  We demonstrate theoretically and experimentally a high level of control of
the four-wave mixing process in an inert gas filled inhibited-coupling guiding
hollow-core photonic crystal fiber in order to generate photon pairs. The
specific multiple-branch dispersion profile in such fibers allows both
entangled and separable bi-photon states to be produced. By controlling the
choice of gas, its pressure and the fiber length, we experimentally generate
various joint spectral intensity profiles in a stimulated regime that is
transferable to the spontaneous regime. The generated profiles cover both
spectrally separable and entangled bi-photons and feature frequency tuning over
17 THz, demonstrating the large dynamic control offered by such a photon pair
source.
