Fig. 1: Experimental setup.

a Energy level structure of ¹⁷¹Yb³⁺:Y₂SiO₅ at zero magnetic field for site II. The AFC quantum memory is created on transition (1) and transitions (2)-(4) were used for spin polarization into state \(|{4}_{g}\rangle\). b Conceptual experimental setup showing the main components of the SPDC source based on two periodically poled lithium niobate (PPLN) waveguides and the ¹⁷¹Yb³⁺:Y₂SiO₅ (Yb:YSO) quantum memory (QM), see main text and Methods for details. To increase absorption, the crystal is mounted in a quadruple-pass configuration, where the backward 979 nm output mode is separated from the input mode via an optical circulator based on a Faraday rotator and a polarization beam splitter. The signal and idler photons were filtered by a volume Bragg grating (VBG) and Fabry-Pérot (FP) cavity, and a ultra narrow fiber Bragg grating (UNFBG), respectively, and detected by superconducting nanowire single photon detectors (SNSPDs) D₁ and D₂. c The experimental sequence consists of a spin polarization step (200 ms) and an AFC preparation step (50 ms), as described in the main text. Many storage trials were made during 50 ms, where for each trial the SPDC source was pumped for a duration of 1/Δ, followed by detection window of the same duration.