  By using trajectory-based approaches to quantum transition, it is found that
laser can agitate the probability flow in atoms to form alternating current
with the frequency of the laser. The detailed physical process of quantum
transition is investigated, during which the alternating current in atomic
probability flow becomes a key role connecting the external electromagnetic
wave with the evolution of the quantum states in atoms. Computer was employed
to simulate the physical process. The atomic alternating current may have the
peak-dip phenomenon.
