Interference of Multimode Photon Echoes Generated in Spatially Separated Solid-State Atomic Ensembles

High-visibility interference of photon echoes generated in spatially separated solid-state atomic ensembles is demonstrated. The solid-state ensembles were ${\mathrm{LiNbO}}_{3}$ waveguides doped with erbium ions absorbing at $1.53\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Bright coherent states of light in several temporal modes (up to 3) are stored and retrieved from the optical memories using two-pulse photon echoes. The stored and retrieved optical pulses, when combined at a beam splitter, show almost perfect interference, which demonstrates both phase preserving storage and indistinguishability of photon echoes from separate optical memories. By measuring interference fringes for different storage times, we also show explicitly that the visibility is not limited by atomic decoherence. These results are relevant for novel quantum-repeater architectures with photon-echo based multimode quantum memories.