Creation of coherent atomic superpositions by fractional stimulated Raman adiabatic passage

We discuss the properties of a simple and robust scheme for preparing atoms and molecules in an arbitrary preselected coherent superposition of quantum states-fractional stimulated Raman adiabatic passage (fractional STIRAP)-first proposed by Marte el al (1991 Phys. Rev. A 44 R4118). As in STIRAP, the Stokes pulse arrives before the pump purse, but unlike STIRAP, here the two pulses terminate simultaneously, while maintaining a constant ratio of amplitudes. We extend earlier research and suggest a realization of this scheme with two smoothly varying delayed laser pulses (which can be derived from a single laser), the parameters of the created superposition being controlled by the polarization of the delayed pulse. Furthermore, we provide simple analytic estimates of the robustness of this process against Variations in the laser intensity, laser frequency and pulse delay. Finally, we discuss an extension to multistate systems which provides a possibility for creating coherent superpositions of more than two states.