15 April 2024

An optical circuit capable of ensuring the security of so-called quantum cryptography – that was an idea that researchers Alvaro Alarcón and Guilherme B. Xavier had five years ago. Now, the idea has finally been tested in an international collaboration, and the results could pave the way for the future of secure communication.

Image of researchers Alvaro Alarcón and Guilherme B. Xavier.
Alvaro Alarcón och Guilherme B. Xavier presenting the photonic integrated chip designed at Linköping University. Simon Höckerbo

The idea of developing a compact research platform for experiments in quantum optics began to take shape when Alvaro Alarcón, now a postdoctoral researcher, began his PhD studies at the Department of Electrical Engineering (ISY). The idea resulted in a photonic integrated chip, which was designed at LiU and then manufactured in the Netherlands.

Designing the platform as a chip was relatively quick, according to the researchers, but due to the Corona pandemic and lead times with the Dutch manufacturer, the chip has not been used in any experiments until recently.

– We are very pleased that this platform is finally in place after all these years, says Alvaro Alarcón.

International collaboration

When the project started five years ago, the Laboratory for Quantum Technology at Linköping University was relatively new. Therefore, it was challenging to conduct high-quality experiments in quantum photonics.

Image of photonic chip.
Simon Höckerbo

The experiment that has now been conducted with the chip in Padua, Italy, was recently published in the scientific journal Optica. The work is a collaboration between the University of Padua, the University of Seville, and Linköping University.

– The collaboration with the University of Padua was motivated by their extensive experience in building and testing sources with entangled photons, while the design and testing of the chip were carried out by us, says Alvaro Alarcón.

Practical application

Guilherme B Xavier is an Associate Professor and head of the division for Information Coding at ISY. He believes that the fact that the idea originated from LiU and that the chip was designed here is a great achievement.

Image of photonic chip.
Simon Höckerbo

– The chip contains an optical circuit that performs joint measurements on energy-time or time-bin entangled photons, and based on the results of these measurements, we can provide certification of the entanglement's quality. A practical application is in quantum cryptography, where it is important to certify that the photons are truly entangled to ensure security, says Guilherme B Xavier.

This work has been funded by ZENITH at Linköping University, Vetenskaprådet, QuantERA and the Wallenberg Center for Quantum Technologies (WACQT).

Experimental post-selection loophole-free time-bin and energy-time nonlocality with integrated photonics; Francesco B. L. Santagiustina, Costantino Agnesi, Alvaro Alarcón, Adán Cabello, Guilherme B. Xavier, Paolo Villoresi, and Giuseppe Vallone; Optica Vol. 11 2024; published online 12 April 2024, DOI: 10.1364/OPTICA.499247



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