Rare-earth doped solids are known for their luminescence properties. Among them erbium takes a special place because the Erbium Doped Fiber Amplifier has revolutionized the telecommunications by giving access to long distance communication. The transposition of this scheme to quantum cryptography is appealing. The core element of the so-called quantum repeater is an optical memory [1] for which direct operation at 1.5mm is desirable. We use an erbium doped crystal in the C-band of telecom. I’ll essentially focus on the material properties: how they impact the memory performances and how they can be controlled in this prospect.

I’ll first introduce the general properties of rare-earth ions inserted in optical crystals. The goal of theses opening remarks is essentially pedagogical. To pay honour to whom honour is due, I’ll give also points of comparison with other atomic systems as atomic vapours (hot or cold), trapped ions or coloured centres in diamond.

I’ll briefly review the recent work that we did on an erbium doped yttrium orthosilicate sample (Er3+:Y2SiO5) by applying an original protocol named Revival of Silenced Echo (ROSE) [2]. These later is quite efficient in Er3+:Y2SiO5 [2] as compared to other protocols. I’ll finally show that these performances are limited by the erbium-erbium electron spin interaction [3].

Although we work on the optical transition, the spin properties are absolutely critical for the coherence time governing the memory storage time. As an illustration, I’ll present a recent study of an Er3+:Y2SiO5 crystal in which we added a controlled level of disorder with scandium as co-dopant [4]. This perturbation can surprisingly increase the coherence time at low magnetic field. This counter intuitive result is due to the reduction of Er-Er spin flip-flop rate because the disorder effectively slows down the flip-flop mechanism by making the magnetic interaction non resonant.