Quantum versus classical chirps in a Rydberg atom

The interplay between quantum-mechanical and classical evolutions in a chirped-driven Rydberg atom is discussed. It is shown that the system allows two continuing resonant excitation mechanisms, i.e., a successive two-level transitions (ladder climbing) and a continuing classicallike nonlinear phase locking (autoresonance). The persistent 1:1 and 2:1 resonances between the driving and the Keplerian frequencies are studied in detail and characterized in terms of two dimensionless parameters P1,2 representing the driving strength and the nonlinearity in the problem, respectively. The quantum-mechanical rotating wave and the classical single-resonance approximations are used to describe the regimes of efficient classical or quantum-mechanical excitation in this two-parameter space.