An ordinary laser amplifies light into a bright beam. Run the process in reverse, and you get an anti-laser — a device that sucks in light without a trace. But such a machine is finicky: even a slight disorder can ruin the absorption.
Physicists have found a way to make the anti-laser invulnerable. They built it on topological edge states — special waves that travel only along the boundary of a material and ignore obstacles, like water in a river that flows around rocks without losing its current.
In the experiment, they used a network of microwave resonators with a non-reciprocal response — the signal travels in one direction, like a one-way street. Even when part of the network was severely damaged, the anti-laser continued to absorb almost all incoming light.
Such robustness opens the way to reliable light detectors that work under imperfect conditions — for example, in dusty atmospheres or inside living tissues.
The anti-laser was first demonstrated in 2010, but until now it remained fragile: the slightest imperfection destroyed perfect absorption. The new topological version withstands severe damage for the first time.