Introduction to Rabi oscillations
Suppose, someone is focusing a laser on an atom. If the electromagnetic (EM) wave is resonant with the atoms transition frequency, it brings the atom from the ground state to its first excited state, i.e. it excites the atom. However, if the atom is in its excited state, another photon of the electromagnetic wave is passing by. Now, the atom de-excites and emits a photon with the same phase and frequency as the EM wave. This is called ‘stimulated emission’. 5,6
The probability of finding the atom in the excited state increases over time. This means that at some point in time the probability of finding the atom in its excited state is one. For as long as this probability is one, you would expect that the atom stays in this state. Nevertheless, the atom is still emitting and absorbing photons through stimulated emission. This process of emitting and re-absorbing is called Rabi oscillations.7,8 The Rabi frequency is the rate at which the oscillations occur and is proportional to the strength of the EM field. During the cycle the atom is usually in a superposition of ground and excited state, which gives a ‘distorted’ wave function.
Rabi Oscillations can be described by two different models. The first is the semi-classical Rabi model. This model assumes a quantized atom and a classical description of the field. The second model is the Jaynes-Cummings model. The main difference between this model and the Rabi model is the assumption of a quantized in stead of a classical description of the field. This fully quantum-mechanical model describes the vacuum-field Rabi oscillations, which are presented in the article of Yoshie et al.