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First demonstration of quantum interference in high dimensions

- Wits University

Researchers in South Africa and Scotland have demonstrated a new approach to quantum state engineering that requires only a beamsplitter.

Professor Andrew Forbes, Wits University, collaborated with scientists from the Council for Scientific and Industrial Research (CSIR), University of KwaZulu-Natal and Heriot-Watt University in Scotland to publish a paper, Engineering two-photon high-dimensional states through quantum interference in the online journal of (SCIENCE), SCIENCE ADVANCES.

Forbes is a Distinguished Professor in the School of Physics and heads the Structured Light Laboratory, with programmes to study both classical and quantum communication.

The team wondered what would happen when entangled particles of light (photons) are brought together onto a partially reflecting mirror (beamsplitter) that is designed to send half the light one way and half the other way. 

Entanglement is that “spooky action at a distance” that Albert Einstein so disliked, giving rise to counterintuitive behavior. It was known that when two single photons are brought into the beamsplitter something strange happens: they either both go one way or noth the other way, they never go through in opposite directions as normal light does.

Independent paths

What the team found was that the photons could be made to take independent paths if they were in a particular quantum state. The consequence of this new finding is that this simple beamsplitter – nothing more than a cube of glass – can be used to engineer high-dimensional quantum states. The team used this device to engineer a quantum state in six dimensions using twisted light carrying orbital angular momentum.

The South African team members have a joint project funded by the Photonics Initiative of South Africa (PISA) to demonstrate real-world quantum communication; with the present work a step towards this goal.

“Our project aims to bring quantum technologies out of the laboratory and into the real-world, to demonstrate a secure link using quantum encryption,” said Forbes.

According to the researchers, the story of the Enigma machine knows that encryption based on human ingenuity is flawed – it is always possible that your adversary is smarter than you. But quantum encryption is based on the very laws of Nature, and so fundamentally secure. 


“The challenge is to make this work in high dimensions and in the real world.  In this recent advance, the team have used so-called ‘twisted’ light, light that carries orbital angular momentum, to reach dimensions beyond the usual two. High-dimensional quantum entanglement is a tricky business, the researchers noted.

“Light can be entangled in many dimensions, but most people stick with two because it is so much easier to do the experiments. It is significant that we work in high dimensions, bringing abstract mathematics to life in the laboratory.” 

A single quantum measurement with a high-dimensional quantum state can take the entire weekend to perform, running 24 hours a day. But the advantages of high-dimensions is that more information can be packed into the light, increasing the rate of data transfer.

Way forward

Professor Stef Roux, team leader at the CSIR laboratories, said they want to use this technology to demonstrate secure quantum communication over a long distance. 

“We are working on several approaches to achieve this, some theoretical and some experimental.”

The next step in the project is to demonstrate UKZN Professor’s Thomas Konrad idea, that it is possible to teleport quantum states in high dimensions. 

“This is still rather far from what we see on Star Trek, but we’re getting there,” said Forbes.