China University of Science and Technology realizes millisecond level integrated quantum memory

Recently, the team led by Academician Guo Guangcan from the University of Science and Technology of China has made significant progress in the field of integrated quantum storage. The research team led by Li Chuanfeng and Zhou Zongquan has improved the storage time of integrated quantum memory from 10 microseconds to milliseconds based on their original noiseless photon echo (NLPE) scheme, while successfully breaking through the efficiency of traditional fiber delay lines. The achievement was published on March 26th in the internationally renowned academic journal Science Progress.

As a core device for overcoming channel loss and building large-scale quantum networks, the large-scale application of optical quantum memory requires the integration of devices to achieve the goal of small size and low power consumption. Since 2011, various processes have been used internationally to prepare integrated quantum memories in rare earth doped crystals. However, due to the difficulty in filtering out noise and limited storage efficiency in integrated devices, existing devices can only achieve storage in atomic excited states, with a storage time of only 10 microseconds. The storage efficiency is far lower than the transmission efficiency of fiber delay lines, fundamentally limiting their practical application in remote quantum communication.

To solve this problem, the research group of Li Chuanfeng and Zhou Zongquan used femtosecond laser microfabrication technology to prepare circularly symmetrical concave cladding optical waveguides in europium doped yttrium silicate crystals, achieving noise filtering based on polarization degrees of freedom. Combined with the team's original NLPE quantum storage solution, the storage efficiency was greatly improved, thus achieving spin wave integrated quantum storage in the atomic ground state [National Science Review 12, nwae 161 (2024)].

Recently, the team integrated a coplanar waveguide on the surface of a crystal and achieved dynamic decoupling control of the spin transition of europium ion nuclei within the waveguide by applying a radio frequency magnetic field, thereby extending the spin wave quantum storage lifetime to the millisecond level. When the storage time of optical qubits reaches 1.021 milliseconds, their storage efficiency reaches 12.0 ± 0.5%, which far exceeds the transmission efficiency of the corresponding delay fiber delay line (only 0.01%), fully proving that integrated quantum storage devices are no longer functionally replaceable by fiber delay lines.

Figure 1. Schematic diagram of long-life integrated quantum storage experiment, illustrating the details of the incident end face of the memory.

Figure 2. Efficiency and lifetime performance of integrated quantum memory. The performance of fiber optic delay lines is represented by blue dashed lines, and the red pentagram represents the performance of this achievement.

This work has increased the lifespan of integrated quantum memory from 10 microseconds to milliseconds, achieving a breakthrough in storage efficiency beyond fiber delay lines for the first time, laying a solid foundation for the practical application of integrated quantum storage in long-range quantum networks. At the same time, this achievement demonstrates the enormous potential of NLPE solutions in solving the signal-to-noise ratio problem of long-lived quantum storage. The reviewer highly praised: 'This is a very important achievement in the field of integrated quantum memories', “this work makes a significant contribution to the development of integrated and long-duration quantum memories”（ This work has made significant contributions to the development of integrated and long-lived quantum memories.

The first author of this paper is Liu Yuping, a doctoral student in the Key Laboratory of Quantum Information, Chinese Academy of Sciences. This work has been supported by the Science and Technology Innovation 2030 Major Project, the National Natural Science Foundation of China, Anhui Province and the Chinese Academy of Sciences. Zhou Zongquan was supported by outstanding members of the Youth Innovation Promotion Association of the Chinese Academy of Sciences.

Source: opticsky