A more robust quantum communication

Senior lecturer Guilherme B Xavier founded a laboratory of quantum technologies at LiU in 2018. His research group’s new results could make quantum communication over long distances more robust.

Alvaro Alarcón, Joakim Argillander and Guilherme B Xavier in the lab at LiU. Magnus Johansson

Alongside artificial intelligence, quantum technology is probably the research area within technology that is stimulating most interest at the moment. Research into quantum technology is attracting huge investment in many countries, such as China and the US, and in Sweden. 

Quantum mechanics describes how particles, including photons, behave at the atomic level. Their behaviour is often remarkably different from how we perceive the world in everyday life. Researchers have, however, shown experimentally that the theory of quantum mechanics describes the real world accurately.  

“We sometimes say that we are experiencing a second quantum revolution. It’s now possible to control individual quantum systems”, says Guilherme B Xavier, senior lecturer in the Department of Electrical Engineering. He set up a laboratory of quantum technologies at Linköping University in 2018.

Suitable for certain types of problems

The quantum computers that are available today are primitive, but there are huge hopes that it will be possible to build superfast quantum computers in the future. Although, just as is the case also for AI, some people believe that these hopes are exaggerated.

“It’s a common misconception that quantum computers will replace conventional computers, but quantum computers are suitable only for certain types of problems. They do not outperform conventional computers for other types”, says Guilherme B Xavier.

Guilherme Xavier.Guilherme B Xavier. Photo credit Magnus Johansson

Neither black nor white

A central concept in quantum mechanics is “superposition”.

“Imagine you have a white ball. According to classical physics, it is white for everybody, and it is white all the time. If you create a quantum superposition of a white ball and a black ball, the ball is neither black nor white, nor is it grey. When you finally observe the ball, it will be white or black, but you don’t know what colour it is until you observe it, “explains Guilherme B Xavier.

Superposition plays an important role in quantum technology since it is behind the security in quantum cryptography. It allows the generation of truly random numbers and is necessary in quantum algorithms to give a few examples.

“One of the tasks that would be ideal for a quantum computer is to simulate large complex systems, such as a large molecule. This is difficult for conventional computers, and could be used in drug development, for example.” 

Guilherme B Xavier and his research group specialise in quantum communication, which is the process by which quantum computers pass information to each other.  A subfield to this. that is particularly closely linked to quantum properties, is quantum cryptography.

“If you send information between two conventional computers, it is possible, in theory, for an unauthorised person to read the information without either the sender or the receiver knowing that security has been breached. Information that is sent by quantum communication is affected when it is read by anyone. This means that you can see if some one has read the information you sent”, says Guilherme Xavier.  

Picture of the Picture of the "donut" spatial mode profile of the light beam that was used in the experiments.

A new way of encoding information 

Together with two PhD students, Alvaro Alarcón and Joakim Argillander, Guilherme B Xavier recently published an article in the scientific journal Physical Review Applied. He describes the two main results in the article.

“We have investigated different ways to transmit information in a quantum state along an optical fibre. One way to encode the information that is particularly suitable for transmission over long distances is known as ‘time-bin’.” 

A problem that arises during transmission across long distances is loss of signal. This can be solved in classical communication by reinforcing the signal, but this is not possible in quantum communication.

“The first main point of our article is that we suggest a new way of encoding information that reduces losses for time-bin quantum communication systems.” 

The light transmitted through the optical fibre has a form known as its “spatial mode”. This form affects the efficiency of the communication. In order to create a particular form, the light must first leave the fibre and pass through an optical component, similar to a lens, and then back into the fibre. This can lead to loss of information.

“The second main point of the article is that we have managed to create ‘modes’ inside the fibre itself, and that all the equipment we have used is commercially available. This makes the communication more robust.”  

"I'm cautiously optimistic"

Guilherme B Xavier and the research group plan to take these results further.  The article they have published shows that their ideas can be carried out in practice. The next step is to build a quantum communication system in which they can transmit encrypted information.

People have huge hopes for quantum technology, for extremely rapid computers and secure information transfer. How do you see the future of this field? 

“I’m cautiously optimistic. And even if it turns out that we cannot achieve as much as we hope, we have even so learnt a lot in the process that can be used also in non-quantum fields.”

The article “Few-mode fiber technology fine-tunes losses of quantum communication systems” is available here.

The research has received financial support from: Ceniit Linköping University, Vetenskapsrådet and the Wallenberg Center for Quantum Technologies.

Translated by George Farrants

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