Based on the self-developed periodic polarization lithium niobate waveguide, Professor Zhang Qiang, Xie Xiuping Gaogong, and Zheng Mingyang associate researchers at the Quantum Detection and Waveguide Device Laboratory of Jinan Institute of Quantum Technology helped the academician Pan Jianwei of the University of Science and Technology of China successfully. A quantum memory was entangled and demonstrated a two-node entanglement via 22 kilometers of external field fiber. This achievement has taken the practical quantum repeater a step forward and laid a solid foundation for building quantum networks based on quantum relays. Related papers have recently been published in the international authoritative academic journal Nature.

NATURE

Constructing a global quantum network and realizing quantum communication on this basis is one of the ultimate goals of quantum information research. At present, the farthest point-to-point ground safety communication distance is only on the order of one hundred kilometers. Through the development of quantum relay technology, it is expected to further expand the safety communication distance significantly.

However, due to technical bottlenecks such as low entanglement of light and atoms, mismatch of atomic memory wavelength with communication fiber, and remote single photon interference, the farthest fiber quantum relay was only on the order of kilometers. Aiming at the above technical problems, the research team first adopted the ring cavity enhancement technology to improve the coupling between single photon and atomic ensemble, and optimized the optical path transmission efficiency, increasing the brightness of the previous entanglement of light and atoms by an order of magnitude. Wavelength loss in the fiber is about 3.5 dB / km. In 50 km fiber, and the optical signal will be attenuated to one billionth of a billion, making quantum communication impossible. The periodically-polarized lithium niobate waveguide independently developed by Professor Zhang Qiang’s team converts the light wavelength of the memory from the near infrared to the communication band through a non-linear difference frequency process. After 50 kilometers of optical fiber, it is attenuated to more than one percent. It is 16 orders of magnitude higher than before. Finally, in order to achieve long-range single-photon interference, the research team designed and implemented a dual phase locking scheme, which successfully controlled the optical path difference caused by 50 kilometers of optical fiber transmission to about 50 nanometers.