7 July 2026

Photo: MPSD, Joerg Harms

Over many years, the use of optical cavities has been perfected to control the behavior of photons. In its simplest realization, an optical cavity consists of two mirrors that can trap photons between them.

One particularly interesting, but at first glance counter-intuitive, idea for controlling the quantum world is to use the quantum fluctuations of the photon field to engineer the properties of materials. In our Cluster of Excellence, this idea is turned into reality by research projects of Prof. Angel Rubio and Dr. Michael Ruggenthaler. Part 3 of our series on the research journal “Advanced Imaging of Matter – Seven Years CUI in Focus” explores quantum fluctuation technology and its path to industrial application.

When the pioneers of quantum physics – Max Planck, Niels Bohr, Erwin Schrödinger and others – demonstrated that the laws of physics had to be revised on the microscopic scale, little did they know that their scientific revolution also implied a complete transformation of technology and society. In retrospect, this epochal shift came to be known as the “first quantum revolution”. More than a hundred years later, based on developments by other brilliant scientists such as Richard Feynman, Serge Haroche, and the 2022 Physics Nobel laureates Alain Aspect, John Clauser and Anton Zeilinger, we are experiencing a “second quantum revolution”. This time, it is the control over the quantum world that promises novel technologies, including quantum computing and quantum sensing technologies. 

But how can we achieve control over quantum fluctuations?

In our everyday life we perceive light and matter as two distinct entities. The first is fleeting and dynamic, the second permanent and stable. At the quantum level, however, there is no strict distinction between the two. Photons, the quantum particles of light, are the glue that holds atoms, molecules and solids together. They even constitute a part of the mass of charged elementary particles, such as electrons and positrons. From this perspective, it no longer seems impossible that, by controlling the quantum fluctuations of the photons, one might be able to tailor properties of materials, such as their conductivity or their basic structure. But how can we achieve such control over quantum fluctuations?

Quantum optics offers an answer. Over many years, the use of optical cavities has been perfected to control the behavior of photons. In its simplest realization, an optical cavity consists of two mirrors that can trap photons between them. However, these mirrors trap not only real photons but also virtual ones. That is, the mirrors control also the quantum fluctuations of the electromagnetic field. If we now place matter, such as molecules or two-dimensional materials, inside this cavity, the “pho-tonic glue” that holds them together is slightly altered and we can observe modifications of these molecules or two- dimensional materials. Most importantly for technological applications, these changes occur without any external energy consumption – the cavity is “dark” – and they are even observed at room temperature. Both of these features make quantum fluctuation technology obviously highly interesting. The confluence of favorable properties seems almost too good to be true. So what is the catch?

What is the catch?

The catch is that the interplay of photon fluctuations with materials on the microscopic scale is extremely difficult to predict and understand from a theoretical perspective. Two completely different fields of physics, electronic structure theory and quantum optics, collide. To combine them consistently, one needs to turn to quantum electrodynamics and develop computational tools to solve the resulting equation in the low-energy regime fully non-perturbatively. This need led to the emergence of the novel field of ab initio quantum electrodynamics, which was pioneered at CUI. Using these computational methods, including quantum-electrodynamical density functional theory, we are learning step by step how quantum fluctuations of light can modify material and chemical properties. Based on this understanding, we can now make theoretical predictions, such as the appearance of cavity-controlled phase transitions or chemical reactions. These can then be tested by experimental collaborators. If our understanding grows at the same rate as it has over the past few years in our Cluster, we might soon see the first industrial applications of quantum fluctuation technology at room temperature. Text: Michael Ruggenthaler, Angel Rubio

Literature

• M. Ruggenthaler et al., Nature Reviews Chemistry, 2, 0118 (2018).
• M. Ruggenthaler et al., Physical Review A 90, 012508 (2014).
• S. Latini et al., Proceedings of the National Academy of Sciences 118, e2105618118 (2021).
• I. Lu et al., Proceedings of the National Academy of Sciences 121, e2415061121 (2024).
• D. Sidler et al., The Journal of Chemical Physics 156, 230901 (2022).
• M. Ruggenthaler et al., Chemical Reviews 123, 11191 (2023).
• D. Sidler et al., Journal of Chemical Physics 156, 230901 (2022).
• M. Kamper Svendsen et al., arXiv:2312.17374 (2023).
• D. Sidler et al., arXiv:2409.08986 (2024).