Gigahertz-bandwidth optical memory in Pr³⁺:Y₂SiO₅

We experimentally study a broadband implementation of the atomic frequency comb (AFC) rephasing protocol with a cryogenically cooled Pr³⁺:Y₂SiO₅ crystal. To allow for storage of broadband pulses, we explore a novel, to the best of our knowledge, regime where the input photonic bandwidth closely matches the inhomogeneous broadening of the material (∼5GHz), thereby significantly exceeding the hyperfine ground and excited state splitting (∼10MHz). Through an investigation of different AFC preparation parameters, we measure a maximum efficiency of 10% after a rephasing time of 12.5 ns. With a suboptimal AFC, we witness up to 12 rephased temporal modes.