Precisely shaping single photons or entangled pairs in space and time for specific applications.

Developing fast methods to characterise complex high dimensional quantum states.

Using quantum correlations of structured light for imaging in challenging conditions.

Key Tools & Techniques

On chip integrated photonics, non-linear optics, multi-plane light conversion.

Spatial light modulators, orbital angular momentum (OAM) modes, custom laser sources.

Hybrid tomography/Bell measurement approaches, digital light projection.

coincidence detection, machine learning integration.

Entangled photon pairs, coincidence detection, machine learning integration.

Quantum Imaging. The lab has developed a quantum camera that uses the correlations between entangled photons to see through scattering media like living tissue or tinted glass. One photon probes the object while its entangled partner is detected by a camera, enabling image reconstruction.

Fast Quantum State Measurement. A major practical hurdle has been the time required to measure (‘certify’) high dimensional states. A Wits PhD student team developed a novel hybrid method that can determine key parameters of a 100-dimensional entangled state in minutes, a process that would take decades with standard methods.

Current Challenges and Future Directions

Despite significant progress, Professor Andrew Forbes notes remaining challenges. A primary one is that long distance transmission of spatially structured photons through real world channels (like turbulent air or optical fibres) is currently difficult, limiting their reach compared to simpler degrees of freedom like polarisation. To overcome this, researchers are exploring more abstract degrees of freedom, such as imbuing quantum states with topological properties, which could offer inherent robustness to disturbances and preserve quantum information even with fragile entanglement.

Their approach does not focus on maintaining perfect entanglement under all conditions. Instead, it engineers quantum states where the key information is tied to a topological number (like a 'skyrmion number'). This number is robust and remains stable as long as some entanglement persists, even while traditional measures of quantum state quality degrade under noise.

Key Topological Structures and Their Robustness

Skyrmions

Characterised by an integer skyrmion (wrapping) number.

The discrete number is unaffected by noise that merely deforms the state. It only flips if entanglement is completely destroyed.

Tested against isotropic background noise and simulated atmospheric turbulence.

High Dimensional OAM Topology

A rich spectrum of thousands of topological invariants hidden in standard orbital angular momentum (OAM) entanglement.

Offers an enormous 'alphabet' for encoding information. The topology emerges naturally from common lab entanglement, requiring no special engineering.

Discovered in entangled photon pairs generated via spontaneous parametric down conversion (SPDC), up to 7 dimensions

Wits University's Research Pillars in Quantum Imaging

Quantum Correlated Imaging

Using entangled photon pairs to image an object indirectly. One photon probes the scene, its entangled partner (which never sees the object) goes to a camera, the image is reconstructed from their quantum correlations.

Quantum camera, coincidence measurement to extract correlations from noise.

Intelligent Imaging Systems

Integrating machine learning (ML) and AI to process the complex data from quantum imaging systems

Algorithms for fast object recognition and image interpretation from sparse quantum .

Automated detection of concealed objects or biological indicators for security and medical diagnostics.

Structural & Topological Light

Engineering the spatial shape and topological properties of photons to encode more information and create inherently robust states.

High dimensional encoding, creating states with topological invariants (like skyrmions) that resist noise.

Noise resilient quantum communication and sensing, enabling reliable operation in real world, turbulent conditions.