ABSTRACT
C oherence arises when amplitudes add with a definite relative phase and one observes a physical quantity proportional to the square of the
total amplitude. A familiar example is the addition of E fields to produce an interference pattern in classical optics. In quantum optics a coherent (linear) superposition of states with definite numbers of quanta is required to produce a classical oscillating E field in one mode of the quantized field. Coherence is also responsible for a number of interesting interference effects present in scattering of atoms and molecules. In this case it is the quantum mechanical amplitudes associated with different “trajectories” that scatter to the same angle which add to give interference in the probability distribution of the scattered particles.
