Classical phase locking in adiabatic rapid passage

When a Rydberg atom is slowly swept through a dipole allowed resonance with an oscillating field adiabatic rapid passage (ARP) from one state to the other can occur. By measuring the time resolved momentum of a Rydberg electron with a half cycle pulse we demonstrate that during ARP the classical oscillating dipole due to the orbital motion of the electron becomes phase locked to the microwave field driving the resonance. Depending on the direction of the sweep through resonance, the atom is in one or the other of the two Floquet eigenstates in which the electron is phase locked in or $\pi$ out of phase with the driving field. In contrast, in Rabi oscillations the electron motion oscillates between leading and lagging the driving microwave field.