**Figure 8.** Time evolution of the population of the D_{3/2} (red dotted line), S_{1/2} (black dashed line) and D_{5/2} (dot-dashed blue line) states during an incomplete STIRAP process driven by Gaussian pulses Ω_{B}(*t*) and Ω_{R}(*t*) kept constant since *t* = 40 μs (see equation (6)). Laser parameters are τ = Δ*t* = 28 μs, Ω_{C}/2π = 50 MHz, Δ_{C}/2π = 10 MHz, \Omega _B^0/2\pi =400 MHz, Δ_{B}/2π = 100 MHz, \Omega _R^0/2\pi =40 MHz, \Delta _R=\Delta _B-\Delta _C(1+\sqrt{1+4\alpha _C^2})/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.