  We briefly discuss four different possible types of transitions from quark to
hadronic matter and their characteristic signatures in terms of correlations.
We also highlight the effects arising from mass modification of hadrons in hot
and dense hadronic matter, as well as their quantum statistical consequences:
the appearance of squeezed quantum states and the associated experimental
signatures, i.e., the back-to-back correlations of particle - anti-particle
pairs. We briefly review the theoretical results of these squeezed quanta,
generated by in-medium modified masses, starting from the first indication of
the existence of surprising particle - anti-particle correlations, and ending
by considering the effects of chiral dynamics on these correlation patterns. A
prerequisite for such a signature is the experimental verification that these
theoretically predicted back-to-back correlation of particle anti-particle
pairs are, in fact, observable in high energy heavy ion reactions. Therefore,
the experimental observation of back-to-back correlations in high energy heavy
ion reactions would be a unique signature, proving the existence of in-medium
mass modification of hadronic states. On the other hand, their disappearance at
some threshold centrality or collision energy would indicate that the hadron
formation mechanism would have qualitatively changed: asymptotic hadrons above
such a threshold are not formed from medium modified hadrons anymore, but
rather by new degrees of freedom characterizing the medium. Furthermore, the
disappearance of the squeezed BBC could also serve as a signature of a sudden,
non-equilibrium hadronization scenario from a supercooled quark-gluon plasma
phase.
