  We show that dynamic spin polarization by collective raising and lowering
operators can drive a spin ensemble from arbitrary initial state to many-body
singlets, the zero-collective-spin states with large scale entanglement. For an
ensemble of $N$ arbitrary spins, both the variance of the collective spin and
the number of unentangled spins can be reduced to O(1) (versus the typical
value of O(N)), and many-body singlets can be occupied with a population of
$\sim 20 %$ independent of the ensemble size. We implement this approach in a
mesoscopic ensemble of nuclear spins through dynamic nuclear spin polarization
by an electron. The result is of two-fold significance for spin quantum
technology: (1) a resource of entanglement for nuclear spin based quantum
information processing; (2) a cleaner surrounding and less quantum noise for
the electron spin as the environmental spin moments are effectively
annihilated.
