Efficient solid state memories for quantum cryptography

Long distance quantum cryptography requires quantum repeaters which use quantum memories. The latter are designed to store and retrieve photon quantum states on demand. Although quantum memories have been demonstrated in atomic vapors and ultra cold gases, a solid state alternative may better fulfill quantum memories requirements. Rare earth based crystals, which exhibit long coherence lifetimes, are actively studied for this purpose. Memory efficiency, i.e. the probability to retrieve a photon after storage, should be close to unity for practical applications. This can be achieved in highly doped crystals. Although Pr-Pr interactions could be detrimental in this case, we show that in a 3% Pr3+ doped La2(WO4)3 crystal ground state hyperfine coherence lifetime is still close to that measured at low Pr concentration. Since the latter determines the memory storage time, this result suggests that highly doped crystals may be useful for efficient quantum memories.