  We propose a method to adiabatically control an atomic ensemble using a
decoherence-free subspace (DFS) within a dissipative cavity. We can engineer a
specific eigenstate of the system's Lindblad jump operators by injecting a
field into the cavity which deconstructively interferes with the emission
amplitude of the ensemble. In contrast to previous adiabatic DFS proposals, our
scheme creates a DFS in the presence of collective decoherence. We therefore
have the ability to engineer states that have high multi-particle entanglements
which may be exploited for quantum information science or metrology. We further
demonstrate a more optimized driving scheme that utilizes the knowledge of
possible diabatic evolution gained from the so-called adiabatic criteria. This
allows us to evolve to a desired state with exceptionally high fidelity on a
time scale that does not depend on the number of atoms in the ensemble. By
engineering the DFS eigenstate adiabatically, our method allows for faster
state preparation than previous schemes that rely on damping into a desired
state solely using dissipation.
