Quantum communication breakthrough to enable secure data transmission across optical fibre networks

Researchers demonstrate multi-dimensional quantum communications with twisted light down legacy fibre networks

A joint team of researchers from South Africa and China has demonstrated that it is possible to transmit multiple quantum patterns of twisted light across conventional fibre networks.

Normally, such a legacy link could transmit only a single pattern of light.

Researchers believe the breakthrough could allow extremely secure data transmission over long distances, by combining multiple quantum properties on the same photons.

The study was led by Professor Andrew Forbes from the School of Physics at the University of the Witwatersrand (Wits) in Johannesburg, South Africa, in collaboration with Huazhang University of Science and Technology in Wuhan, China.

The researchers made the breakthrough using quantum entanglement phenomena, which occurs when subatomic particles, such as protons, become linked and start to influence one another regardless of the distance between them.

In their experiments, the researchers engineered quantum entanglement between two photons of light - one in polarisation (photon A) and the other (photon B) in orbital angular momentum (a twisted light or pattern).

Photon A was passed through the fibre, while photon B was kept in the air. The research team then measured the polarisation of photon A and the orbital angular momentum (OAM) patterns of photon B.

Because both photons were entangled, photon A also carried through the fibre the information about photon B.

"The trick was to twist the one photon in polarisation and twist the other in pattern, forming 'spirally light' that is entangled in two degrees of freedom," said Forbes.

"Since the polarisation entangled photon has only one pattern it could be sent down the long-distance single-mode fibre (SMF), while the twisted light photon could be measured without the fibre, accessing multi-dimensional twisted patterns in the free-space. These twists carry orbital angular momentum, a promising candidate for encoding information."

"The trick works because the photons don't know what they are until we measure them, so the state is unaware that we have multiple patterns in the game."

"The novelty in the published work is the demonstration of multi-dimensional entanglement transport in conventional single-mode fibre," Forbes highlighted.

The team successfully demonstrated the transmission of "multiple quantum patterns of twisted light" down a 250-metre-long conventional fibre link.

In future, Forbes expects the research team to achieve transmission distances over 250 metres.

"It should be possible to get across 100 km of fibre, which would make it practical," he said.

The detailed findings of the study are published in the journal Science Advances.