December 17, 2025 -- Researchers from the University of the Witwatersrand in South Africa, in collaboration with Huzhou University, discover that the entanglement workhorse of most quantum optics laboratories can have hidden topologies, reporting the highest ever observed in any system: 48 dimensions with over 17 000 topological signatures, an enormous alphabet for encoding robust quantum information.

Most quantum optics laboratories produce entangled photons by a process of spontaneous parametric downconversion (SPDC), which naturally produces entanglement in “space”, the spatial degrees of freedom of light.  Now the team have found that hidden in this space is a world of high-dimensional topologies, offering new paradigms for encoding information and making quantum information immune to noise.  The topology was shown using the orbital angular momentum (OAM) of light, from two dimensional to very high dimensions.

Reporting in Nature Communications, the team showed that if one measures the OAM of two entangled photons it can be shown to have a topology: an underlying feature of the entanglement itself.  Since OAM can take on an infinite number of possibilities, so too can the topology.  “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 polarisation,” says Professor Andrew Forbes, from the Wits School of Physics. “The consequence is that since OAM is high-dimensional, so too is the topology, and this let us report the highest topologies ever observed.”  The team showed that once the topology exceeds two dimensions, a spectrum of topological numbers are needed rather than just one, as is seen in usual optical topologies.  

A major benefit of this discovery is that the resource needed to make it work is common to most quantum optics laboratories and doesn’t need any special “quantum engineer”.  Pedro Ornelas explains, “You get the topology for free, from the entanglement in space.  It was always there, it just had to be found.”

Prof. Robert de Mello Koch, lead author from Huzhou University explains further, “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!”

Orbital angular momentum entanglement has been studied and used in many quantum systems, but thus far has suffered from fragility.  Now the team believe that OAM entanglement can be revisited from the perspective of its underlying topology, opening new avenues for its use in real-world quantum systems.