Distributing entanglement and single photons through an intra-city, free-space quantum channel

Entangled photons are distributed directly through the atmosphere to a receiver station 7.8 km away at night over the city center of Vienna, Austria. Without a time-stable connection, the two stations found coincidence counts in the detection events through the cross-correlation of locally-recorded time stamps shared over a public Internet channel. As such, the sending and receiver stations were completely independent. The polarization correlations contained in the measured time tags are sufficient to convincingly violate a CHSH-Bell inequality and demonstrate entanglement between the two city buildings.