“We used a silicon-vacancy colour centre, which is a special defect in the material that strongly interacts with light,” explains team member Ruffin Evans. “We placed silicon atoms inside nanophotonic diamond devices that guide photons using the same principle as optical fibres. The cross section of these devices is so small that a single colour centre can block the flow of light. In other words, the light is ‘focused’ into a device so small that a single colour centre can interact with it.

“We also use a set of special nanoscale mirrors – known as a photonic crystal cavity – to increase the interaction strength even more by bouncing the light back and forth over the colour centre thousands of times. Once we have this strong interaction, we can control the internal states of the centre to control the flow of light and make a switch from it.”

Two different electronic states

To go into more detail, the silicon colour centre can be prepared in two different electronic states by using laser fields, explains team member Alp Sipahigil. One of these states strongly interacts with light and reflects photons while the other state does not interact with light and is transparent to photons. “By choosing which state the colour centre is prepared in, we can switch the light transmission either on or off.”

One of the biggest advantages of the new system is that it is possible to create millions of the devices on a single chip using semiconductor fabrication techniques. “In our experiments, we created several thousand devices, but industry could do much better,” says Evans. “So these systems could be used to fabricate complicated on-chip networks or even optical quantum processors with many devices, all communicating using photons.”

And that is not all: the devices might even be employed in classical information processing. How? “Because they could be used to switch, control and store light without first converting it into an electrical signal,” answers Evans. “And since all long-distance communication is done with optical fields in fibre-optic cables, being able to control the propagation of light directly this way would have distinct advantages.”

Towards on-chip networks

The researchers, led by Mikhail Lukin, say they are now busy working on a new experiment in which two silicon colour centres in a single device interact directly by exchanging a photon. Here, a photon created by one colour centre will be “caught” by the second colour centre, explains Sipahigil. “This would allow us to directly and efficiently generate interesting (entangled) quantum states between multiple colour centres and would be a major step forwards in the quest towards on-chip networks.”

It is not all plain sailing though, he admits. “One potential drawback of our system is its short memory time. To switch the systems, we need to ‘store’ a photon in the colour centre. This changes the state of the centre and switches the incident light field. In our current set up, the photon stored in the centre is lost after tens of nanoseconds. We are developing techniques to extend this storage time (up to one second) such that a single colour centre can be used as a long-lived memory for photons.”

The work is detailed in Science DOI: 10.1126/science.aah6875.