Light is used to illuminate our world and to send information, but light does not typically interrupt or interact with itself. The smallest unit of light transmission, the photon, carries electromagnetic energy in light as a particle that behaves like a wave. Although the coupling of photons has been observed within certain materials, the energy required for this to occur is extremely high.

Researchers show for the very first time the alteration of a beam of light with just one photon from a different light source. Light has been altered in non-linear media previously, but the technique required an enormous quantity of photons to facilitate. The alteration of light by a single photon, observed by researchers at the Vienna University of Technology (TU Wien), is ground-breaking for the field of quantum optics.

A photon was sent through a fiber optic cable and then part-way through a resonator, a device that can alter the phase of the photon. The photon was then sent back through the cable. On the return trip, the researchers observed that there was a trough where a wave crest would normally be present.

“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,” said Arno Rauschenbeutel of the Institute for Atomic and Subatomic Physics at TU Wien. “There cannot be a more extreme change in the pendulum’s oscillation. We achieve the strongest possible interaction with the smallest possible intensity of light.”

The researchers observed that a single rubidium atom was enough to shut down the resonator. When a pair of photons arrived at the rubidium atom, one was absorbed, and the other was inverted.

“That way, a maximally entangled photon state can be created. Such states are required in all fields of quantum optics – in quantum teleportation, or for light-transistors which could potentially be used for quantum computing,” Rauschenbeutel said in a statement.

The results were published in the journal Nature Photonics.