Quantum communication holds the promise of provably secure transmission of information. One of the key components required for quantum communication is a non-classical light source that is compatible with a quantum memory. The protocol of rephased amplified spontaneous emission (RASE) provides an elegant solution in which entangled light is generated in a rare-earth ion-doped crystal with an in-built quantum memory.

Spontaneous emission from an ensemble of excited state atoms generates entanglement between the emitted optical field and the collective state of the ensemble. The RASE scheme enables the collective atomic state to be read out as a second optical field, which is entangled with the initial spontaneous emission. Here we demonstrate that the entanglement between the two optical fields is preserved after the collective atomic state is stored on the long-lived spin states of a Pr³⁺:Y₂SiO₅ crystal. In addition, we show that temporally multimode entanglement can be generated and stored, with high distinguishability between the modes.

The ability to store and generate entangled light in a single protocol paves the way to making scalable, solid state quantum information processing architectures.

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