  Single-walled carbon nanotubes (SWCNTs) are promising absorbers and emitters
to enable novel photonic and optoelectronic applications but are also known to
severely suffer from low optical quantum yields. Here we demonstrate SWCNTs
excitons coupled to plasmonic nanocavities reaching deeply into the Purcell
regime with FP=234 (average FP=76), near unity quantum yields of 70% (average
41%), and a photon emission rate of 1.7 MHz into the first lens. The measured
ultra-narrow exciton linewidth (18 micro eV) implies furthermore generation of
indistinguishable single photons from a SWCNT. To demonstrate utility beyond
quantum light sources we show that nanocavity-coupled SWCNTs perform as
single-molecule thermometers detecting plasmonically induced heat (Delta
T=150K) in a unique interplay of excitons, phonons, and plasmons at the
nanoscale.
