How Time Works in Quantum Systems: Overview of time ordering and time   correlation in weakly perturbed atomic collisions and in strongly perturbed   qubits
  Time ordering may be defined by first defining the limit of no time ordering (NTO) in terms of a time average of an external interaction, V(t). Previously, time correlation was defined in terms of a similar limit called the independent time approximation (ITA). Experimental evidence for time correlation has not yet been distinguished from experimental evidence for time ordering. 
J. H. McGuire, L. Kaplan, Kh. Kh. Shakov, A. Chalastaras, A. M. Smith,   A. Godunov, H. Schmidt-B\"ocking and D. Uskov