  Entanglement distribution over quantum networks has the promise of realizing
fundamentally new technologies. Entanglement between separated quantum
processing nodes has been achieved on several experimental platforms in the
past decade. To move towards metropolitan-scale quantum network test beds, the
creation and transmission of indistinguishable single photons over existing
telecom infrastructure is key. Here we report the interference of photons
emitted by remote, spectrally detuned NV center-based network nodes, using
quantum frequency conversion to the telecom L-band. We find a visibility of
0.79$\pm$0.03 and an indistinguishability between converted NV photons around
0.9 over the full range of the emission duration, confirming the removal of the
spectral information present. Our approach implements fully separated and
independent control over the nodes, time-multiplexing of control and quantum
signals, and active feedback to stabilize the output frequency. Our results
demonstrate a working principle that can be readily employed on other platforms
and shows a clear path towards generating metropolitan scale, solid-state
entanglement over deployed telecom fibers.
