Emitters placed in an optical cavity experience an environment that changes their coupling to
light. In the weak-coupling regime light extraction is enhanced, but more profound effects
emerge in the single-molecule strong-coupling regime where mixed light-matter states
form1,2
. Individual two-level emitters in such cavities become non-linear for single photons,
forming key building blocks for quantum information systems as well as ultra-low power
switches and lasers3–6
. Such cavity quantum electrodynamics has until now been the preserve
of low temperatures and complex fabrication, severely compromising their use5,7,8
. Here, by
scaling the cavity volume below 40 nm3
and using host-guest chemistry to align 1-10
protectively-isolated methylene-blue molecules, we reach the strong-coupling regime at
room temperature and in ambient conditions. Dispersion curves from >50 plasmonic
nanocavities display characteristic anticrossings, with Rabi frequencies of 300 meV for 10
molecules decreasing to 90 meV for single molecules, matching quantitative models.
Statistical analysis of vibrational spectroscopy time-series and dark-field scattering spectra
provide evidence of single-molecule strong coupling. This dressing of molecules with light can
modify photochemistry, opening up the exploration of complex natural processes such as
photosynthesis9
and pathways towards manipulation of chemical bonds10
.