Normally, photons want nothing to do with one another. Light waves just pass through each other like ghosts. But now, for the first time ever, scientists at the University of Vienna have coaxed a strong interaction between two single photons. It's an achievement that opens up radical new possibilities for a number of quantum technologies.

Above: In an optical fiber — about half as thick as a human hair — light can be seen running around its axis. In this case the light can not escape along the fiber because the diameter decreases to both sides. (University of Vienna)

Photons normally pass right through other beams of light. Indeed, light is this funky mixture of classical and quantum phenomenon, exhibiting properties of both waves and particles. This is great for engineers who wish to exploit these anti-social attributes, allowing them to create such things as optical fibre cables that stretch for miles. But it's a constraint if you want to transmit information through secure quantum channels, or for building optical gates. This latest breakthrough could change that.

A team of researchers at the University of Vienna created a strong interaction between two photons by using an ultra-thin glass fibre. The interaction was so strong that the phase of the photons was altered 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", noted study co-author Arno Rauschenbeutel in a statement. "We achieve the strongest possible interaction with the smallest possible intensity of light."

The light in a fiber is coupled to a bottle-shaped resonator. (University of Vienna)

To make it happen, the photon was sent on a rather unconventional journey. An ultra-thin glass fibre was joined to a tiny bottle-like optical resonator so that light could enter into the resonator, twist about in circles, and then return to the glass fibre. It was this detour through the resonator that inverted the phase of the photon. And in fact, a wave crest appeared where a wave trough was expected.

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But when the researchers added a single rubidium atom to the resonator, the system changed dramatically. Because of the atom, hardly any light entered into the resonator and the oscillation phase of the photon remained unchanged. Essentially, the addition of the atom got both photons to talk to each other.

As far as quantum mechanics is concerned, the two photons are indistinguishable. As noted by the University of Vienna release:

They have to be considered as a joint wave-like object, which is located in the resonator and in the glass fibre at the same time. Therefore, one cannot tell which photon has been absorbed and which one has passed. When both hit the resonator at the same time, they thus experience a joint phase shift of 180 degrees. Hence, two simultaneous photons that interact show a completely different behaviour than single photons.

This resulted in the creation of a "maximally entangled photon state" — a state that's required in quantum optics, quantum teleportation, and for the creation of light-transistors which could be used for quantum computing.

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All this said, the system wasn't perfect. The noise rate was at 50%, which is a far cry from what's required in a quantum computer. But it's a remarkable achievement nonetheless, one that, if it can be refined, could result in scalable quantum computers.

Read the entire study at Nature Photonics: "Nonlinear π phase shift for single fibre-guided photons interacting with a single resonator-enhanced atom".