Multimode capacity of atomic-frequency comb quantum memories

Ensemble-based quantum memories are key to developing multiplexed quantum repeaters, able to overcome the intrinsic rate limitation imposed by finite communication times over long distances. Rare-earth ion doped crystals are main candidates for highly multimode quantum memories, where time, frequency and spatial multiplexing can be exploited to store multiple modes. In this context the atomic frequency comb (AFC) quantum memory provides large temporal multimode capacity, which can readily be combined with multiplexing in frequency and space. In this article, we derive theoretical formulas for quantifying the temporal multimode capacity of AFC-based memories, for both optical memories with fixed storage time and spin-wave memories with longer storage times and on-demand read out. The temporal multimode capacity is expressed in key memory parameters, such as AFC bandwidth, fixed-delay storage time, memory efficiency, and control field Rabi frequency. Current experiments in europium- and praseodymium-doped Y2SiO5 are analyzed within this theoretical framework, which is also tested with newly acquired data, as prospects for higher temporal capacity in these materials are considered. In addition we consider the possibility of spectral and spatial multiplexing to further increase the mode capacity, with examples given for praseodymium doped Y2SiO5.

Automated summary

Ensemble-based quantum memories are crucial for overcoming the intrinsic rate limitation in long-distance communication. Rare-earth ion doped crystals, with the ability to utilize time, frequency, and spatial multiplexing, are the main candidates for highly multimode quantum memories. This article focuses on atomic frequency comb (AFC) quantum memories, providing theoretical formulas to quantify their temporal multimode capacity. Experimental analysis and prospects for higher capacity in europium- and praseodymium-doped Y2SiO5 crystals are presented, as well as the potential for spectral and spatial multiplexing to further increase mode capacity.