The team of Academician Guo Guangcan of our university has made important progress in the field of solid-state quantum storage. The research group of Li Chuanfeng and Zhou Zongquan based on the self-machining laser direct writing waveguide achieved the integrable solid-state quantum storage of photon polarization states, and the storage fidelity was as high as 99.4±0.6%, which significantly promoted the application of integrable quantum memory in quantum networks. The relevant results were recently published in the internationally renowned academic journals Science Bulletin and Physical Review Letters.

The polarization state of photon has the characteristics of high operation precision and strong anti-interference ability, and has a wide range of applications in quantum information tasks. Realizing integrable quantum storage of polarization states is a basic requirement for constructing large-scale integrable quantum networks. As a kind of solid quantum memory medium with excellent properties, rare earth doped crystals can be combined with various micro and nano processes to prepare integrated quantum memory. However, the existing integrated solid-state quantum memory cannot realize the quantum storage of polarization state, because the light absorption of rare earth doped crystals is generally dependent on the polarization state, and the micro-nano waveguide structure does not support the transmission of arbitrary polarization state.

Eu3+:YSO (europium-doped yttrium silicate crystal) is an important candidate material for the implementation of mobile quantum flash drives, and the research group of Li Chuanfeng and Zhou Zongquan have achieved coherent optical storage for up to 1 hour based on this material [Nature Communications 12, 2381 (2021)]. In recent work, the team noted that Eu3+ (site-2Eu3+), which occupies the second yttrium substitution in yttrium silicate crystals, can achieve uniform absorption of any polarization state. The research team first used the spectral hole burning technique to determine the quasi-energy level structure of the substituted dieuropium ion, and combined with the original "noise-free photon echo (NLPE)" quantum storage scheme [Nature Communications 12, 4378 (2021)] to overcome the weak absorption problem of substituted dieuropium ion. Finally, the quantum storage of the polarization state is realized based on a single crystal with a single pass. This work, which proposes and confirms that substitutional dieuropium ions can achieve quantum storage of polarization states, was recently published in the Science Bulletin.

The research group further uses femtosecond laser direct writing technology to fabricate concave cladding waveguides in europium-doped yttrium silicate crystals. This kind of waveguide has a circular symmetric structure and can support low-loss transmission in any polarization state. The research team has successfully realized the quantum storage of polarization states based on waveguide structure by using the spectral hole burning technique to increase the absorption depth of the substituted europium ion by 2.6 times, and combined with the atomic frequency comb quantum storage scheme modulated by electric field. The quantum storage fidelity of 99.4±0.6% validates the high reliability of this integrable device. The work was published May 2 in the journal Physical Review Letters.

This work applies the polarization degree of freedom of photons to the field of integrated quantum memory, and lays a foundation for the construction of quantum networks based on polarization coding. At the same time, the polarization degree of freedom provides an effective filter degree of freedom for noise suppression of integrated devices, which is of great significance for the practical application of integrated quantum memory.

a, experimental device diagram; b, energy level structure diagram of substituted dieuropium ion in yttrium silicate crystal; c, top view of the crystal, both sides of the metal for integrated coplanar electrodes; d, crystal side view, dotted line frame is the optical waveguide; e: Cross section of the waveguide transmission mode.

The work was highly praised by reviewers: “I think that the paper shows important results,  as it demonstrates for the first time the compatibility of laser-written waveguides inYSO with polarization encoding,  and it broads the technological applicability of this relatively new integrated platform. (The paper reports important results, Because it demonstrates for the first time the compatibility of laser direct written waveguides and polarization coding in yttrium silicate crystals, expanding the technical applicability of this new integrated process platform); “The realization is clearly at the state of the art with the highest level of complexity and  technicality. (This experiment is clearly at the state of the art, of the highest complexity and technicality.)"

The first authors of the two papers are Ming Jin and Tianxiang Zhu, PhD candidates at the Key Laboratory of Quantum Information of the Chinese Academy of Sciences. Sample electrode processing is supported by teacher Ye Yang and teacher Li Wenjuan from Micro and Nano Processing Center of China University of Science and Technology. The work was funded by the Ministry of Science and Technology, the Hefei National Laboratory, the National Natural Science Foundation of China, and Anhui Province. Zhou Zongquan received funding from the Youth Innovation Promotion Association of the Chinese Academy of Sciences.

Paper link:

https://www.sciencedirect.com/science/article/pii/S2095927322000330?via%3Dihub

https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.128.180501

(Key Laboratory of Quantum Information, Academy of Quantum Information and Quantum Technology Innovation, Scientific Research Department, Chinese Academy of Sciences)