Forced Synchronization of Quantum System

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As shown in the plots of Figs. 6 and 7 of Ref. [1], atoms subjected to an external oscillating field tend to be pulled into the 1/2 order subharmonic resonance state when initially excited in one of states nearby the 1/2 subharmonic resonance. The authors of Ref. [1] have suggested that the motion of the Rydberg electron is phase locked to the applied microwave field. Later Maeda and Gallagher have demonstrated that a nondispersing Rydberg wave packet can be made by exposing a Rydberg atom to a weak microwave whose frequency nearly equals to the Kepler frequency of the Rydberg atom [2,3]. Orbital motion of the Rydberg wave packet was found to be phase locked to the microwave field, which remained for an extremely long time [2,3]. In the present study, we have measured state distribution of Rydberg Li atoms exposed to a linearly polarized 17.5-GHz microwave pulse in the regime where the microwave frequency is nearly equal to the classical Kepler frequency of n =72 Rydberg atoms (Kep=17.2 GHz), i.e., the nondispersing wave packets have been observed [2,3]. In case Li atoms are initially excited in the vicinity of the resonance state, i.e., n=72 Rydberg state in the present experiment, the measurements show that state population is in fact pulled into the resonance state at certain microwave field intensities. When excited in other states, on the other hand, final state distribution of the atoms after the microwave pulse irradiation is spread around the resonance state. The later case is well described by one-dimensional classical as well as quantum-mechanical Floquet theory, whereas there are significant differences between our observation of population of the state nearby n=72 moving into the resonance state and the calculated results based on the one-dimensional theory.