  Coherence and correlations represent two related properties of a compound
system. The system can be, for instance, the polarization of a photon, which
forms part of a polarization-entangled two-photon state, or the spatial shape
of a coherent beam, where each spatial mode bears different polarizations.
Whereas a local unitary transformation of the system does not affect its
coherence, global unitary transformations modifying both the system and its
surroundings can enhance its coherence, transforming mutual correlations into
coherence. The question naturally arises of what is the best measure that
quantifies the correlations that can be turned into coherence, and how much
coherence can be extracted. We answer both questions, and illustrate its
application for some typical simple systems, with the aim at illuminating the
general concept of enhancing coherence by modifying correlations.
