  Interferometers with atomic ensembles constitute an integral part of modern
precision metrology. However, these interferometers are fundamentally
restricted by the shot noise limit, which can only be overcome by creating
quantum entanglement among the atoms. We used spin dynamics in Bose-Einstein
condensates to create large ensembles of up to $10^4$ pair-correlated atoms
with an interferometric sensitivity $-1.61^{+0.98}_{-1.1}$ dB beyond the shot
noise limit. Our proof-of-principle results point the way toward a new
generation of atom interferometers.
