Light can get stuck inside tiny structures, like sound in a perfectly tuned bell: the energy does not escape, even though there are no walls around. Such states are called 'bound states in the continuum'—light is trapped but not confined in a box. Until now, they were produced in ordinary crystals, where left and right directions are indistinguishable.

Now, a team of researchers has for the first time created such states in a magnetic photonic crystal that operates without an external magnet: it is enough to magnetize the material in advance. It turns out that the trapped light acquires chirality: it begins to 'prefer' one direction of rotation, like a right-handed screw. The most interesting part is that this chirality can be switched to the opposite one simply by flipping the crystal's magnetization, without altering its structure.

The big surprise is the incredible robustness of these states. Ordinary 'bound states in the continuum' light modes are very sensitive to the slightest imperfections: one scratch, and the trap breaks. Here, thanks to magnetism, the light remains trapped even with noticeable damage to the crystal. It is like a bell that keeps ringing with the same clarity even after being struck with a hammer.

These properties open the door to reliable optical chips that could distinguish 'left-handed' and 'right-handed' molecules — which is critically important in pharmaceuticals, since mirror-image versions of a drug can act completely differently.