Interaction between photons is vital for a number of applications in quantum technology. Unfortunately, a pair of photons in free space would not normally interact with light waves being able to get through each other without influencing each other at all.

A group of scientists, however, appear to have found a solution to this conundrum. Using glass fiber, researchers from the Vienna University of Technology (TU Wien) in Austria, appear to have successfully found a way to establish a strong interaction between a couple of single photons, a method that could open up new possibilities in the field of quantum optics.

Arno Rauschenbeutel from the Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien, and colleagues built a system using a super thin glass fiber that was coupled with a bottle-like device called a resonator to allow the light to enter the resonator, move in circles and go back to the glass fiber. The system produces an interaction between two photons so strong that the photons' phase is altered by 180 degrees.

The system undergoes dramatic changes when the researchers coupled a rubidium atom to the resonator. The presence of this atom hardly allowed light to enter the resonator. The oscillation phase of the photon can't also be inverted.

"We employ a whispering-gallery-mode resonator, interfaced by an optical nano fiber, where the presence of a single rubidium atom in the resonator mode results in a strongly nonlinear response. We show that this results in entanglement of initially uncorrelated incident photons," the researchers reported in their study published in the journal Nature Photonics on Nov. 2.

Rauschenbeutel explained that the atom serves as an absorber that can be saturated. It absorbs a photon for a while before it is released into the resonator, at which time, the atom cannot absorb another photon. Two photons arriving at the resonator simultaneously changes things in that only one of the photons can be absorbed and the other can still be a phase shifter.

No one can tell which of these two photos is absorbed and which has passed. Both experience a phase shift by 180 degrees when they hit the resonator at the same time. A couple of interacting photons that arrive simultaneously exhibited a different behavior than single photons.

The researcher's experiment can create a maximally entangled photon state, which is crucial in the field of quantum optics and could potentially be applied on quantum computing.

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