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Physicists have developed a method to detect hidden topological properties in light, even when energy leaks away from the system. Topology describes features that remain stable despite small changes, often labeled by whole numbers. While these properties usually exist in abstract momentum space rather than physical location, researchers previously inferred them indirectly by observing edge states. This approach becomes difficult in non-Hermitian systems where energy loss is significant, as strong dissipation weakens the signals carrying this information.

To solve this, a team led by Ali W. Elshaari at KTH Royal Institute of Technology created a programmable photonic integrated circuit. The chip uses light traveling through reconfigurable waveguides to simulate specific points in momentum space. By using a mathematical technique called unitary dilation, the researchers added extra channels to absorb energy that would normally be lost, allowing the signal channels to behave exactly like the lossy system they were studying. They used interferometry to mix light with reference beams and measure output intensities to reconstruct the hidden phase information.

The team successfully tested their method on a model called the non-Hermitian Su–Schrieffer–Heeger model, confirming that the topological quantity known as the Zak phase matched theoretical predictions. They also observed an exceptional point, where system modes merge, and verified its unique fingerprint through phase measurements. Furthermore, they extended the technique to a two-dimensional space using a Rice–Mele pump scheme, accurately determining the Chern number for both topological and trivial cases.

This work was published in Physical Review Letters by Andrea Cataldo and colleagues from institutions including Stockholm University and Singapore University of Technology and Design. The researchers note that while the chip simulates rather than represents real materials, its programmability allows it to explore entire families of topological phases on a single device. This flexibility offers a new way to study complex physics in regimes that are difficult to access with fixed-geometry devices.