New form of light: Newly observed optical state could enable quantum computing with photons

February 15, 2018, Massachusetts Institute of Technology
New form of light: Newly observed optical state could enable quantum computing with photons
The setup. Credit: Science (2018). 10.1126/science.aao7293

Try a quick experiment: Take two flashlights into a dark room and shine them so that their light beams cross. Notice anything peculiar? The rather anticlimactic answer is, probably not. That's because the individual photons that make up light do not interact. Instead, they simply pass each other by, like indifferent spirits in the night.

But what if could be made to interact, attracting and repelling each other like atoms in ordinary matter? One tantalizing, albeit sci-fi possibility: sabers - beams of light that can pull and push on each other, making for dazzling, epic confrontations. Or, in a more likely scenario, two beams of light could meet and merge into one single, luminous stream.

It may seem like such optical behavior would require bending the rules of physics, but in fact, scientists at MIT, Harvard University, and elsewhere have now demonstrated that photons can indeed be made to interact - an accomplishment that could open a path toward using photons in quantum computing, if not in light sabers.

In a paper published today in the journal Science, the team, led by Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT, and Professor Mikhail Lukin from Harvard University, reports that it has observed groups of three photons interacting and, in effect, sticking together to form a completely new kind of photonic matter.

In controlled experiments, the researchers found that when they shone a very weak laser beam through a dense cloud of ultracold rubidium atoms, rather than exiting the cloud as single, randomly spaced photons, the photons bound together in pairs or triplets, suggesting some kind of interaction - in this case, attraction - taking place among them.

While photons normally have no mass and travel at 300,000 kilometers per second (the speed of light), the researchers found that the bound photons actually acquired a fraction of an electron's mass. These newly weighed-down light particles were also relatively sluggish, traveling about 100,000 times slower than normal noninteracting photons.

Vuletic says the results demonstrate that photons can indeed attract, or entangle each other. If they can be made to interact in other ways, photons may be harnessed to perform extremely fast, incredibly complex quantum computations.

"The interaction of individual photons has been a very long dream for decades," Vuletic says.

Vuletic's co-authors include Qi-Yung Liang, Sergio Cantu, and Travis Nicholson from MIT, Lukin and Aditya Venkatramani of Harvard, Michael Gullans and Alexey Gorshkov of the University of Maryland, Jeff Thompson from Princeton University, and Cheng Ching of the University of Chicago.

Biggering and biggering

Vuletic and Lukin lead the MIT-Harvard Center for Ultracold Atoms, and together they have been looking for ways, both theoretical and experimental, to encourage interactions between photons. In 2013, the effort paid off, as the team observed pairs of photons interacting and binding together for the first time, creating an entirely new state of matter.

In their new work, the researchers wondered whether interactions could take place between not only 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," Vuletic says. "So it was an open question: Can you add more photons to a molecule to make bigger and bigger things?"

To find out, the team used the same experimental approach they used to observe two-photon interactions. The process begins with cooling a cloud of rubidium atoms to ultracold temperatures, just a millionth of a degree above absolute zero. Cooling the atoms slows them to a near standstill. Through this cloud of immobilized atoms, the researchers then shine a very weak laser beam - so weak, in fact, that only a handful of photons travel through the cloud at any one time.

The researchers then measure the photons as they come out the other side of the atom cloud. In the new experiment, they found that the photons streamed out as pairs and triplets, rather than exiting the cloud at random intervals, as having nothing to do with each other.

In addition to tracking the number and rate of photons, the team measured the phase of photons, before and after traveling through the atom cloud. A photon's phase indicates 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 team observed that as three-photon particles exited the atom cloud simultaneously, their phase was shifted compared to what it was when the photons didn't interact at all, and was three times larger than the phase shift of two-photon molecules. "This means these photons are not just each of them independently interacting, but they're all together interacting strongly."

Memorable encounters

The researchers then developed a hypothesis to explain what might have caused the photons to interact in the first place. Their model, based on physical principles, puts forth the following scenario: As a single photon moves through the cloud of , it briefly lands on a nearby atom before skipping to another atom, like a bee flitting between flowers, until it reaches the other end.

