Physicists show that real-time error correction in quantum communications is possible

January 23, 2017, Wits University

First author and PhD student, Bienvenu Ndagano, in the Structured Lab at Wits University in Johannesburg. Credit: Wits University
Nature Physics today, Monday, 23 January 2017, published online the research by a team led by physicists from the School of Physics at Wits University. In their paper titled: Characterising quantum channels with non-separable states of classical light the researchers demonstrate the startling result that sometimes Nature cannot tell the difference between particular types of laser beams and quantum entangled photons.

In essence, the research show that sometimes Nature cannot tell the difference between the and the classical (or real) worlds, and that a grey area does exist between the two worlds called classical entanglement.

Classical and quantum worlds

Present communication systems are very fast, but not fundamentally secure. To make them secure researchers use the laws of Nature for encoding by exploiting the quirky properties of the quantum world, such as in the case of the use of Quantum Key Distribution (QKD) for secure communication.

"Quantum" refers to the small, and in the photonics world this means one photon - a single particle of light. The rules of the quantum world are vastly different from that of the classical world, and experiments are traditionally much harder due to the difficulty in handling just a few photons.

"In the classical world our intuition holds true. There are no surprises and experiments can be done with many photons (billions and billions of them), such as laser light," explains Professor Andrew Forbes, team leader of the collaboration and Distinguished Professor in the School of Physics where he heads up the Wits Structured Light Laboratory.

"But not so in the quantum world, where things are never quite as they seem. Here waves sometimes look like particles, particles like waves, and measurements change the properties of the very thing you are trying to measure."

Real-time quantum error correction is possible

Now researchers have shown that there is a grey area where Nature cannot tell the difference between the classical and the quantum. This opens the possibility of first performing quantum experiments with a type of classical light called "classically entangled" light.

Atmospheric turbulence is displayed here as a grayscale image for simulation on a spatial light modulator. Credit: Credit: Wits University

For example, establishing a secure quantum communication link over long distance is very challenging: "Quantum links (as in fibre optics) using patterns of light languish at short distances precisely because there is no way to protect the link against noise (interference from, for instance, fog or a bend in a cable) without detecting the photons. Yet, once they are detected their usefulness is destroyed," says Forbes.

This catch 22 situation has been a seemingly insurmountable obstacle. Now the team has shown that this can be overcome using classical (many photon) light fields, enabling real-time .

By preparing and sending a so-called "classically entangled" beam the team could show that this was identical to sending a . This means that the observed quantum entanglement decay due to noise in the link can be reversed, paving the way for major advances in secure quantum links in fibre and free-space.

"We showed for the first time that classical light can be used to analyse a quantum link, acting as a direct equivalent to the behavior of the quantum state," says Bienvenu Ndagano, lead author and PhD student at Wits University.

"Not similar, or mimicking, but equivalent. To show this, we exploited a particular type of laser beam, called vector beams, that have the property of being non-separable and sometimes called 'classically entangled'."

Ndagano explains that the quintessential property of quantum entanglement is the non-separability of the state, meaning that one part of the system cannot be separated from the other. "But non-separability is not unique to the quantum world: you can find it in weather maps where the locations on the map and the temperatures at those locations can't be separated."

Classically entangled light

More intriguingly, classical vector beams have this property too, which the team calls "classically entangled" light.

Says Forbes, "What we asked was: does this mean that classical light can be used in quantum systems - a grey area between the two worlds that we call classical entanglement?".

Animation of the effects of turbulence on various patterns of light. Credit: Wits University
"The notion of classical entanglement is hotly contested in the physics community with some arguing that it is merely a mathematical construct," says Thomas Konrad (UKZN), co-author on the paper. "This work shows that there is physical meaning to it too, and we offer the first side-by-side data of the equivalence of classical and ".

Previously, to fix an error in the quantum state used for secure communication would mean measuring the photon sent, which in turn would mean losing the information that one was trying to send.

This work allows for long distance quantum links to be established and tested with classically entangled light: as there is no shortage of photons in the , all the measurements needed to fix the errors in the quantum state can be done in real-time without destroying the quantum information.

Thus, real-time error correction is possible as you can run experiments in the that will tell you how to fix the error in the .

Fast and secure data transfer over real-world link

The team are working on packing as much information into photons using patterns of as a means to encode the information. Since there are an unlimited number of patterns, the amount of information that can be sent securely is also in principle at least, unlimited.

While all patterns are equivalent in terms of information capacity, this work suggests that the choice of pattern also plays an important role in analysing and correcting the errors experienced by passing over the link.

"By working in this grey area between the classical and the quantum we can show fast and secure data transfer over real-world links," says Forbes.

Explore further: Bridging the gap between the quantum and classical worlds

More information: Characterizing quantum channels with non-separable states of classical light, Nature Physics,

Related Stories

Bridging the gap between the quantum and classical worlds

August 2, 2016

In the quantum world, physicists study the tiny particles that make up our classical world - neutrons, electrons, photons - either one at a time or in small numbers because the behaviour of the particles is completely different ...

Quantum physics mimics spooky action into the past

April 23, 2012

Physicists of the group of Prof. Anton Zeilinger at the Institute for Quantum Optics and Quantum Information (IQOQI), the University of Vienna, and the Vienna Center for Quantum Science and Technology (VCQ) have, for the ...

Breaking the limits of classical physics

June 7, 2012

( -- With simple arguments, researchers show that nature is complicated. Researchers from the Niels Bohr Institute have made a simple experiment that demonstrates that nature violates common sense – the world ...

Three 'twisted' photons in 3 dimensions

February 29, 2016

Researchers at the Institute of Quantum Optics and Quantum Information, the University of Vienna, and the Universitat Autonoma de Barcelona have achieved a new milestone in quantum physics: they were able to entangle three ...

Recommended for you

Engineers invent groundbreaking spin-based memory device

December 7, 2018

A team of international researchers led by engineers from the National University of Singapore (NUS) have invented a new magnetic device to manipulate digital information 20 times more efficiently and with 10 times more stability ...

Multichannel vectorial holographic display and encryption

December 7, 2018

Holography is a powerful tool that can reconstruct wavefronts of light and combine the fundamental wave properties of amplitude, phase, polarization, wave vector and frequency. Smart multiplexing techniques (multiple signal ...

A new 'spin' on kagome lattices

December 7, 2018

Like so many targets of scientific inquiry, the class of material referred to as the kagome magnet has proven to be a source of both frustration and amazement. Further revealing the quantum properties of the kagome magnet ...

1 comment

Adjust slider to filter visible comments by rank

Display comments: newest first

not rated yet Jan 23, 2017
Of course. In quantum mechanics anything is possible. Even poop can stick to the ceiling.
Can you do it consistently?

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.