  We show that a pinning quantum phase transition for photons could be observed
in a hollow-core one-dimensional fiber loaded with a cold atomic gas. Utilizing
the strong light confinement in the fiber, a range of different strongly
correlated polaritonic and photonic states, corresponding to both strong and
weak interactions can be created and probed. The key ingredient is the creation
of a tunable effective lattice potential acting on the interacting polaritonic
gas which is possible by slightly modulating the atomic density. We analyze the
relevant phase diagram corresponding to the realizable Bose-Hubbard (weak) and
sine-Gordon (strong) interacting regimes and conclude by describing the
measurement process. The latter consists of mapping the stationary excitations
to propagating light pulses whose correlations can be efficiently probed once
they exit the fiber using available optical technologies
