Spontaneous focusing as an emergent phenomenon
  We analyze the emergence of diffractive focusing in the transition from discrete to continuous space-time variables. Three types of dynamical equations are studied in a top-to-bottom approach, starting with the most general system. First we solve a linear cellular automaton of two species, then a nearest neighbour tight-binding array and finally the time-dependent Schr\"odinger equation. All models are shown to produce diffractive solutions for square packet distributions. The main result of this paper is that in discrete variables, the nature of the solutions depends strongly on the size of wavepackets, whereas in continuous variables, diffraction due to discontinuities exists at every scale. A transition in the number of participants or cells is identified by means of a measure and the corresponding phenomenon is further analyzed by a generalization of Wigner functions to discrete variables and crystals. 
E. Sadurn\'i
quant-ph cond-mat.other