Time ordering and time correlation in atomic collisions and in quibts

Time ordering may be defined by first defining the limit of no time ordering in terems of the time average of the external interaction, $V(t)$. Time correlation is defined in terms of a similar limit called the independent time approximation. Several cases are discussed including weakly perturbed atomic collisions and strongly perturbed qubits. Experimental evidence for time correlation has not yet been distinguished from evidence for time ordering.