantum repeaters and lays the structure for the building of high-speed quantum networks. Reviewers mentioned”The present work concentrates on the ensemble method, which has a variety of benefits in the context of quantum repeater applications, multiplexing for example.” They extremely suggest this work as”a substantial achievement that will form the basis for additional research study” and “a major step forward in the development of a practical quantum repeater.”

Prof. LI Chuanfeng stated that the group will continue to enhance the signs of absorptive QM, “we will use deterministic entanglement source to greatly improve the entanglement distribution rate, and to achieve practical quantum repeaters beyond direct transmission of optical fiber.”

For future advancements, the research study group will continue to enhance the efficiencies of the absorptive QMs, and embrace deterministic entanglement sources, so regarding significantly boost the entanglement circulation rate, and to accomplish an useful quantum repeater that outshines the direct transmission of photons.

Reference: “Heralded entanglement distribution between two absorptive quantum memories” by Xiao Liu, Jun Hu, Zong-Feng Li, Xue Li, Pei-Yun Li, Peng-Jun Liang, Zong-Quan Zhou, Chuan-Feng Li and Guang-Can Guo, 2 June 2021, Nature.
DOI: 10.1038/s41586-021-03505-3

LIU Xiao and HU Jun from CAS Key Laboratory of Quantum Information and CAS Center for Excellence in Quantum Information and Quantum Physics are the co-first authors. The matching authors are Prof. LI Chuanfeng and Prof. ZHOU Zongquan.