  We demonstrate a cryo-compatible, fully fiber-integrated, alignment-free
optical microresonator. The compatibility with low temperatures expands its
possible applications to the wide field of solid-state quantum optics, where a
cryogenic environment is often a requirement. At a temperature of 4.6 K we
obtain a quality factor of $\mathbf{(9.9 \pm 0.7) \times 10^6}$. In conjunction
with the small mode volume provided by the nanofiber, this cavity can be either
used in the coherent dynamics or the fast cavity regime, where it can provide a
Purcell factor of up to 15. Our resonator is therefore suitable for
significantly enhancing the coupling between light and a large variety of
different quantum emitters and due to its proven performance over a wide
temperature range, also lends itself for the implementation of quantum hybrid
systems.
