  Preparation of a non-classically correlated state is the first step of any
quantum-enhanced interferometric protocol. An efficient method is the one-axis
twisting, which entangles a collection of initially uncorrelated particles by
means of two-body interactions. Here we investigate the limits of the quantum
improvement which can be reached with this method in realistic experimental
conditions. We demonstrate that the usefulness of this entangling mechanism is
a result of fine structures introduced into the quantum state. The scale at
which these structures vary allows us to identify the minimal requirements for
the precision of the complete interferometric sequence. Our
results---especially the explanation of the underlying principle of the
entangling method---may help to develop ultra-precise interferometers.
