Entanglement is the core resource of quantum information processing. The experimental preparation of entanglement states is a basic task in the field of quantum information. High dimensional quantum entanglement has stronger nonlocality, larger information storage capacity, and better security performance.

The high-dimensional entanglement can be realized through the independent coding of a single degree of freedom of the photons, such as path and orbital angular momentum. While by joint regulation of multi-degree of freedom, more possibility and flexibility can be offered. Usually, the generation of multiple degrees of freedom or high-dimensional entanglement involves photons going through bulky optical elements such as beam displacer, polarization elements, spiral phase plate, etc.

Metasurface, with the advantages of high efficiency and flexible regulation ability, has become a powerful tool in quantum state manipulation and have been preliminarily applied in experiments in recent years.

To address the problems, a joint research group led by Prof. Yanqing Lu at Nanjing University in collaboration with Prof. Prof. Lijian Zhang, Prof. Keyu Xia and Prof. Ting Xu, has demonstrated and experimentally verified the a high-dimensional entanglement generation scheme based on a dielectric metasurface. The relevant research results were published in Photonics Research, Volume 12, 2022 (Zhi-Xiang Li, Dong Zhu, Pei-Cheng Lin, Peng-Cheng Huo, Hong-Kuan Xia, Ming-Ze Liu, Ya-Ping Ruan, Jiang-Shan Tang, Miao Cai, Hao-Dong Wu, Chao-Ying Meng, Han Zhang, Peng Chen, Ting Xu, Ke-Yu Xia, Li-Jian Zhang, Yan-Qing Lu. High-dimensional entanglement generation based on a Pancharatnam–Berry phase metasurface[J]. Photonics Research, 2022, 10(12): 2702).

The designed dielectric metasurface is a Pancharatnam-Berry phase-based fork-polarization gratings. It was used for joint modulation of the path, pin angular momentum (SAM) and orbital angular momentum (OAM) of photons. In this work, they send one of the entangled photons to interact with the metasurface for multi-dimensional information encoding. Experimentally a four-qubit state was generated.

They experimentally demonstrated the nonlocal correlations between different degrees of freedom by correlated imaging (Fig.1). They also performed the quantum state tomography to further verify the entanglement.

Fig. 1 Experimental results of the coincidence images.

If two metasurfaces are used separately to modulate Alice's and Bob's photons, one can easily obtain a six-qubit state. Moreover, with a different design of the metasurface, one can encode higher topological charges of the OAM mode or the number of the paths, thus various high-dimensional entanglement schemes can be achieved.

This work proves that metasurface offers an excellent platform for high-dimensional quantum information processing. This scheme has the potential to achieve on-chip quantum state manipulation, which is promising in an overall integrated high-capacity quantum communication system.