We argue that long optical storage times are required to establish entanglement at high rates over large distances using memory-based quantum repeaters. Triggered by this conclusion, we investigate the 795.325 nm 3 H 6 ↔ 3 H 4 transition of Tm∶Y 3 Ga 5 O 12 (Tm:YGG). Most importantly, we find that the optical coherence time can reach 1.1 ms, and, using laser pulses, we demonstrate optical storage based on the atomic frequency comb protocol during up to 100 μs as well as a memory decay time T m of 13.1 μs. Possibilities of how to narrow the gap between the measured value of T m and its maximum of 275 μs are discussed. In addition, we demonstrate multiplexed storage, including with feed-forward selection, shifting and filtering of spectral modes, as well as quantum state storage using members of nonclassical photon pairs. Our results show the potential of Tm:YGG for creating multiplexed quantum memories with long optical storage times, and open the path to repeater-based quantum networks with high entanglement distribution rates.

operating at different wavelengths through the exchange of photons. However, so far, only a few investigations have been reported [16-18], and none of them has included a quantum memory functioning at telecommunication wavelength. In this work, we demonstrate entanglement between two atomic frequency comb (AFC)-based quantum memories [19] in ensembles of cryogenically cooled rare-earth ions, one for 794-nmand one for 1535-nm-wavelength photons. The first memory employs a thulium-doped lithium-niobate (Tm 3+ :LiNbO 3) crystal, the second an erbium-doped fiber (Er 3+ :SiO 2). Entanglement is created through the interaction with entangled photons created by spontaneous parametric down-conversion. Both memories allow buffering and re-emitting multiplexed quantum data in feed-forward-controlled spectral or temporal modes, either of which makes them suitable for quantum repeaters [20,21]. It is significant that our experiment involves two classes of ions: Kramers and non-Kramers. Due to their specific electronic configurations, Kramers ions are strongly coupled to the local magnetic environment while non-Kramers ions are relatively immune to magnetic-field fluctuations. These characteristics make Kramers ions strong candidates for quantum sensors while non-Kramers are generally more suitable for quantum memory with long storage time.