From loophole-free violation of Bell's inequality toward device-independent quantum communication protocols

Summary form only given. Here we present our experiment using heralded entanglement between two 87Rb-atoms separated by 400 m violating Bell's inequality [1]. First each atom is entangled with a photon, which is coupled into single-mode fiber. The interference of the two photons at a beam splitter enables one to project the state onto a Bell state, which leaves the atoms in an entangled state [2]. For space-like separation, the state measurements on the two atoms together with the random choice of the measurement basis, have to be performed in a time shorter than 1.33μs. The atomic state readout itself is based on a highly efficient state-selective ionization and subsequent detection of the ionization fragments by channel-electron-multipliers (CEM) [3]. Altogether the time for the state readout is below 1μs and the locality condition is fulfilled. Evaluation of 5000 events for each of two entangled atom-atom states shows a significant violation of Bell's inequality with S-parameters of 2.240 ± 0.047 and 2.204 ± 0.047.