The hall is silent, but the conductor has already raised the baton. The orchestra is quiet, yet the score is changing. So it is in an optical resonator: there is no light, but the quantum vacuum—that emptiness filled with short-lived field fluctuations—is already conducting the matter.
A material placed between mirrors begins to follow new rules. Virtual photons, particles that exist only for a fleeting moment, are born and vanish, yet they manage to interfere with electronic and vibrational states. In the ultrastrong coupling regime, where the light-matter interaction is comparable to their own frequencies, vacuum Rabi splitting and the Bloch-Siegert shift are observed. These are traces of the breakdown of the rotating-wave approximation, a hint that the familiar picture of interaction is incomplete.
Experiments have already shown that vacuum fluctuations weaken the topological protection of the quantum Hall effect and shift the phase transition temperature in 1T-TaS₂. Theoretically, ground-state squeezing and a superradiant phase transition are predicted, but unambiguous confirmation of ground-state modification is still lacking. The A² term, the field energy that depends on the presence of matter, imposes constraints that prevent the system from falling into an uncontrolled state.
Previously, it was unclear whether quantum fluctuations could be used to controllably change material properties. Now it is clear: in the ultrastrong coupling regime, vacuum fields indeed affect electronic and vibrational states, although ground-state modification is not yet proven. The authors have systematized theoretical models and experimental data, identified key limitations (the A² term, gauge invariance), and proposed criteria for future tests. Already today, the results help interpret cavity spectroscopy experiments and guide the search for materials for quantum technologies.
The conductor's baton is raised. The orchestra is silent, but the score is already different. Perhaps in a few years we will learn to control this emptiness, and then the silence will sound anew.
In a picocavity between a gold substrate and a nanoparticle, the fluctuating field strength can exceed 100 MV/cm, more than the breakdown strength of many materials, but the photons are virtual and do not damage the sample.
The idea of using the quantum vacuum to alter matter echoes the concept of "null space" in science fiction, for example, in Greg Egan's novel Quarantine, where vacuum fluctuations become a source of technology.