For many years engineers have wanted to use light instead of electricity to build circuits with. It moves slightly faster, but more importantly signals can pass one another without interfering. This lack of interference makes it very difficult to make light beams interact when you want them to.
One of the simplest electronic devices is the diode, effectively a one way 
valve for electricity, in light they would be a true one way mirror, and are very difficult to build. So far they have been mm or even cm across and it hasn't been possible to integrate them into a silicon chip. But Caroline Ross and collegues at MIT have managed to do this.
They have built a structure which involves a tiny conducting silicon loop electrical resonator which can absorb energy from light which is passing through a nearby piece of garnet covering half of the loop.
This garnet is magnetic as well as transparent, so applying a magnetic field means that the loop will absorb a different colour of light in one direction to the other, so if you pick your colour it will be absorbed 100 times more in one direction but than the other.
In the first case this type of device would probably be built in front of a laser to stop reflections interfering in its operation, but a diode is a vital component for building more complex photonic circuits in the future. Which amongst other things should help route the optical signals which move most of the data around the internet.
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