The best candidates for necessary conditions of chaos (stretching, folding, exponential divergence) are absent from the quantum domain due to linearity

Before looking at these questions, there is the thorny problem of defining quantum chaos. The difficulties in establishing an agreed definition of quantum chaos are actually more challenging than for classical chaos (§1). Recall that there were several subtleties involved in attempting to arrive at a consensus definition of classical chaos. One important proposal for a necessary condition is the presence of some form of stretching and folding mechanism associated with a nonlinearity in the syste