Metal ions 2 Rare earths 2 Rare-earth-ion doped crystals rare-earth ion doped crystals 2 Atomic frequency 1 Atomic frequency comb 1 Atoms 1 Crystals 1 Fabry-Perot interferometers 1 Frequency combs 1 Frequency reference 1 Frequency sensitivity 1 Frequency stability 1 Laser frequency stabilization 1 Light effects 1 Light frequency 1 Multimode quantum memory 1 Multimodes 1 Noise abatement 1 Optical cavities 1 Proof of concept 1 Quantum memory 1 Quantum repeaters 1 Silicon compounds 1 Slow light 1 Slow-light effect 1 Spatial multiplexing 1 Spin waves 1 State of the art 1 Storage time 1