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Experiment Paves the Way to Directly Explore High-Dimension Topological Physics in a Compact Platform

Prof. LI Chuanfeng, Prof. XU Jinshi, Prof. HAN Yongjian, and their colleagues from Prof. GUO Guangcan's group at the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) have first synthesized a one-dimensional (1D) lattice via introducing photon's spin-orbit coupling in a cavity. This work was published in Nature Communications.

The dimensions of physical models in real space are generally smaller than or equal to their geometric dimensions (in our daily settings, the geometric dimension is three). To understand the physical phenomenon in spaces of different dimensions, scientists are working on methods to synthesize dimensions previously.

There're two typical ways to synthesize dimensions. One is to couple discrete physical states to create artificial lattice structure of a certain dimension. The other is to introduce external degrees of freedom, or external parameters, as extra dimensions.

Introducing the internal degrees of freedom of photons or atoms as the synthetic dimensions has got much attention, as the approach avoids complex experimental requirements. For example, the (D+d)-dimensional system is a system involving D geometric dimensions and d synthetic dimensions.

Photonic orbital angular momentum (OAM) has infinite topological charge numbers, making it an ideal degree of freedom for constructing the synthetic lattice. Researchers from USTC firstly proposed the scheme to realize quantum simulation based on the synthetic photonic OAM dimension in 2015. Then they successively built different degenerative optical cavities based on plane mirrors (2017), spherical mirrors (2018), and elliptic mirrors (2019).

Based on the previous researches, the researchers first realized the coupling between the photonic OAM and the internal spin angular momentum (SAM) in a standing-wave degenerate cavity (which is usually considered as a 0-dimensional system in optics) based on the plane mirror. The OAM of photons in the cavity corresponded to the 1D discrete lattice, and photons with different OAM served as quasi-particles in the lattice sites. The coupling of SAM allowed the simulation of transition between sites.

By detecting the transmission intensity of the degenerate cavity, many physical quantities were directly measured, including the density of states, energy band structures, and topological windings of the simulated periodically driven topological system.

This experiment paves the way to directly explore high-dimension topological physics with synthetic dimensions in a highly compact platform. The degenerate cavity containing OAM is also helpful for constructing all-optical devices such as quantum memory and optical filters.


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