Towards satellite-suited noise-free quantum memories

The use of satellites as transmission links between the nodes of a quantum network could push the current distance limit for quantum key distribution QKD [1] . We realise a technologically simple, satellite-suited quantum memory in Caesium vapour, based on electromagnetically induced transparency (EIT) on the D1 line. Here, the Caesium atoms act as three-level Lambda systems, as depicted in the inset of Fig. 1a ). A signal/control laser configuration with orthogonal polarisations on the |g 〉- |e 〉 and the | s 〉- |e 〉 transitions respectively, is used in order to transfer the incoming signal pulse onto a spatially-distributed, long-lived, coherent superposition of the states | g 〉 and |s 〉, referred to as a spin wave [2] .