A future quantum network will allow distributing entanglement over in principle arbitrarily long distances. This topic holds importance for applications in quantum information science as well as for fundamental investigations and currently receives significant attention around the world. The most promising approach for achieving long-distance quantum communication is to use quantum repeaters, many of which employ optical quantum memories to store quantum information. Among the candidates for building such quantum memories, rare earth ion-doped crystals exhibit noteworthy promise. In this context, our research delves into the exploration of a thulium-doped yttrium gallium garnet crystal (Tm: YGG) at ultralow temperatures, as low as 500 mK. This crystal offers an optical coherence time exceeding one millisecond and a ground-state Zeeman-level lifetime as long as tens of seconds. We take advantage of such exceptional features to show a series of pivotal experiments. These include the storage of optical pulses for durations of up to 100 μs, frequency-multiplexed storage of multiple frequency modes, and a proof-of-principle demonstration showcasing the selective retrieval of stored frequency modes. Furthermore, I will also shed some light on the optical coherence and relaxation dynamics of Tm: YGG, which is crucial for the development of an efficient quantum memory. Our results suggest that Tm: YGG exhibits substantial potential as a multimode optical quantum memory within the framework of a frequency-multiplexed quantum repeater architecture.
