Dynamical Crossing of an Infinitely Degenerate Critical Point

Abstract

We study the evolution of a driven harmonic oscillator with a time-dependent frequency $\omega _t \propto |t|$. At time $t=0$, the Hamiltonian undergoes a point of infinite spectral degeneracy. If the system is initialized in the instantaneous vacuum in the distant past, then the asymptotic future state is a squeezed state, whose parameters are explicitly determined. We show that the squeezing is independent on the sweeping rate. This manifests the failure of the adiabatic approximation at points, where infinitely many eigenvalues collide. We extend our analysis to the situation, where the gap at $t=0$ remains finite. We also discuss the natural geometry of the manifold of squeezed states. We show that it is realized by the Poincaré disk model viewed as a Kähler manifold.