It is like a valve for light – the first material that transmits electromagnetic radiation in one direction only.

Zheng Wang and colleagues at the Massachusetts Institute of Technology have made what’s known as a photonic crystal from an array of ferrite rods.

Ordinarily the rods reflect microwaves in all directions, but when Wang adds a strong magnetic field, everything changes. Microwaves bouncing off the magnetised rods in one direction are modified to become “evanescent”, fading as they travel. Waves heading the other way carry on unhindered.

Wang is now working on plans to adapt his device for higher frequency radiation, moving intothe terahertz and eventually the visible part of the spectrum. These photonic crystals may help create new components for optical communication and computing, inciuding simple one-way routers analogous to electrical diodes, as well as wave-guides for light that are more efficient than optical fibres. “It raises a lot of intriguing pOSSibilities,” says optical phYSicist Duncan Haldane of Prince ton University.