Remote Entanglement between a Single Atom and a Bose-Einstein Condensate

Entanglement between stationary systems at remote locations is a key resource for quantum networks. Here we report on an experiment where entanglement is established between a single atom inside a cavity and a Bose-Einstein condensate (BEC) located in two different laboratories. To achieve this, a single photon is generated from the atom-cavity system, such that the polarisation qubit of the photon is entangled with the atomic spin state. The photon is transported through a fibre to the BEC and converted into a collective excitation by a Raman process based on electromagnetically induced transparency. After a variable delay, the produced matter-matter entanglement is converted into photon-photon entanglement by creating two single photons, one from each atomic system. The observed overall fidelity of 95\% and the matter-matter entanglement lifetime of 100 $\mu$s demonstrates the excellent performance of this novel hybrid system.