An ultrafast pulse shaping scheme is presented that transiently modifies a two-state superposition on the $E{}^1{\Sigma }_g^{+}$ curve of the lithium dimer. At short time delays, the wave-function amplitude of one of the states is forced to undergo a sign change, while the sign of the second state is programmed to remain static, analogous to the operation of a quantum-computational Z gate. This is observed as a π phase shift in the time-dependent wave-packet signal for pump-probe delays <1.5 ps, relative to longer time delays, greater than 2.5 ps. This shift in sign is accomplished by taking advantage of the separability of the resonant and nonresonant light field effects in the creation of the excited-state wave function. The results show that, for a single state, the resonant and nonresonant light field effects can either be added or subtracted to create the total time evolution of the excited-state wave-function coefficient. If the nonresonant contributions are subtracted from the resonant ones at a time delay when the nonresonant term dominates, then for a short time the excited-state coefficient will have a sign opposite to that at long time, where the resonant term dominates. A desired phase function is derived to produce the opposition in sign of resonant versus nonresonant contributions, and the experimental transient Z-gate matrix elements are quantified.

• Received 21 May 2003

DOI:https://doi.org/10.1103/PhysRevA.68.043409