  Unlike photons, which are conveniently handled by mirrors and optical fibres
without loss of coherence, atoms lose their coherence via atom-atom and
atom-wall interactions. This decoherence of atoms deteriorates the performance
of atomic clocks and magnetometers, and also hinders their miniaturisation.
Here we report a novel platform for precision spectroscopy. Ultracold strontium
atoms inside a kKagome-lattice hollow-core photonic crystal fibre (HC-PCF) are
transversely confined by an optical lattice to prevent atoms from interacting
with the fibre wall. By confining at most one atom in each lattice site, to
avoid atom-atom interactions and Doppler effect, a 7.8-kHz-wide spectrum is
observed for the $^1 S_0-{}^3P_1$ (m=0) transition. Atoms singly trapped in a
magic lattice in hollow-core photonic crystal fibresHC-PCFs improve the optical
depth while preserving atomic coherence time.
