In physics, the Rabi cycle is the cyclic behaviour of a twostate quantum system in the presence of an oscillatory driving field. A twostate system has two possible states, and if they are not degenerate energy levels the system can become "excited" when it absorbs a quantum of energy. The effect is important in quantum optics, nuclear magnetic resonance and quantum computing. The term is named in honour of Isidor Isaac Rabi. When an atom (or some other twolevel system) is illuminated by a coherent beam of photons, it will cyclically absorb photons and reemit them by stimulated emission. One such cycle is called a Rabi cycle and the inverse of its duration the Rabi frequency of the photon beam. This mechanism is fundamental to quantum optics. It can be modelled using the JaynesCummings model and the Bloch vector formalism. For example, in a twostate atom (an atom in which an electron can either be in the excited or ground state), the probability of finding the atom in the excited state is found from the Bloch equations to be:  cb(t)  2 = cos(ωt)2 where ω is the Rabi frequency. More generally, one can consider a system where the two levels under consideration are not energy eigenstates. Therefore if the system is initialized in one of these levels, time evolution will make the population of each of the levels oscillate with some characteristic frequency, whose angular frequency[1] is also known as the Rabi frequency. Links * http://www.itp.tuberlin.de/en/menue/lehre/owl/quantenmechanik/zweiniveau/?H=1 A Java applet that visualizes Rabi Cycles of twostate systems (laser driven). * http://www.itp.tuberlin.de/index.php?id=6854&L=1 extended version of the applet. Includes electron phonon interaction. Notes See also * Atomic coherence * Bloch sphere * Laser pumping * Optical pumping * Rabi problem * Vacuum Rabi oscillation Retrieved from "http://en.wikipedia.org/"

