YOU’LL never catch a rainbow, but it might just be possible to trap one inside a specially designed grating. If the technique works, the advance would revolutionise computing and telecommunication networks.

In existing networks and computers, signals have to be converted back and forth from optical to electrical. Devices that use optical signals alone could be much faster and more efficient, but doing this means being able to stop or slow down light long enough to carry out the necessary computations or operations.

Light has been slowed down, and even stopped, inside gases of ultra-cold atoms and inside specially …