  We report optical studies of quantum dot excitons in individual suspended
carbon nanotubes at cryogenic temperatures. Narrow optical linewidths, strongly
suppressed spectral wandering, and photoluminescence lifetimes in the range of
nanoseconds emerge as key signatures of exciton localization. We infer exciton
quantum dot formation with a characteristic length of a few exciton Bohr radii.
Localization inhibits exciton diffusion and protects the exciton from dephasing
by structural or environmental inhomogeneities as well as from exploring
nonradiative quenching sites. In consequence, quantum dot excitons in carbon
nanotubes exhibit intrinsic radiative lifetimes, long coherence times and a
quantum yield of 100%. Our results underpin the potential of carbon nanotube
excitons for both fundamental studies and applications that scale
advantageously with enhanced spectral resolution and coherence.
