Light isn't much of a building material. A photon, the quantization (or particle) of light, zips around the vacuum unencumbered by mass and, thus, inertia. A photon has no real size, nor does it have a structure, let alone the atomic structure of a positively charged nuclei immersed in electron clouds, as is usually required to form proper molecules. It's hard to picture what a photon-based molecule might even look like, just as it's hard to imagine bricks formed from liquid water.
As fundamentally wrong as it might seem, physicists at the National Institute of Standards and Technology (NIST) have shown theoretically that photon-based molecules may in fact be possible. The (theorized) result consists of pairs of photons bound together at fixed distances in a way not unlike the side-by-side pairings of hydrogen atoms that constitute hydrogen molecules.
The group's work is detailed in a study recently accepted for publication in the Physical Review Letters. The paper can be viewed now in open-access (and free) form at the arXiv pre-print server.
"Photons are fundamental massless particles which are essentially non-interacting for optical frequencies," the NIST paper explains. "However, a medium that couples light to its atomic constituents can induce interactions between photons. This interaction may lead to exotic, many-body states of light, or can be used as a basis for realizing deterministic quantum gates between two photons." So, yes, the interaction is weird enough to (possibly) be useful in quantum computers.
"We're learning how to build complex states of light that, in turn, can be built into more complex objects."
The first big milestone in light-as-molecule research came in 2013 when physicists from Harvard, Caltech, and MIT succeeded in superimposing pairs of photons such that they would travel together one stacked on top of another. The new work expands on this by tweaking a few parameters in the coupling process with the result being photons paired up at fixed distances side by side, an arrangement analogous to how atoms pair up in molecules, according to the NIST researchers.
"It's not a molecule per se, but you can imagine it as having a similar kind of structure," offers Alexey Gorshkov, one of the co-authors behind the current study. "We're learning how to build complex states of light that, in turn, can be built into more complex objects. This is the first time anyone has shown how to bind two photons a finite distance apart."
The photon arrangements are produced with help from what are known as Rydberg states. The basic idea is of storing photons within small atomic clouds known as Rydberg polaritons. Fire some lasers at just the right batch of atoms in just the right way and you can induce the group to act as a sort of photon trap. Each polariton then stores exactly one photon and the polaritons can be bound together using regular old electrostatic interactions as given by Coulomb's law.
The main thing is that these bound states can be combined together into increasingly more complex arrangements, a prospect that offers a powerful new way of manipulating photons at individual scales. "It's a cool new way to study photons," Gorshkov says. "They're massless and fly at the speed of light. Slowing them down and binding them may show us other things we didn't know about them before."