  Advances in the distribution of quantum information will likely require
entanglement shared across a hybrid quantum network. Many entanglement
protocols require the generation of indistinguishable photons between the
various nodes of the network. This is challenging in a hybrid environment due
to typically large differences in the spectral and temporal characteristics of
single photons generated in different systems. Here we show, for the first
time, quantum interference between photons generated from a single atomic ion
and an atomic ensemble, located in different buildings and linked via optical
fibre. Trapped ions are leading candidates for quantum computation and
simulation with good matter-to-photon conversion. Rydberg excitations in
neutral-atom ensembles show great promise as interfaces for the storage and
manipulation of photonic qubits with excellent efficiencies. Our measurement of
high-visibility interference between photons generated by these two, disparate
systems is an important building block for the establishment of a hybrid
quantum network.
