Technology forum – laser – photonics

04 Jul Quantum communication on its way into practice

Fundamental technical hurdles still need to be overcome to ensure that quantum communication not only works in the laboratory but can also be used under real-life conditions. Researchers from Jena in Germany and Canada have investigated two of these together with an international team: How can more information be transmitted per particle of light? And how can the signal remain stable over long distances – despite the physical effects that occur when propagating through optical fibers?

Professor Mario Chemnitz is researching new approaches at Leibniz IPHT in Jena to transfer secure quantum communication via optical fiber into practical applications. Image: Stela Todorova / Leibniz-IPHT

Higher data rates: Time-bin coding

A central starting point is time-bin coding: photons carry information about their exact arrival time – i.e. the tiny time window (time bin) in which they are detected. Until now, only two time bins could usually be distinguished. The research team has now developed a photonic platform that can use up to eight such time bins per photon simultaneously, thereby significantly increasing the data rate. “You can think of it like a drawer system,” explains Professor Mario Chemnitz from the Leibniz Institute of Photonic Technology (Leibniz IPHT) and Friedrich Schiller University Jena. “Instead of just one drawer, several can now be opened in parallel – allowing more information to be transmitted at the same time.”

Photonic chip as a platform

The platform was developed as part of a study led by the Institut National de la Recherche Scientifique (INRS) in Canada and the Leibniz IPHT. It is based on a specially designed photonic chip with tiny interferometers made of silicon nitride, a material that is particularly suitable for integrated light guidance. This structure generates and processes entangled light in the smallest of spaces, using standard components from the telecommunications industry. According to the researchers, the system was successfully tested over 60 kilometers of optical fiber – a typical distance between two network nodes. In future, more users could communicate simultaneously – securely and at high data rates.

Stable over long distances

A second challenge that was addressed: As the distance increases, the signal becomes more vulnerable – partly due to dispersion, a physical effect that spreads light pulses apart in time. This makes it difficult to precisely distinguish the time windows. This effect can be compensated for: The team analyzed not only the distance between two photons, but also their common arrival time. This so-called sum correlation remains stable even with strong dispersion and could now be used in a targeted manner for the first time. The researchers report that the range of an encrypted quantum connection could be extended to up to 200 kilometers of fiber optic equivalent with higher signal quality and security.

From basic research to application

The two approaches are interlinked: while one increases the amount of information, the other ensures greater stability. “We are working on making quantum communication suitable for practical use – with systems that can be integrated into existing fiber optic networks,” summarizes Chemnitz. For him, the focus is on the interplay between basic research and technical application: “What we develop should also prove itself in everyday life at some point – in diagnostics, in communication, perhaps even in autonomous sensors.”

Original publications:
[Yu, H., Sciara, S., Chemnitz, M., Montaut, N., Crockett, B., Fischer, B., Helsten, R., Wetzel, B., Goebel, T. A., Krämer, R. G., Little, B. E., Chu, S. T., Nolte, S., Wang, Z., Azaña, J., Munro, W. J., Moss, D. J., & Morandotti, R. (2025). Quantum key distribution implemented with d-level time-bin entangled photons. Nature Communications, 16, Article 171. DOI: https://doi.org/10.1038/s41467-024-55345-0]

Source: www.leibniz-ipht.de

Image: Stela Todorova / Leibniz-IPHT