  We report observation of quasiparticle pair-production and characterize
quantum entanglement created by a modulational instability in an atomic
superfluid. By quenching the atomic interaction to attractive and then back to
weakly repulsive, we produce correlated quasiparticles and monitor their
evolution in a superfluid through evaluating the in situ density noise power
spectrum, which essentially measures a 'homodyne' interference between ground
state atoms and quasiparticles of opposite momenta. We observe large amplitude
growth in the power spectrum and subsequent coherent oscillations in a wide
spatial frequency band within our resolution limit, demonstrating coherent
quasiparticle generation and evolution. The spectrum is observed to oscillate
below a quantum limit set by the Peres-Horodecki separability criterion of
continuous-variable states, thereby confirming quantum entanglement between
interaction quench-induced quasiparticles.
