Ordinarily, photons—particles of light—don't interact with each other. They interfere, but that's a characteristic that doesn't alter their wavelength or cause them to attract or repel. However, if photons can be induced to interact, it could open up a wide number of applications in quantum computing and optical materials. This sort of radical change can't happen under ordinary circumstances but is possible in special environments.

Researchers fabricated just such a medium and produced photons that simultaneously act as though they are massive and mutually attractive. The key to this weird behavior involved passing light through a cold diffuse gas with strong inter-atomic interactions, properties that are usually exclusive but which can be induced in some circumstances. The atoms in the gas acted as an intermediary, causing photons to form bound pairs. Such behavior could have interesting applications for optical switches and quantum logic networks based on photons.

Ordinary matter primarily interacts via the electromagnetic force: electrons repel each other and are attracted to protons, thanks to their electrical charges. Photons, on the other hand, are both electrically neutral and massless in free space. The lack of charge means they don't "see" each other electromagnetically, and their lack of mass means they move at the maximum possible speed, that of light. Among other things, the masslessness reduces the possible time for photon pairs to interact with each other.

Prior experiments showed that photons can behave as though they have mass, at least when light passes through a medium with unusual properties. The effective mass of a photon in this type of system is an altered relationship between its energy and momentum, behavior that's separate from the question of whether photons might have a tiny intrinsic mass. These conditions are related to experiments where electrons can behave as massless particles.

The new experiment, conducted by Ofer Firstenberg and colleagues, involved a low-density gas of rubidium atoms at very low temperatures. These atoms were then excited with a laser so that their outermost electrons jump into orbits where they are only loosely bound to the nuclei, a state known as a Rydberg atom. A Rydberg rubidium atom is physically large, and the loosely attached electrons change something that would ordinarily be a very diffuse gas into a system with atoms that repel each other strongly.

The interactions between the Rydberg atoms produce a bizarre environment. The researchers saturated the atoms with laser light, holding them in their Rydberg states, which prevented them from absorbing any more photons (something known as the Rydberg blockade). As a result, additional polarized photons entering the medium were slowed by interacting with it. That gave them a significant effective mass and—thanks to the Rydberg blockade—created a channel where the photons' paths were bent.

The net effect: two photons behaved as though they were massive particles and found themselves attracted to each other. Ultimately they became correlated, behaving like a single coupled system.

An appropriate analogy: in many superconductors, vibrations of atoms in the material act to make electrons (which ordinarily repel thanks to having the same electric charge) attractive to each other, forming Cooper pairs. While the photons are not bound quite like an atom or molecule, they still behave like a single quantum system, a property that the researchers confirmed as the pairs left the Rydberg gas.

Comparison between these results and the behavior of unpaired, non-interacting photons demonstrated that these particles were indeed coupled. The experimenters tuned the properties of the pairs, demonstrating entanglement of polarization and other interesting characteristics.

All of this opens up the possibility of bizarre quantum systems: "matter" comprised of interacting massive photons, optical switches based on paired light, quantum logic using the entangled polarization states, or even a single-photon "transistor." While these systems—as with most involving weird quantum phenomena—require very cold temperatures, the implications for quantum logic, networks, and computing are fascinating.

Nature, 2013. DOI: 10.1038/nature12512 (About DOIs).