We are just a few steps toward IRL lightsabers. A team of physicists has created a new form of light that permits up to three photons to bind together. The technology isn’t quite ready to defeat the Dark Side, but it could be a major boon to photon-based quantum computers.
The two lead researchers on the project, MIT physicist Vladan Vuletic and Harvard physicist Mikhail Lukin, head up the joint MIT-Harvard Center for Ultracold Atoms and have spent the last few years trying to make photons interact with each other. Their first major success was in 2013, when the researchers managed to get two photons to bind together to create a new form of light—but they wanted to know if this was the limit to photon interactions.
“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 said in a statement. “So it was an open question: Can you add more photons to a molecule to make bigger and bigger things?”
Most particles acquire mass by interacting with the Higgs field, which is a ubiquitous field of energy. Photons, on the other hand, don’t interact with the Higgs field and have no mass, which is why two photons are able to pass through one another like ghosts if you were to, say, shine two flashlight beams at one another.
In order to get these massless particles of light to bind together like normal matter, Vuletic and Lukin created an experimental set-up that involves shining a laser through some very cold atoms. In particular, they were using a cloud of rubidium atoms chilled to just a millionth of a degree above absolute zero. This makes it so the rubidium atoms in the cloud are hardly moving. Then they shine a weak laser beam through the supercooled atomic cloud, so that only a few photons pass through to be measured on the other side of the apparatus.
They found the photons emerging on the other side were strongly bound together with one another in groups of three, and had actually acquired a very small amount of mass (equal to just a fraction of the mass of an electron). As a result, these photon triplets moved 100,000 times slower than the speed of a normal photon, which travel at 300,000 kilometers per second.
But wait—there’s more.
Vuletic, Lukin and their colleagues developed a theory for what caused these photons to bind together like this in the first place. In this model, the photons basically skip from one rubidium atom to the next. While a photon is ‘on’ a rubidium atom, it can create a hybrid atom-photon called a polariton. If multiple polaritons are formed in the cloud, they can interact with one another by way of the rubidium element of the hybrid as the polaritons continue to move through the rubidium cloud. When the polaritons reach the ‘edge’ of the cloud, the rubidium atoms remain in the clouds while the still-bound-together photons exit. According to the researchers, this entire process occurs within a millionth of a second.
The important thing here is that this process allows photons to interact with one another when they otherwise wouldn’t. They are essentially entangled, a property that is integral to manipulating qubits in quantum devices. The photon triplets created by Vuletic and Lukin are an improvement over other photonic qubits, however, because they are much more strongly bound together and are, as a result, better carriers of information.
Given the highly experimental nature of Vuletic and Lukin’s research, it will likely be a while before it is put to any practical use. In fact, the researchers said they themselves often don’t know what to expect from their experiments. Going forward, they said they intend to figure out ways to cause other interactions among photons, such as making them repel one another.
“It’s completely novel in the sense that we don’t even know sometimes qualitatively what to expect,” Vuletic said in a statement. “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.”