Heralded Single-Magnon Quantum Memory for Photon Polarization States

We demonstrate a heralded quantum memory where a photon announces the mapping of a light polarization state onto a single collective-spin excitation (magnon) shared between two atomic ensembles. The magnon can be converted at a later time into a single polarized photon with polarization fidelity over 90(2)% for all fiducial input states, well above the classical limit of (2/3). The process can be viewed as a nondestructive quantum probe where a photon is detected, stored, and regenerated without touching its - potentially undetermined - polarization.