  Rare-earth ions in dielectric crystals are interesting candidates for storing
quantum states of photons. A limiting factor on the optical density and thus
the conversion efficiency is the distortion introduced in the crystal by doping
elements of one type into a crystal matrix of another type. Here, we
investigate the system Pr3+:La2(WO4)3, where the similarity of the ionic radii
of Pr and La minimizes distortions due to doping. We characterize the
praseodymium hyperfine interaction of the ground state (3H4) and one excited
state (1D2) and determine the spin Hamiltonian parameters by numerical analysis
of Raman-heterodyne spectra, which were collected for a range of static
external magnetic field strengths and orientations. On the basis of a crystal
field analysis, we discuss the physical origin of the experimentally determined
quadrupole and Zeeman tensor characteristics. We show the potential for quantum
memory applications by measuring the spin coherence lifetime in a magnetic
field that is chosen such that additional magnetic fields do not shift the
transition frequency in first order. Experimental results demonstrate a spin
coherence lifetime of 158 ms - almost three orders of magnitude longer than in
zero field.
