New Form of Light Created

Scientists at MIT, Harvard University, and elsewhere have now demonstrated that photons can be made to interact. An accomplishment that could open a path toward using photons in quantum computing. (Source: C. Daniloff, MIT)

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 light particles could be made to interact, attrac­ting and repel­ling each other like atoms in ordinary matter? One tanta­lizing, albeit sci-fi possi­bility: light 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. The researchers report that they have observed groups of three photons interacting and, in effect, sticking together to form a completely new kind of photonic matter.

In controlled experi­ments, they 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 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.

Vladan 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 and Mikhail 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, 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, 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 single photons 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,” Aditya Venkatramani of Harvard Univer­sity 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.”

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 rubidium atoms, 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 atoms 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 photon 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,” Sergio Cantu, MIT, 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 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,” 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.” (Source: MIT)

Reference: Q.-Y. Liang et al.: Observation of three-photon bound states in a quantum nonlinear medium, Science 359, 783 (2018); DOI: 10.1126/science.aao7293

Links: Experimental Atomic Physics, Research Laboratory of Electronics, Massachusetts Institute of Technology MIT, Cambridge, USA • Quantum Optics Lab., Dept. of Physics, Harvard University, Cambridge, USA

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