Vacuum-dressed superconductivity in NbN observed in a high-$Q$ terahertz cavity

TL;DR

This study demonstrates that embedding NbN superconducting films in high-Q terahertz cavities can modify their superconducting properties through vacuum electromagnetic fluctuations, revealing a new way to engineer material ground states.

Contribution

First experimental observation of cavity-induced modifications in superconductivity of NbN films driven by vacuum fluctuations.

Findings

13% reduction in superfluid density within the cavity

2% decrease in superconducting gap due to cavity effects

Significant changes in optical conductivity observed in cavity environment

Abstract

Emerging theoretical frameworks suggest that physical properties of matter can be altered within an optical cavity by harnessing quantum vacuum electromagnetic fluctuations, even in the total absence of external driving fields. Among the most intriguing predictions is the potential to noninvasively manipulate superconductivity. Here, we experimentally observe modified superconductivity in niobium nitride (NbN) thin films within high-quality-factor ($Q$) terahertz cavities. Using terahertz time-domain spectroscopy, we characterize the NbN response both in free space and within a high-$Q$ photonic-crystal cavity. Our analysis reveals significant cavity-induced modifications to the optical conductivity. A theoretical model indicates that these changes originate from a substantial ($\sim13\,\%$) reduction in the superfluid density and a minor ($\sim2\,\%$) reduction in the superconducting gap, driven by cavity vacuum fluctuations. These results demonstrate a platform for engineering ground states via vacuum--matter coupling, opening frontiers in cavity materials science.