  We show that a single quantum emitter can efficiently couple to the tunable
plasmons of a highly doped single-wall carbon nanotube (SWCNT). Plasmons in
these quasi-one-dimensional carbon structures exhibit deep subwavelength
confinement that pushes the coupling efficiency close to 100% over a very broad
spectral range. This phenomenon takes place for distances and tube diameters
comprising the nanometer and micrometer scales. In particular, we find a beta
factor ~1 for QEs placed 1-100 nm away from SWCNTs that are just a few
nanometers in diameter, while the corresponding Purcell factor exceeds 10^6.
Our finding not only holds great potential for waveguide QED, in which an
efficient interaction between emitters and cavity modes is pivotal, but it also
provides a way of realizing quantum strong coupling between several emitters
mediated by SWCNT plasmons, which can be controlled through the large
electro-optical tunability of these excitations.
