  Stunning progresses in the experimental resolution and control of natural or
man-made complex systems at the level of their quantum mechanical constituents
raises the question, across diverse subdisciplines of physics, chemistry and
biology, whether that fundamental quantum nature may condition the dynamical
and functional system properties on mesoscopic if not macroscopic scales. But
which are the distinctive signatures of quantum properties in complex systems,
notably when modulated by environmental stochasticity and dynamical
instabilities? It appears that, to settle this question across the above
communities, a shared understanding is needed of the central feature of quantum
mechanics: wave-particle duality. In this Perspective, we elaborate how
randomness induced by this very quantum property can be discerned from the
stochasticity ubiquitous in complex systems already on the classical level. We
argue that in the study of increasingly complex systems such distinction
requires the analysis of single incidents of quantum dynamical processes.
