We report a novel source of twin beams based on modulational instability in
high-pressure argon-filled hollow-core kagom\'e-style photonic-crystal fibre.
The source is Raman-free and manifests strong photon-number correlations for
femtosecond pulses of squeezed vacuum with a record brightness of ~2500 photons
per mode. The ultra-broadband (~50 THz) twin beams are frequency tunable and
contain one spatial and less than 5 frequency modes
