Manipulating the singlet-triplet transition in ion strings by nonresonant dynamic Stark effect

Using strong laser pulses, we show that it is possible to control the spin state in a model system based on a two-electron extension with spin couplings of the Shin-Metiu Hamiltonian, truncated to account for the lowest electronic energy states. We consider two different models depending on the number of electronic states included in the calculation. The initial electronic state determines when the spin state is stable or not in the absence of an external field. In the latter case, by non-resonant dynamic Stark effect, we show that it is possible to avoid spin transitions with strong fields, using different pulse frequencies. This effective spin locking requires minimizing absorption to excited singlets as well as decoupling the singlet and triplet electronic states. In the first case, we show that it is possible to force the spin transition by a combination of two pulses, a chirped pulse and a transform limited pulse, where the time-delay must be chosen to maximize spin switching on a different electronic state. Our results show that forcing the spin switching is a more difficult goal than avoiding it and that this goal becomes highly restricted when many electronic pathways or multi-photon processes are available.