Tapp-proof communication is regarded as one of the great promises of quantum physics. In this process, information is transmitted using individual particles of light – known as photons – and is encoded and decoded using a special key that is generated with every measurement. The key feature of quantum cryptography is that if an external attempt is made to intercept the signal, the key changes. Until now, it has only been possible to a limited extent to transfer certain properties of quantum light – which is required for quantum cryptography – into the classical world.
A team of researchers from the University of Graz, the University of Vienna and the Technical University of Munich has now developed a method enabling classical, commercial light simulations to capture key properties of quantum light. The new approach is intended to facilitate the development of components for eavesdropping-proof quantum communication, quantum computers and quantum sensors. The results have been published in the prestigious journal Nature Communications.
“Simulation programmes, as we know them from classical physics, can calculate very precisely how light travels through nanostructured materials. However, what they do not adequately capture are the actual quantum properties of light – such as its changes and interactions,” explains Felix Hitzelhammer, a PhD student at the Institute of Physics at the University of Graz and lead author of the article. He collaborated on the project with his colleague and co-supervisor Gaby Slavcheva. The key question the researchers therefore ask is: “How can the effects from the quantum world be transferred to classical electrodynamics to make them measurable?”
Noise: not a disruptive factor, but a tool
After numerous calculations and experiments, the scientists have found a way to extend classical light simulations so that they also correctly describe important properties of quantum light. The trick: deliberately generated noise. “In this case, noise is not a disruptive signal, but the carrier of crucial information,” explains physicist Ulrich Hohenester, supervisor of Hitzelhammer’s PhD thesis. In the process, specific noise components are ‘imprinted’ onto the simulated light sources during transmission. These are selected in such a way that they contain the quantum properties of the light. The light can then be ‘observed’ and further analysed using established classical simulation programmes. The sought-after information about the quantum light is contained within the noise itself.
The method was tested on a well-known test case in quantum optics: the so-called “Mollow triplet”. This characteristic light spectrum arises when a quantum system is strongly excited by laser light. It shows particularly clearly whether a model correctly describes the fluctuations of a quantum system. The result: the simulations matched measurements on a semiconductor quantum dot made of indium gallium arsenide perfectly. Such a quantum dot is an artificial light source for individual photons.
This is still a proof of concept. Nevertheless, the work points to a promising avenue: quantum light does not need to be calculated entirely using extremely complex quantum models. Under certain conditions, its properties can also be transferred to classical light simulations – provided the correct noise is incorporated.
Publication
Hitzelhammer, F., Stowasser, J., Hanschke, L. et al. Bridging quantum noise and classical electrodynamics with stochastic methods. Nat Commun (2026). https://doi.org/10.1038/s41467-026-73066-4