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First author of the paper, Bienvenu Ndagano, School of Physics, University of the Witwatersrand

Physicists at the University of the Witwatersrand in Johannesburg, South Africa, have shown that real-time error correction in quantum communications is possible. The work indicates that sometimes nature cannot tell the difference between the quantum and the real world and that a grey area — called classical entanglement — exists. The findings could help to boost speed and security of optical data transfer and to advance technologies to secure quantum communication links over long distances.

“One of the problems in quantum mechanics is that you once you measure something, say a photon, the very thing you measured now disappears,” says Professor Andrew Forbes. The distinguished professor and head of the Wits Structured Light Laboratory in the School of Physics led the research work. 

This means it is impossible to gain information about a photon and keep that photon in the system, e.g., traveling from one person to another.  

“In the ‘classical’ world, we can measure as much as we like and not destroy anything. For example, when we measure the speed of a car the car does not disappear,” Forbes notes. “But with photons, the photon does disappear.”

Hence, the researchers wondered, “Could we get nature to give us the best of both worlds?”

How real-time error correction in quantum communications is possible

“We showed that we could construct a classical light beam to behave in an identical manner to the quantum state — a single photon — so since the classical world allows many measurements, we can find out all the problems with the quantum state without having to measure it,” Forbes says. “This means we can fix all the problems and still keep the information.”

Solving a fundamental problem

The physicist explains that when quantum information is sent from one person to another, the light has to travel over some distance, perhaps in optical fibre or in air. In doing so, some distortions are inevitable, resulting in errors, such as in the information that was sent. The issue scientists had faced was that in order to fix the errors, one needs to measure and find out what the problem is. However, as Forbes explains, measuring something in the quantum world results in the consequential loss of whatever it was one tried to measure. 

“Now with our approach, you can send this classical beam, which we call classically entangled light, and measure this instead,” Forbes says. “Since nature cannot tell the difference between it and the quantum state, the errors are the same. So now fixing the quantum errors is possible without destroying information.”

Performing quantum experiments with classically entangled light

As a result of this potentially paradigm-shifting research work, real-time error correction in quantum communication is now possible by measuring the error on the classically entangled light. “The significance is that we can send quantum information securely over distance, even if the air or fibre causes distortions,” Forbes confirms.

Impact on future of data transfer and design of next generation of light-based communication technologies

Forbes reinforces that quantum communication is fundamentally secure because it uses the laws of nature for the encoding. “But the quantum states it uses are fragile, easily distorted or destroyed when they experience noise,” he points out. “This has been a major hurdle to overcome for realizing long distance secure quantum communication. We are able to get around this problem with our discovery.”

Even the professor, an expert in the field of light technologies, admits he perceived the outcome of the research with wonder: “I knew mathematically that classically entangled light looked the same as quantum states but still I was amazed to see that this is not only a mathematical construct but a reality — nature really does work that way.”

Next steps

Moving forward, Forbes and his colleagues are now trying show that it is possible to send information in a way that is both fast and secure, over long distances. “In particular, high-dimensional quantum entanglement has not been demonstrated over any appreciable distance,” the expert notes. “We want to show that it is possible, realizing the potential of this new technology.”

The work is detailed in the paper "Characterising quantum channels with non-separable states of classical light," published in Nature Physics.

Written by Sandra Henderson, Research Editor, Novus Light Technologies Today

Labels: University of the Witwatersrand,quantum communication,classical light entanglement,Professor Andrew Forbes,Wits Structured Light Laboratory,quantum information,quantum data,quantum channels

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