Abstract: Overcoming the physical distance barrier in quantum communications requires quantum memories capable of storing photonic states without decoherence. This paper evaluates rare-earth-doped crystal (Eu3+:Y2SiO5) and warm alkali vapor memories engineered for satellite platforms.
Eu3+:Y2SiO5 crystals operating at 3.5 Kelvin achieve coherence times exceeding 6 hours when combined with zero-first-order-Zeeman (ZEFOZ) magnetic field tuning. We demonstrate an orbital cryocooler payload occupying less than 3U volume with 150 milliwatt cooling capacity at 4K.
For satellite platforms where cryogenic payloads are impractical, rubidium-87 vapor cells utilizing electromagnetically induced transparency (EIT) store single-photon wavepackets for over 1.2 milliseconds at room temperature, sufficient for low-earth-orbit cross-link hops.
Storage-and-retrieval fidelity exceeding 96.8% was demonstrated with polarization-encoded single photons, opening the path for deterministic quantum repeater chains in orbit that operate independently of instantaneous line-of-sight availability.