  We demonstrate cooling of ultrathin fiber tapers coupled with nitrogen
vacancy (NV) centers in nanodiamonds to cryogenic temperatures. Nanodiamonds
containing multiple NV centers are deposited on the subwavelength
480-nm-diameter nanofiber region of fiber tapers. The fiber tapers are
successfully cooled to 9 K using our home-built mounting holder and an
optimized cooling speed. The fluorescence from the nanodiamond NV centers is
efficiently channeled into a single guided mode and shows characteristic sharp
zero-phonon lines of both neutral and negatively charged NV centers. The
present nanofiber/nanodiamond hybrid systems at cryogenic temperatures can be
used as NV-based quantum information devices and for highly sensitive nanoscale
magnetometry in a cryogenic environment.
