  Entangled many body systems have recently attracted significant attention in
various contexts. Among them, spin squeezed atoms and ions have raised interest
in the field of precision measurements, as they allow to overcome quantum noise
of uncorrelated particles. Precise quantum state engineering is also required
as a resource for quantum computation, and spin squeezing can be used to create
multi-partite entangled states. Two-mode spin squeezed systems have been used
for elementary quantum communication protocols. Until now spin squeezing has
been always achieved via generation of entanglement between different atoms of
the ensemble. In this Letter, we demonstrate for the first time ensemble spin
squeezing generated by engineering the quantum state of each individual atom.
More specifically, we entangle the nuclear and electronic spins of $10^{12}$
Cesium atoms at room temperature. We verify entanglement and ensemble spin
squeezing by performing quantum tomography on the atomic state.
