Multiplicative semiclassical dynamics and the quantization time
  We study smooth, caustic-free, chaotic semiclassical dynamics on two-dimensional phase space and find that the dynamics can be approached by an iterative procedure which constructs an approximation to the exact long-time semiclassical propagator. Semiclassical propagation all the way to the Heisenberg time, where individual eigenstates are resolved, can be computed in polynomial time, obviating the need to sum over an exponentially large number of classical paths. At long times, the dynamics becomes quantum-like, given by a matrix of the same dimension as the quantum propagator. This matrix, however, differs both from the quantum and the one-step semiclassical propagators, allowing for the study of the breakdown of the semiclassical approximation. The results shed light on the accuracy of the Gutzwiller trace formula in two dimensions, and on the source of long-time periodic orbit correlations. 
L. Kaplan