Topological Defects and Geometrical Frustration in Fourier Photonic Simulator.

The XY models with continuous spin orientation play a pivotal role in understanding topological phase transitions and emergent frustration phenomena such as superconducting and superfluid phase transitions. However, the complex energy landscapes arising from frustrated lattice geometries and competing spin interactions make these models computationally intractable. To address this challenge, we designed a programmable photonic spin simulator capable of emulating the XY models with tunable lattice geometries and spin couplings, allowing systematic exploration of their statistical properties. We experimentally observed the Berezinskii-Kosterlitz-Thouless transition in a square-lattice XY model with nearest-neighbor interactions and determined its critical temperature. Expanding to frustrated systems, we implemented the approach in triangular and honeycomb lattices, uncovering sophisticated phase transitions and frustration effects, which were consistent with theoretical predictions. This versatile platform opens avenues for probing unexplored XY model phenomena across diverse geometries and interaction schemes, with potential applications in solving complex optimization and machine learning problems.