  We fabricate an extremely thin optical fiber that supports a super-extended
mode with a diameter as large as 13 times the optical wavelength, residing
almost entirely outside the fiber and guided over thousands of wavelengths (5
mm), in order to couple guided light to warm atomic vapor. This unique
configuration balances between strong confinement, as evident by saturation
powers as low as tens of nW, and long interaction times with the thermal atoms,
thereby enabling fast and coherent interactions. We demonstrate narrow coherent
resonances (tens of MHz) of electromagnetically induced transparency for
signals at the single-photon level and long relaxation times (10 ns) of atoms
excited by the guided mode. The dimensions of the guided mode's evanescent
field are compatible with the Rydberg blockade mechanism, making this platform
particularly suitable for observing quantum non-linear optics phenomena.
