Even if the parameters determining a system's state are varied smoothly, the
behavior of quantum correlations alike to quantum discord, and of its classical
counterparts, can be very peculiar, with the appearance of non-analyticities in
its rate of change. Here we review this sudden change phenomenon (SCP)
discussing some important points related to it: Its uncovering,
interpretations, and experimental verifications, its use in the context of the
emergence of the pointer basis in a quantum measurement process, its appearance
and universality under Markovian and non-Markovian dynamics, its theoretical
and experimental investigation in some other physical scenarios, and the
related phenomenon of double sudden change of trace distance discord. Several
open questions are identified, and we envisage that in answering them we will
gain significant further insight about the relation between the SCP and the
symmetry-geometric aspects of the quantum state space.Comment: Lectures on General Quantum Correlations and their Applications, F.
F. Fanchini, D. O. Soares Pinto, and G. Adesso (Eds.), Springer (2017), pp
309-33