Bonded photons present a new form of matter

The invention of real life Star Wars-style light sabres just got a tiny little bit closer – but would-be Jedi shouldn’t hold their collective breath yet.

In a paper published in the journal Science a team led by physicist Vladan Vuletić of the Massachusetts Institute of Technology, US, reports inducing for the first time ever the successful interaction of photons.

This is important because normally photons have nothing to do with each other: in company, they pass by, utterly unaffected by proximity to others. That’s why light sabres are just wishful thinking for geeks: they clearly rely on beams of light that can collide and recoil from each other.

Vuletić and colleagues, however, have successfully induced groups of two and three individual photons to bind together – in effect, creating a completely new kind of phototonic matter.

To do so, they began with a cloud of rubidium atoms, which they chilled to just a millionth of a degree above absolute zero. The ultra-chilly conditions meant that the atoms slowed to almost a complete standstill.

Into this frozen wasteland the scientists shone an extremely weak laser beam. Its power was so low that it pumped only half a dozen or so photons into the cloud at any given time.

Having made sure the individual photons entered the cloud, Vuletić and his team then turned their attention to watching them come out at the other end. The standard model predicts that they should exit individually at random intervals, but this turned out not to be the case. The scientists found that they popped out joined together in pairs and triplets.

As an additional inquiry, the team measured the phase of the photons – the direction of their oscillation – before and after they entered the rubidium cloud.

“The phase tells you how strongly they’re interacting, and the larger the phase, the stronger they are bound together,” explains co-author Aditya Venkatramani.

The researchers found that in the exiting photon groups the phase had been shifted compared to that of the individual photons. Furthermore, the phase shift was three times larger in the group of three than in the group of two.

“This means these photons are not just each of them independently interacting, but they’re all together interacting strongly,” says Venkatramani.

The other thing that was significantly affected by the bonding was speed. Photons, axiomatically, travel at the speed of light. The joined-up particles, however, moved 100,000 times more slowly.

The fact that Vuletić and his colleagues observed photons effectively forming into molecules raised an awkward question – how were they doing it, when the laws of physics say they can’t?

The team believe they have a plausible explanation. Passing through the cloud, a photon may ever-so-briefly rest on a stationary rubidium atom, forming a hybrid called a polariton. Should two (or more) of these polaritons then collide, the atomic parts of the hybrid will interact and bond.

At the far edge of the cloud, the rubidium atoms get left behind, leaving only the photons.

“What’s neat about this is, when photons go through the medium, anything that happens in the medium, they ‘remember’ when they get out,” says team member Sergio Cantu.

The emerging photon pairs or triplets, thus, are not bonded in some impossible chemical way, but instead they are very closely entangled – in line with quantum theory. No laws of physics are broken.

Star Wars fans may read the team’s results and indulge in fantasies about somehow adapting the work to form the basis of a real-world light sabre, but Vuletić sees more pedestrian – and undoubtedly more beneficial – applications arising.

“Photons can travel very fast over long distances, and people have been using light to transmit information, such as in optical fibres,” he says.

“If photons can influence one another, then if you can entangle these photons, and we’ve done that, you can use them to distribute quantum information in an interesting and useful way.”