A general two-level systems consists of two states with energies E_1 and E_2 coupled by a matrix element W. For an energy difference Delta = E_1 - E_2 large compared to |W|, the eigenvectors and energies are approximately the same as they were without any coupling; however, for a degenerate system, E_1 = E_2, the coupling produces a gap with one eigenvector having energy -|W| (bonding) and the other having energy +|W| (antibonding).

If at t=0 the wave function starts out in one of the uncoupled states, say phi_1, them due to the coupling, W, at a later time there will be a probability that one will be in the other state, phi_2. The probability for being in state phi_2 oscillates periodically in time (Rabi oscillations).