  Optical waveguides in the form of glass fibers are the backbone of global
telecommunication networks. In such optical fibers, the light is guided over
long distances by continuous total internal reflection which occurs at the
interface between the fiber core with a higher refractive index and the lower
index cladding. Although this mechanism ensures that no light escapes from the
waveguide, it gives rise to an evanescent field in the cladding. While this
field is protected from interacting with the environment in standard optical
fibers, it is routinely employed in air- or vacuum-clad fibers in order to
efficiently couple light fields to optical components or emitters using, e.g.,
tapered optical fiber couplers. Remarkably, the strong confinement imposed by
the latter can lead to significant coupling of the light's spin and orbital
angular momentum. Taking advantage of this effect, we demonstrate the
controlled directional spontaneous emission of light by quantum emitters into a
sub-wavelength-diameter waveguide. The effect is investigated in a paradigmatic
setting, comprising cesium atoms which are located in the vicinity of a
vacuum-clad silica nanofiber. We experimentally observe an asymmetry higher
than 10:1 in the emission rates into the counterpropagating fundamental guided
modes of the nanofiber. Moreover, we demonstrate that this asymmetry can be
tailored by state preparation and suitable excitation of the quantum emitters.
We expect our results to have important implications for research in
nanophotonics and quantum optics and for implementations of integrated optical
signal processing in the classical as well as in the quantum regime.
