Fast qubits of optical frequencies on the rare-earth ions in fluoride crystals and color centers in diamond

Fluoride crystals doped with rare-earth ions (REI) and pair centers in diamond for fast (${10}^{-9}\mathrm{s}$) quantum computers (FQC) are proposed. As specific systems for REI doping, we propose $C{a}_{1-x}S{r}_x{F}_2$ crystals and their analogues. The ${}^{4}f$-states with different total orbital angular moments of these ions serve as two-level systems (qubits). Suitable REIs are proposed as well. It is established that the pair $SiV$ and $GeV$ centers in diamond are promising systems for fast optical quantum computers operating at elevated temperatures.