Light is comprised of photons, which are particles of electromagnetic energy. They are considered elementary particles (the smallest constituents of matter and energy) despite their lack of mass.
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Photons typically do not interact with one another. Contrary to what is shown in most sci-fi movies, beams of light do not repel one another or interact in any way. Flashlights and laser beams aimed at one another would continue shining as if nothing out of the ordinary had occurred. However, researchers have recently set out to change that through experiments designed to force an interaction between particles.
Physicists at the Massachusetts Institute of Technology (MIT) and Harvard University have collaborated a project where photons became entangled. Forcing a connection caused the photons to slow down and gain mass, thus creating a new form of light.
The team, led by MIT’s Lester Wolfe Professor of Physics Vladan Vuletic and Harvard University’s Professor Mikhail Lukin, published their findings in the journal Science. “We report the observation of traveling three-photon bound states in a quantum nonlinear medium where the interactions between photons are mediated by atomic Rydberg states. Photon correlation and conditional phase measurements reveal the distrinct bunching and phase features associated with three-photon and two-photon bound states.”
The study builds on the physicists’ earlier research, wherein the team coaxed pairs of photons into interactions in order to create photonic molecules. Upon successful completion of that study, the researchers wondered if the results could be replicated on a larger scale. Lead researcher Vladan Vuletic explained that oxygen molecules can be combined to form O2 and O3 but not O4 and in some cases, you cannot even form a three-particle molecule. “So it was an open question: Can you add more photons to a molecule to make bigger and better things?”
The project involved replicating earlier experiments. A cloud of rubidium atoms was cooled to nearly absolute zero, freezing them in place. The team then shone a weak laser through the cloud of immobile atoms, effectively sending only a few photons through at a time. These photons were then measured as they emerged from the cloud.
Although this would typically produce streams of individual photons exiting at random intervals, this experiment resulted in photon attraction and increased mass. Photons normally travel at the speed of light and contain no mass. However, the experiment created photon molecules which had acquired a fraction of an electron’s mass. While this increase seems miniscule, it was a significant gain for an otherwise weightless particle, causing them to move approximately 100,000 times more slowly than their noninteracting counterparts.
The researchers also measured the photons’ phase, or their oscillation frequency. “The phase tells you how strongly they’re interacting,” said physicist and co-author Aditya Venkatramani of Harvard University, “…the larger the phase, the stronger they are bound together.” The team was surprised to discover that the phase of three-photon molecules was three times larger than that of photon pairs. “This means these photons are not just each of them independently interacting, but they’re all together interacting strongly.”
According to Vuletic, the formation of these “triplets” was interesting on its own, and they did not know whether the molecules would be “equally, less, or more strongly bound compared with photon pairs.”
The researchers hypothesized that photons form polaritons through contact with rubidium atoms. “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’s neat about this is, when photons go through the medium, anything that happens in the medium, they remember when they get out,” says MIT’s Sergio Cantu. “We send the light into the medium, it gets effectively dressed up as if it were atoms, and then when it turns back into photons they remember interactions that happened in the medium.”
“The interaction of individual photons has been a very long dream for decades,” said Vuletic in a statement, explaining that photons travel quickly over distances and have been used to transmit light in the past, such as in the development of optical fibres. He also added that the ability to entangle photons and have them influence one another could result in the use of these photons in distributing quantum information in interesting and useful ways.
This groundbreaking discovery opens doors for a multitude of uses, particularly in regards to quantum optics and quantum computing. Theoretically, computer systems could make use of quantum-mechanical phenomena such as superposition and entanglement to perform operations much more quickly than current systems. Though basic quantum computers have been developed, scalable systems for practical use are eons away. However, research such as this could make them a reality much sooner than previously thought.
Going forward, researchers plan to branch out from simple attraction and experiment with other types of interactions, such as forcing photons to repel one another. “It’s completely novel in the sense that we don’t even know sometimes qualitatively what to expect,” said Vuletic. “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.”