Quantum interference of photon pairs from two remote trapped atomic ions

Trapped atomic ions are among the most attractive implementations of quantum bits for applications in quantum-information processing, owing to their long trapping lifetimes and long coherence times. Although nearby trapped ions can be entangled through their Coulomb-coupled motion, it seems more natural to entangle remotely located ions through a coupling mediated by photons, eliminating the need to control the ion motion. A promising way to entangle ions via a photonic channel is to interfere two photons emitted from the ions and then detect appropriate photon coincidence events. Here, we report the pivotal element of this scheme in the observation of quantum interference between pairs of single photons emitted from two atomic ions residing in independent traps.