Reading hidden topology in light, even when energy leaks away
When I explain topology to students, I start with a knot in a rope. You can stretch it, twist it or shake it, but the knot stays until you cut the rope. Physicists have found that some materials and devices carry similar "knots" in how waves move through them. These are topological properties, labeled by whole numbers that don't change under small imperfections. That robustness is why topology…
Topology, a central concept in modern physics, is represented by whole numbers that remain constant despite small imperfections. These numbers are found in materials and devices, promising electronics, photonics, and quantum devices that can tolerate defects and noise. However, it is challenging to study these topological numbers directly in momentum space, which describes how waves travel, as most experiments infer them from consequences like special edge states.
Non-Hermitian systems, which lose energy to their surroundings, pose an even greater challenge, as strong loss weakens the topological signal. To address this, researchers developed a programmable photonic integrated circuit that can scan momentum space and read out topology directly, even in lossy systems. The chip, a small device with a reconfigurable network of waveguides, can reproduce the behavior of a system in one point of momentum space and then another point.
Loss was handled using a mathematical technique called unitary dilation, which embeds the lossy evolution inside a larger, loss-free one. By using interferometry, the researchers measured the output intensities and reconstructed the signal's strength and phase, where the topological information resides. The phase revealed the topological properties, such as the Zak phase and the Chern number, with high accuracy.
This method allows for exploring a wide range of topological phases on a single device, overcoming the limitations of fixed-geometry devices.
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