In a simple experiment, when a pair of flashlights is shone in a dark room such that their light beams cross each other, does one notice anything strange? The rather anticlimactic answer is maybe not. The reason for this is the individual photons that make up light merely pass each other by – similar to indifferent spirits in the night – and do not interact in any way.
However, what will happen if light particles are allowed to interact, repelling and attracting each other similar to atoms in ordinary matter? Light sabers provide one interesting, although sci-fi possibility. These are beams of light that are capable of pulling and pushing on each other, making for stunning, epic confrontations. Or, in a more likely case, two light beams could meet and combine into a single, luminous stream.
This would mean that the rules of physics may need to be tweaked to realize such optical behavior, but actually, researchers at Harvard University, MIT, and elsewhere, have now shown that photons can certainly be made to interact — a major breakthrough that could pave the way for applying photons in quantum computing, if not in light sabers.
The research team, headed by Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT, and Professor Mikhail Lukin from Harvard University, published the results of the study in the Science journal. The scientists reported that they have viewed groups of three photons interacting and, in effect, binding together to create an entirely new kind of photonic matter.
During controlled experiments, the team observed that when a very weak laser beam is shone via a dense cloud of ultracold rubidium atoms, the photons stick together in triplets or pairs instead of exiting the cloud as single, arbitrarily spaced photons. This indicates that some kind of interaction — in this case, attraction — is occurring among them.
Usually, photons lack mass and can travel at 300,000 kilometers per second (the speed of light), the research team noted that the bound photons have in fact attained a fraction of an electron’s mass. The newly weighed-down light particles were also quite sluggish, and compared to normal non-interacting photons, travel about 100,000 times slower.
According to Vuletic, the results show that photons can certainly entangle, or attract, each other. If the photons can be made to interact in other different ways, they may be harnessed to perform very fast, extraordinarily complex quantum computations.
“The interaction of individual photons has been a very long dream for decades,” Vuletic says.
Co-authors of Vuletic include Sergio Cantu, Qi-Yung Liang, and Travis Nicholson from MIT, Aditya Venkatramani and Lukin of Harvard University, Alexey Gorshkov and Michael Gullans of the University of Maryland, Cheng Ching of the University of Chicago, and Jeff Thompson from Princeton University.
The MIT-Harvard Center for Ultracold Atoms is headed by Vuletic and Lukin and together they have been exploring ways – both experimental and theoretical – to promote interactions between photons. The effort finally paid off in 2013, because for the first time, the scientists observed the interaction and binding between pairs of photons, producing a whole new state of matter. In their latest study, the team wondered if interactions could occur between not just two photons, but more.
For example, you can combine oxygen molecules to form O2 and O3 (ozone), but not O4, and for some molecules you can’t form even a three-particle molecule. So it was an open question: Can you add more photons to a molecule to make bigger and bigger things?
In order to find out, the researchers used the same experimental method which they utilized to observe the interactions between two photons. In this process, a cloud of rubidium atoms is first cooled to ultracold temperatures, i.e., just a millionth of a degree above absolute zero. When the atoms are cooled, they slow down to a near standstill. The researchers then shone an extremely weak laser beam through this cloud of immobilized atoms — the laser beam was so weak that only a handful of photons were able to pass through the cloud at any single time. They then determined the photons as they exit the other side of the atom cloud.
In the latest experiment, it was observed that the photons streamed out as triplets and pairs, instead of coming out of the cloud at haphazard intervals, since single photons have nothing to do with each other.
Besides tracking the rate and number of photons, the researchers measured the photons’ phase, both before and after passing through the cloud of immobilized atoms. The phase of a photon suggests its frequency of oscillation.
“The phase tells you how strongly they’re interacting, and the larger the phase, the stronger they are bound together,” Venkatramani explains. The researchers noted that when three-photon particles simultaneously exited the atom cloud, their phase was moved compared to what it was before when there was no interaction between the photons, and was in fact three times larger than the phase shift of two-photon particles.
“This means these photons are not just each of them independently interacting, but they’re all together interacting strongly.”
The team then came up with a theory to describe what might have made the photons to interact in the first place. The researchers’ model, based on physical principles, presents the following scenario: As one photon moves via the cloud of rubidium atoms, it shortly lands on an adjoining atom prior to skipping to next atom, similar to a bee flitting from one flower to another, until it comes to the other end.
Similarly, if another photon is concurrently traveling through the cloud of rubidium atoms, it can also briefly land on a rubidium atom and form a polariton, a hybrid that is part atom and part photon. The two polaritons can then interact with each other through their atomic component. The atoms remain where they are at the edge of the cloud, whilst the photons exit, still bound together. The team noted that this same phenomenon can take place with three photons, producing an even stronger bond than the two-photon interactions.
“What was interesting was that these triplets formed at all,” Vuletic says. “It was also not known whether they would be equally, less, or more strongly bound compared with photon pairs.”
The whole interaction inside the atom cloud takes place over a millionth of a second, and this interaction activates the photons to stay bound together, even after they have exited the cloud.
“What’s neat about this is, when photons go through the medium, anything that happens in the medium, they ‘remember’ when they get out,” Cantu says.
This means that photons that have interacted with one another, in this case via an attraction between them, can be believed to be as strongly entangled, or correlated — an important property for any quantum computing bit.
Photons can travel very fast over long distances, and people have been using light to transmit information, such as in optical fibers. 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.
In the future, the researchers will investigate ways to force other interactions, for example, repulsion, where photons might scatter off each other similar to billiard balls.
“It’s completely novel in the sense that we don’t even know sometimes qualitatively what to expect,” Vuletic says. “With repulsion of photons, can they be such that they form a regular pattern, like a crystal of light? Or will something else happen? It’s very uncharted territory.”
The National Science Foundation partly supported the research.