Did Scientists Really Just Make Lightsabers?

What did the scientists say?

The team were interested in the properties of photons. Photons are individual particles of light (or electromagnetic radiation, to be more precise). Individual photons have no mass, and normally will not interact with each other – two photons will pass by each other without being affected.

However, theoretical physicists have long thought that there could be circumstances when photons would affect each other if they became close. In fact, this would make them appear to have mass!

The scientists attempted to make photons interact inside a chamber filled with rubidium gas. They used a laser to cool the gas to very low temperatures. The light from the laser is absorbed by the rubidium atoms, and is then re-emitted at a higher energy. This extra energy is extracted from the atoms, lowering the temperature of the gas.

Once they have this very cool gas, they use another, very weak laser to shoot individual photons through it. The photons travel through the ultra-cold gas, interacting with the atoms and eventually exiting the gas from the other side as single photons.

However, when they shoot two photons at a time, they interact with the gas and leave the chamber as a single entity. This is because of something called a Rydberg blockade – if a rubidium atom absorbs and gets excited by photon 1, photon 2 can’t excite nearby atoms to the same level. This means the photons have to “co-operate” with each other as they are absorbed and re-emitted by the rubidium atoms. If you’re feeling brave, you can read more about Rydberg Blockades here.

This co-operation effect results in the photons becoming attracted to each other; just as atoms can be attracted to each other to form molecules, the photons have become attracted to each other to form a photonic molecule!

This is a new state of matter, the stuff of theory made real. Photons that interact with each other could form incredibly small transistors, or logic gates for quantum computers. In the lab, the photonic molecules didn’t last very long, but the researchers are confident that future experiments can make longer lasting molecules.

What did the Media Say?

The journal article doesn’t mention lightsabers at all. They do mention the various quantum computing applications, but unsurprisingly shy away from proposing the production of Jedi weapons as a potential source of research impact.

It seems to have been mentioned in the press release (which I have struggled to find), and the authors do address it in various quotes. Lukin is quoted as saying:

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

Unsurprisingly, specialist websites like Nature World News and phys.org acquit themselves honourably. Other outlets, especially tech sites like Gizmodo are keen to include lightsabers in the article title (which notes this is still early work), along with images from the Star Wars films, but they do caution towards the end of the article that real lightsabers are unlikely to appear anytime soon. The Independent also takes a measured approach to the reporting of this story. Some sites, like Fox News, brashly state “MIT, Harvard scientists accidentally create real-life lightsaber”, and only later explain that this photonic molecule experiment is miles away from a real-life lasersword.

The reporting on this story isn’t particularly inaccurate, but overly focused on a few comments in the press release, rather than the science itself. This is perhaps more the fault of those who drafted the release, who may have used the lightsaber concept as a hook to get column inches. As a result, readers may miss the really interesting part of the story in the process – that a new state of matter has been created in the lab, and that this could take quantum computing to places that were never before deemed to be possible.

Ofer Firstenberg, Thibault Peyronel, Qi-Yu Liang, Alexy V. Gorshkov, Mikhail D. Lukin, Vladan Vuletic(2013). Attractive Photons in a quantum nonlinear medium. Nature, 502, doi: 10.1038/nature12512