  Recent years have witnessed revolutionary improvement in the production,
manipulation, characterization and quantification of multiatom (multiqubit)
states - because of their promising applications in high precision atomic
clocks, atomic interferrometry, quantum metrology and quantum information
protocols. In the characterization and quantification of non-classical atomic
correlations, two concepts namely, spin squeezing and quantum entanglement
emerge. While spin squeezing originates from the uncertainty relation between
atomic collective angular momentum operators, entanglement is a peculiar
phenomena arising due to superpositions in the multiparticle Hilbert space.
Here, we outline different quantitative measures of spin squeezing and discuss
the intrinsic relation of spin squeezing with quantum entanglement in
multiqubit systems.

Quantum Physics