Time Ordering in Kicked Qubits
  We examine time ordering effects in strongly, suddenly perturbed two-state quantum systems (kicked qubits) by comparing results with time ordering to results without time ordering. Simple analytic expressions are given for state occupation amplitudes and probabilities for singly and multiply kicked qubits. We investigate the limit of no time ordering, which can differ in different representations. 
L. Kaplan, Kh. Kh. Shakov, A. Chalastaras, M. Maggio, A. L. Burin, and   J. H. McGuire