Two photons hanging out in free space want nothing to do with each other: Waves of light simply pass through each other without either one having an effect on the other. Now, researchers have found a way to spark a strong interaction between two single photons by using an ultra tiny fiber of glass. The findings were published in Nature Photonics this week. 

For many applications in quantum technology, the interaction between photons is a crucial prerequisite: from transmitting information through tap-proof quantum channels to building light-transistors for quantum computing, maybe even quantum teleportation. “In order to have light interact with light, one usually uses so-called nonlinear media,” says Arno Rauschenbeutel of TU Wien, the Vienna University of Technology. Light affects certain properties of these materials, which in turn influences the light, leading to an indirect coupling between photons. This only works at high light intensities involving innumerable photons. 

So, a trio of TU Wien researchers set about building a system that creates a strong interaction between only two photons. In fact, they created an interaction that’s so strong, the phase of the photons is changed by 180 degrees. “It is like a pendulum, which should actually swing to the left, but due to coupling with a second pendulum, it is swinging to the right. There cannot be a more extreme change in the pendulum’s oscillation,” Rauschenbeutel explains in a news release. “We achieve the strongest possible interaction with the smallest possible intensity of light.”

The photons were sent on a journey through an ultra-thin glass fiber that’s coupled to a tiny bottle-like optical resonator (pictured above). When light enters the resonator, it moves in circles, then returns to the glass fiber. This detour through the resonator inverts the phase of the photon: A crest appears in the wave where a trough would normally be expected. Pictured here, lighting running around a bottle-shaped glass fiber that’s about half as thick as a human hair.

However, when a single rubidium atom is coupled to the resonator, hardly any light will enter the resonator anymore and the oscillation phase of the photon remains unchanged. Because the atom is an absorber that can be saturated, when two photons arrive at the same time, one of them is absorbed by the atom for a short while, then released again into the resonator; but in that time, it can’t absorb any other photons. “Only one can be absorbed, while the other can still be phase shifted,” Rauschenbeutel explains. Two simultaneous photons that interact end up showing a completely different behavior than single photons. 

The result is “a maximally entangled photon state,” he adds, which is needed for every field of quantum optics. Another advangage is that the technique uses glass fiber technology, which is already being used for optical communication anyway.

Images: TU Wien