  Quantum information transfer from light to atom ensembles and vice versa has
both basic and practical importance. Among the relevant topics let us mention
entanglement and decoherence of macroscopic systems, together with applications
to quantum memory for long distance quantum cryptography. Although the first
experimental demonstrations have been performed in atomic vapors and clouds,
rare earth ion doped crystals are also interesting media for such processes. In
this paper we address Tm3+ ions capability to behave as three-level lambda
systems, a key ingredient to convert optical excitation into a spontaneous-
emission-free spin wave. Indeed Tm3+ falls within reach of light sources that
can be stabilized easily to the required degree. In the absence of zero-field
hyperfine structure we apply an external magnetic field to lift the nuclear
spin degeneracy in Tm3+:YAG. We experimentally determine the gyromagnetic
tensor components with the help of spectral hole-burning techniques. Then
appropriate orientation of the applied field enables us to optimize the
transition probability ratio along the two legs of the lambda. The resulting
three-level lambda system should suit quantum information processing
requirements.
