Physicists at the Harvard-MIT Center for Ultracold Atoms have figured out a way to make photon particles bind together to form molecules. Photons have typically been thought of as massless particles that don’t interact with each other, and any binding of light particles has only been discussed theoretically. “It’s not an in-apt analogy to compare this to light sabers,” says Mikhail Lukin, Professor of Physics at MIT. “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,” Lukin says. A special type of medium has been created 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. But are light sabers really right around the corner?
Well, to begin, this whole process works by researchers pumping rubidium atoms into a vacuum. Then, with the use of lasers, they cooled the cloud of atoms just a few of degrees above absolute zero. Once that’s done they used very weak laser pulses to blast in photons. As these photons travel through this cloud the cooled atoms become excited by the energy along the photon’s path causing the photons to slow down. As the photon travels, its energy is exchanged from atom to atom and eventually exits the cloud with the photon.
Researchers discovered that when they fired two protons into the cloud they exited as one molecule.
The explanation of why this happens can be explained by something called the Rydberg blockade. This states that if an atom is excited then nearby atoms can’t be excited to the same degree. And in the case of this experiment the photon travels through the cloud, it then excites an atom, but then it must move forward before the next photon can excite nearby atoms. As this happens the photons push and pull each other as they excite the nearby atoms. This interaction makes the photons act like molecules and when they exit the cloud together they’re much more likely to exit as one instead of two.
However, this discovery doesn’t exactly mean that light sabers will soon be available at your local Hot Topic or Cabela’s. The application has other possibilities though.
“…photons remain the best possible means to carry quantum information. The handicap, though, has been that photons don’t interact with each other.” Lukin says. For example, in building a quantum computer researchers need a means to preserve the quantum information and then be able to process the information with quantum logic operations. However, in order to process this information there needs to be a system that requires the quantum systems to be switched because quantum logic requires the interaction between individual quanta.
This experiment is a proof-of-concept in which a practical quantum switch or photonic logic gate can be constructed.
It may even be able to create three dimensional complex crystals completely out of light. It could even be used in classical computing considering the power-dissipation challenges chip-makers now face.
Lukin says, “what it will be useful for we don’t know yet, 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.”