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.