A research team at the Vienna University of Technology has succeeded in producing a strong interaction between two single photons. This development opens up a completely new avenue for quantum optics.

A pair of photons free in space do not normally interact. When passing through each other, light waves do not have any influence on each other and while this may benefit some, scientists studying quantum technology find photon interactions crucial.

“In order to have light interact with light, people have been using so-called nonlinear media,”

Prof. Arno Rauschenbeutel from the Institute for Atomic and Subatomic Physics, TU Wien said.

Results from the experiment have been published in the journal Nature Photonics. According to Prof. Rauschenbeutel, light has a particular effect on the properties of the materials he mentioned. In turn, these materials influence the light which then causes an indirect coupling between photons.

There is one disadvantage to this technique such strong couplings can only be obtained at strong light intensities, when countless photons are involved. As such, prof. Rauschenbeutel and his colleagues designed a system that would allow them to create the same strong interactions but only between two photons. A reaction so strung occurred between the photons that their phase 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 swings to the right. There cannot be a more extreme change in the pendulum’s oscillation,”

Rauschenbeutel said.

To make such an interaction possible, the photon must be sent on an unimaginable journey. Researchers use ultra-thin glass fiber and coupled it to a tiny resonator that forces light to partly enter, move in circles through the resonator and return to the glass. As a result of such a complicated retour, the photon’s phase is inverted and a wave crest appears where the wave through would have been expected.

“We achieve the strongest possible interaction with the smallest possible intensity of light.”

Prof. Rauschenbeutel said.

The team went further to couple one single rubidium atom to this resonator, so that the system no longer allows light to enter the resonator. When two photons arrive simultaneously, the atom works as an absorber, guarding one photon for a short while and then releasing it into the resonator.

“During that time, it cannot absorb any other photons. If two photons arrive simultaneously, only one can be absorbed, while the other can still be phase shifted.”

Rauschenbeutel said.

What is the use for such technology, one would ask. And although it isn’t a quest of ours to achieve teleportation yet, there are important applications to such a system, Rauschenbeutel claims.

“Such states are required in all fields of quantum optics – in quantum teleportation, or for light-transistors that could potentially be used for quantum computing.”