A research team at Wits University has discovered a way to improve data transmission across fibre networks.
The team comprising of a PhD student at the university, as well as several colleagues from Wits and Huazh University of Science and Technology in Wuhan, China.
This research uses quantum physics to improve data security across fibre networks without the need to replace legacy fibre infrastructure.
“Our team showed that multiple patterns of light are accessible through conventional optical fibre that can only support a single pattern,” said Wits PhD student, Isaac Nape.
“We achieved this ‘quantum trick’ by engineering the entanglement of two photons. We sent the polarised photon down the fibre line and accessed many other patterns with the other photon.”
Entanglement refers to particles interacting in a way that the quantum state of each particle cannot be described without reference to the state of others – even if the particles are separated by large distances.
In this scenario, the researchers manipulated the qualities of the photon on the inside of the fibre line by changing the qualities of its entangled counterpart in free space.
“In essence, the research introduces the concept of communicating across legacy fibre networks with multi-dimensional entangled states, bringing together the benefits of existing quantum communication with polarised photons with that of high-dimension communication using patterns of light,” said team leader Wits Professor Andrew Forbes.
Quantum entanglement has been explored extensively over the past few decades, with the most notable success story being increased communications security through Quantum Key Distribution (QKD).
This method uses “qubits” – 2D quantum states – to transfer a limited amount of information across fibre links by using polarisation as a “degree of freedom”.
Another degree of freedom is the spatial pattern of light, but while this has the benefit of high-dimensional encoding, it requires a custom fibre optical cable – making it unsuitable to already existing networks.
“Our team found a new way to balance these two extremes, by combining polarisation qubits with high-dimensional spatial modes to create multi-dimensional hybrid quantum states,” said Nape.
“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, 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 (or spin), a promising candidate for encoding information.”