Photons are the smallest particles which carry units of electromagnetic energy, or light, and these units of energy rarely interact with each other. Coupling of the particles can occur within certain materials, at extremely high energy.

Light waves easily pass through each other without interfering with the other beam. Quantum technology, an emerging field that could revolutionize electronics, is dependent on the ability to influence one beam of light with another. Optical logic gates would become possible, as would the ability to transmit information without any chance of interception.

Non-linear media has previously been used to indirectly alter one beam of light using another. This process involves electromagnetic energy striking the material, which then alters the second beam. However, this technique involves the use of vast quantities of light particles. This is the first time one photon of light has been made to directly alter another.

Vienna University of Technology (TU Wien) researchers have brought together a pair of photons in the strongest coupling possible during a recent experiment. The interaction was great enough to change the phase of each photon by 180 degrees.

A photon of light is sent through a fiber optic cable, then partly sent through a resonator, much like a bottle. This device can alter the phase of the photon, before sending it back through the cable. Where there would normally be a crest of a wave, the photon now exhibits a trough.

"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. We achieve the strongest possible interaction with the smallest possible intensity of light," Arno Rauschenbeutel of the Institute for Atomic and Subatomic Physics at TU Wien said.

A single atom of rubidium is enough to "shut down" the resonator, researchers found. When a single particle of the metallic element is placed in the device, almost all light is blocked in the resonator, preventing the effect from occurring.

The atom acts in such a way that a photon can be absorbed, saturating the system, before the photon is released back into the resonator. This prevents the device from accepting additional particles of light. When a pair of photons arrives in the resonator simultaneously, one is absorbed, while the other is 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 told the press.

Fiber optics are already in common use around the world in global communications systems.

Photon coupling and interaction was detailed in the journal Nature Photonics.

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