The research team of Li Chuanfeng and Zhou Zongquan, Academician Guo Guangcan of the University of Science and Technology of China, has realized on-demand quantum storage of photons in the communication band based on erbium-doped waveguides, taking an important step toward the construction of large-scale optical fiber quantum networks. The results were recently published in Physical Review Letters.

Quantum memory is the core device of quantum network. By reading entangled photons on demand, exponential loss can be reduced to polynomial loss in long-distance optical fiber transmission. In order to construct a quantum network using the existing optical fiber network, the quantum memory should work in the communication band. The rare earth erbium ion has a unique optical transition in communication band and is an important candidate material for realizing quantum memory in communication band. However, the readout time of the existing communication band quantum memory is pre-set before the photon is written, so it cannot be read on demand.

The research group of Li Chuanfeng and Zhou Zongquan independently fabricated optical waveguides on erbium-doped yttrium silicate crystals using laser direct writing technology, and pasted ordinary single-mode fibers directly on both ends of the waveguides. In order to realize the on-demand reading, the research team further uses electron evaporation technology to fabricate on-chip electrodes on both sides of the waveguide, thus using the electric field-induced Stark effect to control the coherent evolution of erbium ions in the waveguide in real time. By polarizing the electron spin of erbium ion and initializing its nuclear spin state, the photon storage efficiency is improved to 10.9%, which is a five-fold increase compared to the previously reported integrated communication band quantum storage. The fidelity of on-demand quantum memory controlled by electric field is 98.3%, which far exceeds the classical limit considering storage efficiency and photon statistics.

This achievement realizes on-demand quantum storage of communication bands based on erbium ions, and this optical fiber integrated device can directly interconnect with existing optical fiber networks. In the field of classical communication, the invention of erbium-doped fiber amplifier made long-distance fiber communication a reality, and similarly, quantum storage based on erbium ions can be used to overcome exponential losses in long-distance quantum communication.

Reviewers commented on the work: "By using erbium ion doping, the memory can operate directly in the communication band and achieve integration with existing optical fiber devices. This work is an important advance compared to previous work, especially the optical fiber directly attached to the optical waveguide, to support stable operation at low temperatures."

Related paper information:

https://doi.org/10.1103/PhysRevLett.129.210501

Journal of Chinese Science (2022-11-25, 1st edition)

https://news.sciencenet.cn/sbhtmlnews/2022/11/372143.shtm