A group of scientists from Harvard and MIT have created a state of matter that until now has only been found in the realms of science fiction.

The physicists were exploring the properties of photons – an elementary particle that is the most basic constituent of light and all other types of electromagnetic radiation – when they managed to create molecules formed from photons bound together.

The discovery is startling as it goes against what scientists have previously believed to be the signature quality of photons: that they are massless particles that do not interact with each other. The capacity to create molecules out of photons has been described by the physicists involved as “pushing the frontiers of science”.

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"Most of the properties of light we know about originate from the fact that photons are massless, and that they do not interact with each other," said Harvard Professor of Physics Mikhail Lukin in a press release published at phys.org.

"What we have done is create a special type of medium in which photons interact with each other so strongly that they begin to act as though they have mass, and they bind together to form molecules.”

“It's not an in-apt analogy to compare this to lightsabers," Lukin added. "When these photons interact with each other, they're pushing against and deflect each other. The physics of what's happening in these molecules is similar to what we see in the movies."

To get the photons to interact with one another the team from the Harvard-MIT Center for Ultracold Atoms (a group led by Lukin alongside MIT Professor of Physics Vladan Vuletic) cooled rubidium atoms in a vacuum chamber to just a few degrees above absolute zero – the coldest possible temperature at which particles do not move.

When two photons were fired at the cloud they did not pass through it and exit individually (as had been expected) but emerged on the other side as a single molecule. This was due to the Rydberg blockade, which states that when an atom is excited (has energy imparted to it) nearby atoms cannot be excited to the same degree.

This meant that as the first photon excited atoms in the cloud but had to move on before the second photon could do the same. Lukin describes the end result as the photons pushing and pulling each other through the cloud.

"It's a photonic interaction that's mediated by the atomic interaction," said Lukin. "That makes these two photons behave like a molecule, and when they exit the medium they're much more likely to do so together than as single photons."

The scientists involved are hoping to use their discovery to aid quantum computing (rather than build futuristic weaponry) as that technology would rely on photons to carry and exchange quantum information.

"What it will be useful for we don't know yet,” said Lukin, “but it's a new state of matter, so we are hopeful that new applications may emerge as we continue to investigate these photonic molecules' properties.”

