Theory Describes 'Molecules' of Light

Theoretical work has shown how two photons can be bound together in much the same way that atoms form molecules.

If demonstrated experimentally, the phenomenon could be exploited to enhance photon detectors and enable quantum computing.

"Lots of modern technologies are based on light, from communication technology to high-definition imaging," said researcher Alexey Gorshkov. "Many of them would be greatly improved if we could engineer interactions between photons."

Two photons, depicted in this artist's rendering as waves at left and right, can be locked together at a short distance. Under certain conditions, the photons can form a state resembling a diatom, represented as the blue dumbbell shape at center. Courtesy of E. Edwards/Joint Quantum Institute.

Gorshkov is a fellow at the Joint Quantum Institute, a collaboration between the University of Maryland and the National Institute of Standards and Technology (NIST).

The findings build on previous research,to which several team members contributed before joining NIST. In 2013, the researchers found a way to bind two photons together so that one would sit atop the other, superimposed as they travel.

The new work shows theoretically that by tweaking a few parameters of the binding process, photons could travel side by side at a specific distance from each other. The arrangement is akin to the way two hydrogen atoms sit next to each other in a hydrogen molecule.

"It's not a molecule per se, but you can imagine it as having a similar kind of structure," Gorshkov said. "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 findings could make it far easier to create a "standard candle" that shines a precise number of photons at a detector, enabling more precise calibration, Gorshkov said.

Perhaps more significant to industry, binding and entangling photons could allow computers to use photons as information processors, reducing energy consumption in the process.

"It's a cool new way to study photons," Gorshkov said of his team's work. "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."

The research was published in Physical Review Letters (doi: 10.1103/PhysRevLett.115.123601).