Multimode capacity of atomic-frequency comb quantum memories

TL;DR

This paper develops theoretical formulas to quantify the temporal multimode capacity of atomic frequency comb quantum memories, analyzing current experiments and exploring prospects for increased capacity through spectral and spatial multiplexing.

Contribution

It provides a theoretical framework for assessing the multimode capacity of AFC quantum memories, including fixed and on-demand storage, and discusses potential enhancements via multiplexing.

Findings

Derived formulas linking multimode capacity to key memory parameters

Analyzed current europium and praseodymium experiments within the framework

Explored potential for increased capacity through spectral and spatial multiplexing

Abstract

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 Y$_2$SiO$_5$ are analyzed within this theoretical framework, and 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 both rare earh ions.