A group of researchers led by Harvard Professor of Physics, Mikhail Lukin, and Vladan Vuletic, Professor of Physics at the Massachusetts Institute of Technology have made a surprising advance in our knowledge of the nature of light as they have managed to create a state of matter which, until only a few weeks ago, existed only in theory: photons behaving as if they had mass.
For decades photons have been described as massless particles that do not interact with each other. For example, if two laser beams are shone so that they cross, they simply pass through one another.
Light Molecules: From the Traditional Laser to Light Sabres
The research team at the Harvard-MIT Center for Ultracold Atoms has been able to create a special kind of medium (which they describe as “extreme”) in which the photons interact so strongly they start behaving as if they have mass and bind together to form molecules.
Lukin has stated that it would not be an inappropriate analogy to compare the discovery with laser sabres like those one sees in films like Star Wars, which previously belonged strictly to the realms of science fiction. The physics that is supposedly behind what one sees in these films is very similar to what they have achieved: photons, on interacting, push against and deflect each other.
The scientists pumped rubidium atoms into a vacuum chamber, thus creating a cloud of atoms which they cooled down to just above absolute zero. Using very weak laser pulses, they then fired single photons into the cloud of atoms. Lukon says that when the photons enter the cloud, their energy excites atoms along their path, which causes the photons to slow down dramatically. As the photons move through the cloud, that energy is passed from atom to atom and eventually comes out of the cloud with the photon. In an article published online in the Harvard Gazette, Lukin explains, “When the photon exits the medium, its identity is preserved.” He continues, “It’s the same effect we see with refraction of light in a water glass. The light enters the water, it hands off part of its energy to the medium, and inside it exists as light and matter coupled together. But when it exits, it’s still light. The process that takes place is the same. It’s just a bit more extreme. The light is slowed considerably, and a lot more energy is given away than during refraction.”
The scientists have expressed their surprise at confirming that when they fired two photons into the cloud, they came out together as a single molecule. This is due to an effect called a Rydberg blockade. When an atom is excited, nearby atoms cannot be excited to the same degree. In practice, this means that, when two photons enter the atomic cloud, the first one excites an atom but it must keep moving forward before the second photon can also excite nearby atoms. Accordingly, the two photons push and pull each other through the cloud as their energy has handed off from one atom to the next. This means that the two photons behave like a molecule and, when they leave the medium, it is much more likely that they will do so together as a molecule than as single photons.
Lukin recognises that the recent discovery has opened up an immense field for exploration but he also states that the physical principles the team has established with his work are undeniably important. Among other applications, they could be used to build quantum computers.