Researchers from the University of the Witwatersrand in South Africa and Huzhou University have discovered hidden topological structures in entangled photons, reaching up to 48 dimensions. These patterns emerge from the orbital angular momentum of light produced via spontaneous parametric downconversion. The findings, published in Nature Communications, suggest new ways to encode quantum information.
Scientists at the University of the Witwatersrand (Wits) in South Africa, collaborating with researchers from Huzhou University, have identified previously unseen topological structures in entangled photons generated by spontaneous parametric downconversion (SPDC), a common quantum optics technique. These structures extend to 48 dimensions and include over 17,000 distinct topological signatures, forming a new 'alphabet' for stable quantum information encoding. The discovery relies solely on the orbital angular momentum (OAM) of light, challenging prior assumptions that multiple properties like OAM and polarization were needed for such topologies. Professor Andrew Forbes from the Wits School of Physics stated, 'We report a major advance in this work: we only need one property of light (OAM) to make a topology, whereas previously it was assumed that at least two properties would be needed -- usually OAM and polarization. The consequence is that since OAM is high-dimensional, so too is the topology, and this let us report the highest topologies ever observed.' Beyond two dimensions, these topologies require a range of values to describe, unlike simpler systems. Pedro Ornelas noted, 'You get the topology for free, from the entanglement in space. It was always there, it just had to be found.' Lead author Prof. Robert de Mello Koch from Huzhou University explained, 'In high dimensions it is not so obvious where to look for the topology. We used abstract notions from quantum field theory to predict where to look and what to look for -- and found it in the experiment!' The effect is accessible in most quantum optics labs without specialized equipment. The study, titled 'Revealing the topological nature of entangled orbital angular momentum states of light,' appears in Nature Communications (2025; 16(1); DOI: 10.1038/s41467-025-66066-3), with authors Robert de Mello Koch, Pedro Ornelas, Neelan Gounden, Bo-Qiang Lu, Isaac Nape, and Andrew Forbes.
