Quantum interference from remotely trapped ions
  We observe quantum interference of photons emitted by two continuously laser-excited single ions, independently trapped in distinct vacuum vessels. High contrast two-photon interference is observed in two experiments with different ion species, calcium and barium. Our experimental findings are quantitatively reproduced by Bloch equation calculations. In particular, we show that the coherence of the individual resonance fluorescence light field is determined from the observed interference. 
S. Gerber, D. Rotter, M. Hennrich, R. Blatt, F. Rohde, C. Schuck, M.   Almendros, R. Gehr, F. Dubin, J. Eschner