On Demand magnetic-Doppler nuclear frequency comb memory for hard X-ray photons

Abstract

Nuclear quantum memories in the hard X-ray regime offer some key advantages over their optical counterparts, such as broader bandwidth and lower background noise. A Doppler frequency comb protocol has been theoretically proposed [X. Zhang \textit{et al.}, Phys. Rev. Lett. \textbf{123}, 250504 (2019)] and recently demonstrated experimentally [S. Velten \textit{et al.}, Sci. Adv. \textbf{10}, eadn9825 (2024)] for the storage and retrieval of X-ray photons. However, achieving on-demand retrieval remains challenging because of the requirement for precise and synchronous mechanical motion of multiple absorbers. We propose a hybrid, magnetic-Doppler nuclear frequency comb composed of Doppler-shifted resonant absorbers with lifted nuclear spin degeneracy, which expands the Doppler comb structure. By synchronously reversing the directions of both the magnetic fields and absorber velocities, the system achieves time-reversed phase evolution dynamics that allows for efficient on-demand photon retrieval with significantly reduced mechanical complexity.