  Active hydrodynamic theories are a powerful tool to study the emergent
ordered phases of internally driven particles such as bird flocks, bacterial
suspension and their artificial analogues. While theories of orientationally
ordered phases are by now well established, the effect of chirality on these
phases is much less studied. In this paper, we present the first complete
dynamical theory of orientationally ordered chiral particles in two-dimensional
incompressible systems. We show that phase-coherent states of rotating chiral
particles are remarkably stable in both momentum-conserved and non-conserved
systems in contrast to their non-rotating counterparts. Furthermore, defect
separation -- which drives chaotic flows in non-rotating active fluids -- is
suppressed by intrinsic rotation of chiral active particles. We thus establish
chirality as a source of dramatic stabilization in active systems, which could
be key in interpreting the collective behaviours of some biological tissues,
cytoskeletal systems and collections of bacteria.

Soft Condensed Matter;Statistical Mechanics;Biological Physics