Quantum maps

We quantize area-preserving maps M of the phase plane q, p by devising a unitary operator U transforming states | φ_n〉 into | φ_n+1〉. The result is a system with one degree of freedom q on which to study the quantum implications of generic classical motion, including stochasticity. We derive exact expressions for the equation iterating wavefunctions ψ_n(q), the propagator for Wigner functions W_n(q,p), the eigenstates of the discrete analog of the quantum harmonic oscillator, and general complex Gaussian wave packets iterated by a U derived from a linear M. For | ψ_n〉 associated with curves L_n in q, p, we derive a semiclassical theory for evolving states and stationary states, analogous to the familiar WKB method. This theory breaks down when L_n gets so complicated as to develop convolutions of area h{stroke} or smaller. Such complication is generic; its principal morphotologies are"whorls" and "tendrils," associated respectively with elliptic and hyperbolic fixed points of M. Under U, ψ_n(q) eventually transforms into a new sort of wave that no longer perceives the details of L_n. For all regimes, however, the smoothed | ψ_n(q)|² appears semiclassically appears to be given accurately by the smoothed projection of L_n onto the q axis, both smoothings being over a de Broglie wavelength. The classical, quantum, and semiclassical theory is illustrated by computations on the discrete quartic oscillator map. We display for the first time stochastic wavefunctions, dominated by dense clusters of caustics and characterized by multiple scales of oscillation.