Breakthrough in Quantum Optics: Wits Team Reveals Hidden Topologies in Entangled Light

In a groundbreaking advancement for quantum physics, researchers from the University of the Witwatersrand (Wits) in Johannesburg, South Africa, have uncovered a vast array of hidden topological structures within conventionally generated quantum entangled photons. Published in Nature Communications on December 12, 2025, the study demonstrates that orbital angular momentum (OAM)—a fundamental property of light—harbors entanglement across an unprecedented 48 dimensions, revealing more than 17,000 distinct topological signatures. This discovery challenges long-held assumptions in quantum optics and opens new pathways for robust quantum technologies.

Quantum entanglement, often dubbed 'spooky action at a distance' by Albert Einstein, occurs when two or more particles become correlated such that the state of one instantly influences the other, regardless of distance. In quantum optics, this is typically produced via spontaneous parametric down-conversion (SPDC), where a laser beam splits into entangled photon pairs. While spatial entanglement is well-known, the Wits team revealed that these states conceal rich topological features when examined through the lens of OAM, the helical phase front of light beams resembling twisted corkscrews.

Led by Professor Andrew Forbes from Wits' School of Physics and in collaboration with Huzhou University in China, the researchers—including Robert de Mello Koch, Pedro Ornelas, Neelan Gounden, Bo-Qiang Lu, and Isaac Nape—used standard lab setups to probe these hidden dimensions. Their work not only confirms theoretical predictions from quantum field theory but also experimentally validates them in high-dimensional spaces.

Understanding Orbital Angular Momentum and Its Role in Entanglement

Orbital angular momentum (OAM) refers to the azimuthal phase structure of light, where photons carry quantized helical wavefronts characterized by a topological charge ℓ. Unlike spin angular momentum (polarization), OAM offers an infinite-dimensional Hilbert space, making it ideal for high-capacity quantum information processing. In entangled OAM states, the sum of the OAM values of paired photons is conserved due to SPDC dynamics, creating a broadband spiral spectrum.

The Wits study shows that mapping these states onto spheres or higher manifolds reveals skyrmion-like textures—vector fields wrapping around the surface with non-trivial winding numbers. For two-dimensional cases, these equate to 't Hooft-Polyakov monopoles in Yang-Mills-Higgs theory, linking optics to particle physics. As dimensions increase, the topology explodes: from simple skyrmions in 2D to complex SU(d) gauge field invariants in higher d.

This single degree-of-freedom approach (OAM alone) surpasses prior methods requiring multiple properties like polarization, simplifying experimental access while amplifying complexity.

The Wits Structured Light Laboratory: A Hub for Quantum Innovation

At the heart of this discovery is Wits' Structured Light Laboratory, directed by Professor Andrew Forbes, a globally recognized expert in structured light. The lab has pioneered quantum optics in Africa, producing over 500 publications and training numerous PhD students. Past achievements include kilometer-scale quantum networks and topological quantum states resilient to noise.

In South Africa's higher education landscape, Wits stands out for quantum research amid limited funding. The South African Research Chairs Initiative supports Forbes' work, fostering collaborations with international partners like Huzhou University. This positions Wits as a leader in continental quantum efforts, alongside institutions like Stellenbosch and UCT.

For aspiring researchers, opportunities abound in South African universities. Explore research jobs or South African academic positions to join such cutting-edge teams.

Methodology: From Theory to Experiment

Theoretically, the team employed non-Abelian SU(d) Yang-Mills theory to model OAM entanglement as gauge fields on high-dimensional manifolds. Topological invariants—quantities unchanged under continuous deformations—were computed via homotopy groups and Chern numbers.

Experimentally, entangled photons were generated via SPDC in a beta-barium borate crystal pumped by a 405 nm laser. OAM states were measured using quantum state tomography, reconstructing density matrices for concurrence, purity, and topological spectra up to d=7, extrapolating to 48 dimensions with 17,296 invariants for d=7 alone.

• Pump laser alignment for broadband OAM spectrum.
• Spatial light modulators for mode projection.
• Coincidence detection for entanglement verification.
• Numerical glueing of disk-to-disk maps for higher topologies.

This rigorous approach confirmed theoretical predictions with high fidelity (>0.95) and signal-to-noise ratios.

Key Findings: A Spectrum of 17,000+ Topological Signatures

The study unveiled:

• Skyrmion numbers from -8 to +5 in 2D OAM states, visualized as vectorial textures on spheres.
• High-dimensional topologies up to 48 dims, dwarfing prior records (e.g., 3-4 dims in spin systems).
• 17,000+ invariants in d=7, scaling factorially with dimension for encoding capacity.
• Robustness: Topologies persist under noise (purity >0.7), emerging even in non-topological subspaces.

"You get the topology for free, from the entanglement in space. It was always there, it just had to be found," notes Pedro Ornelas.

Theoretical Insights from Quantum Field Theory

Drawing from quantum field theory, the OAM-entangled state mimics the asymptotic Higgs field, with skyrmions as monopoles. In higher dims, Lie algebra vector fields yield a 'topological spectrum'—a complete set of invariants classifying states. This framework predicts infinite topologies given OAM's unbounded nature, verified experimentally.

For South African students, this exemplifies how abstract math underpins experimental physics, aligning with Wits' strong theoretical physics programs.

Implications for Quantum Technologies

This discovery revolutionizes quantum information science:

• Encoding: 17k+ signatures per state enable ultra-high-capacity quantum bits (qudits).
• Noise Resistance: Topological protection shields data, vital for noisy intermediate-scale quantum (NISQ) devices.
• Communication: Enhanced OAM quantum key distribution (QKD) for secure networks.
• Computing: Topological qubits for fault-tolerant systems.

"In high dimensions it is not so obvious where to look... We used abstract notions from quantum field theory to predict... and found it," says Prof. Robert de Mello Koch. Access the full paper here.

South Africa's Growing Quantum Ecosystem

South Africa invests in quantum via the National Quantum Strategy, with Wits at the forefront. The South African Quantum Initiative fosters hubs at Wits, Stellenbosch, and UKZN. This discovery bolsters SA's position, attracting funding and talent amid global quantum race (e.g., US Quantum Economic Development Consortium, EU Quantum Flagship).

Statistics: SA produces ~200 quantum PhDs annually; Wits contributes 20%. Impacts include job creation in research assistant roles and spin-offs.

Challenges and Future Directions

Challenges: Scaling to infinite dims requires better detectors; decoherence in real channels. Future: Integrate with Wits' prior noise-resilient topologies; explore temporal OAM; continental quantum network.

"We report a major advance... only need one property of light (OAM)," Prof. Forbes. Wits plans extensions to qudits and multimode fibers.

Photo by Buddha Elemental 3D on Unsplash

Opportunities for Higher Education and Careers

This underscores Wits' excellence, drawing students globally. For South Africans, programs like MSc in Quantum Optics offer entry. Explore academic CV tips or scholarships.

In conclusion, Wits' 48-dimensional topologies discovery elevates SA quantum research. Stay informed via Rate My Professor, pursue higher ed jobs, or university jobs. Share insights in comments below.