  As a direct consequence of the no-cloning theorem, the deterministic
amplification as in classical communication is impossible for quantum states.
This calls for more advanced techniques in a future global quantum network,
e.g. for cloud quantum computing. A unique solution is the teleportation of an
entangled state, i.e. entanglement swapping, representing the central resource
to relay entanglement between distant nodes. Together with entanglement
purification and a quantum memory it constitutes a so-called quantum repeater.
Since the aforementioned building blocks have been individually demonstrated in
laboratory setups only, the applicability of the required technology in
real-world scenarios remained to be proven. Here we present a free-space
entanglement-swapping experiment between the Canary Islands of La Palma and
Tenerife, verifying the presence of quantum entanglement between two previously
independent photons separated by 143 km. We obtained an expectation value for
the entanglement-witness operator, more than 6 standard deviations beyond the
classical limit. By consecutive generation of the two required photon pairs and
space-like separation of the relevant measurement events, we also showed the
feasibility of the swapping protocol in a long-distance scenario, where the
independence of the nodes is highly demanded. Since our results already allow
for efficient implementation of entanglement purification, we anticipate our
assay to lay the ground for a fully-fledged quantum repeater over a realistic
high-loss and even turbulent quantum channel.
