Quantum storage in rare-earth-doped crystals for secure networks

Quantum storage of photons in an atomic ensemble can be obtained by using three-level Λ systems. In these systems, two levels are coupled by optical transitions to a third one. Rare-earth ion-doped crystals are attractive materials for quantum storage because their hyperfine levels can have coherence lifetimes longer than 100 μs and thus can be used to build Λ systems. Tm3+ ions are especially interesting since they can be excited by ultra-stable laser diodes. The influence of an external magnetic field has been studied in order to obtain an efficient three-level Λ system with the hyperfine levels of the rare earth. The particular case of the Tm3+ ion in the Y3Al5O12 host is discussed.