Mapping photonic entanglement into and out of a quantum memory

Recent developments of quantum information science critically rely on entanglement, an intriguing aspect of quantum mechanics where parts of a composite system can exhibit correlations stronger than any classical counterpart. In particular, scalable quantum networks require capabilities to create, store, and distribute entanglement among distant matter nodes via photonic channels. Atomic ensembles can play the role of such nodes. So far, in the photon counting regime, heralded entanglement between atomic ensembles has been successfully demonstrated via probabilistic protocols. However, an inherent drawback of this approach is the compromise between the amount of entanglement and its preparation probability, leading intrinsically to low count rate for high entanglement. Here we report a protocol where entanglement between two atomic ensembles is created by coherent mapping of an entangled state of light. By splitting a single-photon and subsequent state transfer, we separate the generation of entanglement and its storage. After a programmable delay, the stored entanglement is mapped back into photonic modes with overall efficiency of 17 %. Improvements of single-photon sources together with our protocol will enable "on demand" entanglement of atomic ensembles, a powerful resource for quantum networking.Comment: 7 pages, and 3 figure

Slow, Stored and Stationary Light

Slow light has received growing interest since 1999 when the propagation velocity of light was reduced in an experiment to 17 m/s, i.e. almost 20 million times slower than in vacuum. Two years later light pulses were stopped, or more specifically stored in an atomic medium and subsequently released after some time. This provided the basis for important applications in photon-based quantum information technology. The present chapter explains what slow light is and what it is good for, how to understand the physics of it and how one can practically make light go so slow. To answer these questions, the chapter uses simple pictures, on the one hand, and supplements them with a little bit of details, on the other hand, for those who want to go slightly deeper into the field. The chapter also discusses more recent generalizations of slow light, such as stationary and spinor slow light which are interesting model system and can be used to understand more complex quantum systems