18 May 2020

A quantum mechanical random generator that functions with high security has been developed by a research group in which LiU researcher Guilherme Xavier from the Division for Information Coding is a member. The result opens the way for high-dimensional quantum communication.

Picture of an optical fibre launching light in a photonic integrated silicon chip at the quantum technologies laboratory at LiU.
Picture of an optical fibre launching light in a photonic integrated silicon chip at the quantum technologies laboratory at LiU.
The telecommunications industry has been using for some years multicore optical fibres to provide greatly increased transfer capacity in fibre networks. Scientists working in quantum communication have, of course, been eager to use multicore optical technology in experiments, looking to fulfil their dream of transferring information in several dimensions.

LiU researcher Guilherme Xavier started to look at this several years ago, working at the University of Concepción in Chile. Together with colleagues in Chile, the UK, Spain and Brazil he has now developed a high-dimensional beam splitter that causes photons to be distributed completely at random across the cores in a fibre. This means that quantum information can be transferred in several dimensions with high security.

Beam splitter inside the fibre

Until now, the most common way to involve several dimensions has been to build a complete matrix of beam splitters, but the researchers have now developed an integrated unit, a high-dimensional beam splitter inside the multicore fibre. This is achieved by heating and stretching the fibre with high precision, which causes the cores to become positioned so close to each other that photons can jump from one core to another – in a completely random manner.
Fiber optical setup for the transmission of single-photons similar to the one used in the experiment with multi-core fibres.“We know that an individual photon is inside the fibre, but we don’t know exactly where it is, or even which core it is in”, says Guilherme Xavier.

Information coded in photons is inserted into the system, and becomes randomly distributed by the beam splitter. It can subsequently be reassembled and withdrawn at the other end, without it ever having been possible to read the information along the way. At a very high rate.

Guaranteed randomness

One of the major problems in quantum communication is to guarantee randomness, which was also an issue in the Big Bell Test (see below). Researchers from all over the world, including LiU, enrolled people of all different ages in the generation of random numbers. This was necessary, since it is difficult to construct a random number generator that is truly random.

“We have here developed a random generator that we trust completely”, says Guilherme Xavier.

Another application is in drawing up protocols for quantum communication that store more information in each individual photon than what is otherwise possible.

The results are highly significant for the development of secure quantum communication in several dimensions, and have been published in Optica, published by The Optical Society of America.

Multi-core fiber integrated multi-port beam splitters for quantum information processing, Jaime Cariñe, Gustavo Cañas, Paul Skrzypczyk, Ivan Šupić, Nayda Guerrero, Tania Garcia, Luciano Pereira, Miguel Solís Prosser, Guilherme B Xavier, Aldo Delgado, Stephen Walborn, Daniel Cavalcanti, and Gustavo Lima, Optica 2020. DOI 10.1364/OPTICA.388912

Translated by George Farrants


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