  We propose a novel entanglement-creation scheme in a multi-atom ensemble,
named entanglement amplification, which converts unentangled states into
entangled states and amplifies less-entangled ones to maximally-entangled
Greenberger-Horne-Zeilinger (GHZ) states. The scheme starts with a multi-atom
ensemble initialized in a coherent spin state. By shifting the energy of a
particular Dicke state, we break the Hilbert space of the ensemble into two
isolated subspaces to tear the coherent spin state into two components so that
entanglement is introduced. After that, we utilize the isolated subspaces to
further enhance the entanglement by coherently separating the two components.
By single-particle Rabi drivings on atoms in a high-finesse optical cavity
illuminated by a single-frequency light, 2000-atom GHZ states can be created
with a fidelity above 80% in an experimentally achievable system, making
resources of ensembles at Heisenberg limit practically available for quantum
metrology.
