CSU achieves on-demand quantum storage in communication bands

Academician Guangcan Guo's group at the Chinese University of Science and Technology has made important progress in the field of solid-state quantum storage. The team's research group, Chuanfeng Li and Zongquan Zhou, has achieved on-demand quantum storage of communication band photons based on erbium-doped waveguides, an important step toward building large scale fiber optic quantum networks. The results were published in the internationally recognized academic journal Physical Review Letters on November 15.

Quantum memory is the core device of quantum networks, and by reading entangled photons on-demand, the exponential loss in long-distance fiber optic transmission can be reduced to polynomial-level loss. In order to build quantum networks using existing optical fiber networks, quantum memories should operate in the communication band. Rare-earth erbium ions have unique communication-band optical transitions and are important candidates for the realization of communication-band quantum memories. However, the readout time of existing communication band quantum memories is pre-set before photon writing, and on-demand readout cannot be realized.

The research group of Chuanfeng Li and Zongquan Zhou has autonomously processed optical waveguides on erbium-doped yttrium silicate (167Er3+:Y2SiO5) crystals using laser direct writing technology, and integrated ordinary single-mode optical fibers directly pasted on both ends of the waveguides. To realize on-demand readout, the research group further processed on-chip electrodes on both sides of the waveguide using electronic vapor deposition technology, thus using the electric field-induced Stark effect to regulate the coherent evolution of erbium ions within the waveguide in real time. By polarizing the electron spin of the erbium ion and initializing its nuclear spin state, the storage efficiency of the photons is enhanced to 10.9%, a 5-fold enhancement of this efficiency compared to the previously reported quantum storage in the integratable communication band. The electric field-modulated on-demand quantum storage fidelity reaches 98.3%, far exceeding the classical limit considering the storage efficiency and photon statistics.

The results enable on-demand quantum storage in the communication band based on erbium ions, and this fiber integrated device can be directly docked to existing fiber networks. Similar to the invention of erbium-doped fiber amplifiers in classical communications, which made long-range fiber optic communications a reality, erbium ion-based quantum storage can be used to overcome exponential losses in long-range quantum communications, making erbium ions expected to play an important role in the construction of quantum networks again.

The work was highly praised by the reviewers: "By transitioning from europium to erbium the memory can be operated directly at the telecommunication wavelength and as By transitioning from europium to erbium the memory can be operated directly at the telecommunication wavelength and as such could integrate their devices with existing fiber optic technologies". "the results show important improvements with respect to previous demonstrations, especially regarding the stable fiber gluing to the waveguide in a cryogenic environment (this work makes important improvements with respect to previous work, especially regarding the stable fiber gluing directly to the optical waveguide to support stable operation in low-temperature environments)."

Co-first authors of the paper are Duancheng Liu, a PhD student at the Key Laboratory of Quantum Information, Chinese Academy of Sciences, and Pei-Wei Li, a postdoctoral fellow. This work was supported by the Ministry of Science and Technology, the National Natural Science Foundation of China, and Anhui Province. Zongquan Zhou was supported by the Youth Innovation Promotion Association of CAS.

Figure 1: Direct fiber-optic connection of the communication band quantum memory. (a). Local detail view of the fiber array connected to the erbium-doped waveguide; (b). Enlarged detail view of the waveguide cross section.

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