**Figure 7.** Non-fidelity 1 − *F* of the full transfer driven by Gaussian pulses (Δ*t* = 28 μs) plus weak coupling decay versus the laser half line-width at half-maximum Γ_{L}. Laser parameters are \Omega _B^0/2\pi =400 MHz and \Omega _R^0/2\pi =40 MHz and Δ_{B}/2π = 100 MHz, Ω_{C}/2π = 10 MHz, Δ_{C}/2π = 100 MHz and Δ_{R} = Δ_{B} − Δ_{C} − α_{C}Ω_{C}/2.

**Abstract**

A stimulated Raman adiabatic passage (STIRAP)-like scheme is proposed to exploit a three-photon resonance taking place in alkaline-earth-metal ions. This scheme is designed for state transfer between the two fine structure components of the metastable D-state which are two excited states that can serve as optical or THz qubit. The advantage of a coherent three-photon process compared to a two-photon STIRAP lies in the possibility of exact cancellation of the first-order Doppler shift which opens the way for an application to a sample composed of many ions. The transfer efficiency and its dependence with experimental parameters are analysed by numerical simulations. This efficiency is shown to reach a fidelity as high as (1–8 **×** 10^{−5}) with realistic parameters. The scheme is also extended to the synthesis of a linear combination of three stable or metastable states.