If another photon is simultaneously traveling through the cloud, it can also spend some time on a rubidium atom, forming a polariton - a hybrid that is part photon, part atom. Then two polaritons can interact with each other via their atomic component. At the edge of the cloud, the remain where they are, while the photons exit, still bound together. The researchers found that this same phenomenon can occur with three photons, forming an even stronger bond than the interactions between two photons.

"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 pairs."

The entire interaction within the atom cloud occurs over a millionth of a second. And it is this interaction that triggers photons to remain bound together, even after they've left 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 each other, in this case through an attraction between them, can be thought of as strongly correlated, or entangled - a key property for any bit.

"Photons can travel very fast over long distances, and people have been using light to transmit information, such as in optical fibers," Vuletic 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."

Going forward, the team will look for ways to coerce other interactions such as repulsion, where photons may scatter off each other like 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."

Explore further: Lens trick doubles odds for quantum interaction

More information: "Observation of three-photon bound states in a quantum nonlinear medium" Science (2018). … 1126/science.aao7293

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Display comments: newest first

1 / 5 (5) Feb 15, 2018
To put it more succinctly, particles don't exist!
1 / 5 (4) Feb 15, 2018
A Photon is a field event; so, yes QM only?
2.6 / 5 (5) Feb 15, 2018
Would these doublets and triplets then be axions? Since they have been through a super-cold medium and the photons show themselves to form doublet and triplet photon ensembles, with very strong bonds yet they also show an attainment of 'mass', such that they are affected by gravity.

That takes us to:

And showing how they are looking at photon-ultracold neutrons, which is what they were effectively doing with the above experiment, and since we have been looking for Axions, bits of photon that take on mass? I think we have a winner folks, but it just shows it is a matter of condensing photons out of the 3D+1T plane-space.

So, essentially, unltra-cold, condensed photons take on the properties of mass although they lose a large fraction of their speed in so doing.

This is going to be terribly upsetting for some folks, I am sure.
3 / 5 (4) Feb 15, 2018
Its not that new.
Excitons, and plasmons and polaritons are the already known optical excitations of matter. These quasi-particles are known to interact with photons in the way that other photons do not.
3 / 5 (4) Feb 15, 2018
I would guess the imprint of mass was a pick up of mass magnetically to the photons construction that made it an unbalanced magnetically construction instead of equal positive and negative mass quantum mass charge it became unbalanced in charge with a heavier positive or negative quantum mass charge connected to the photons construction of it parts.
not rated yet Feb 15, 2018
Pay-walled, of course...
Hmm: How resilient are these doublets and triplets ? Could such be formed by natural masers ??
3 / 5 (2) Feb 15, 2018
Quantum magnetic particle attachment from the environment the photon traveled thru
not rated yet Feb 15, 2018
I have read in the past that theoretical predictions, such as the existence of gravity waves, or the energy of the Higgs boson, have sometimes been stated years or even decades in advance of experimental confirmation. But in this report, it appears that the experimenters did not know in advance what they might see.

("It's completely novel in the sense that we don't even know sometimes qualitatively what to expect," Vuletic says.)

Yet clearly some theory must have been required to set up the experiment. How did it come about that the experimenters knew they needed cold Rubidium atoms, but did not know that triplets or other (usually very improbable) states could be reached?
3 / 5 (4) Feb 15, 2018
Pay-walled, of course...
Hmm: How resilient are these doublets and triplets ? Could such be formed by natural masers ??

Wow Nik what's up? You are getting lazy.
5 / 5 (2) Feb 16, 2018
#T: "Wow Nik what's up?"
Tired unto tears, and Winter bronchitis, too...
5 / 5 (3) Feb 16, 2018
#T: "Wow Nik what's up?"
Tired unto tears, and Winter bronchitis, too...

I can relate,I had had been afficted this winter too. I hope you get well soon.
5 / 5 (1) Feb 19, 2018
"These newly weighed-down light particles were also relatively sluggish, traveling about 100,000 times slower than normal non-interacting photons."

What? Their velocity is only 3 km/s?

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