Quantum Interference of Photon Pairs from Two Trapped Atomic Ions
  We collect the fluorescence from two trapped atomic ions, and measure quantum interference between photons emitted from the ions. The interference of two photons is a crucial component of schemes to entangle atomic qubits based on a photonic coupling. The ability to preserve the generated entanglement and to repeat the experiment with the same ions is necessary to implement entangling quantum gates between atomic qubits, and allows the implementation of protocols to efficiently scale to larger numbers of atomic qubits. 
P. Maunz, D. L. Moehring, M. J. Madsen, R. N. Kohn Jr., K. C. Younge,   C. Monroe