  We propose a scheme for performing an entanglement-swapping operation within
a quantum communications hub (a Bell like measurement) using an NV centre's
$|\pm 1\rangle \leftrightarrow | A_2 \rangle$ optical transition. This is based
on the heralded absorption of a photon resonant with that transition. The
quantum efficiency of a single photon absorption is low but can be improved by
placing the NV center inside a micro cavity to boost the interaction time and
further by recycling the leaked photon back into the cavity after flipping its
phase and/or polarisation. Throughout this process, the NV is repeatedly
monitored via a QND measurement that heralds whether or not the photon
absorption has succeeded. Upon success we know a destructive Bell measurement
has occurred between that photon and NV center. Given low losses and a high
per-pass absorption probability, this scheme allows the total success
probability to approach unity. With long electron spin coherence times possible
at low temperatures, this component could be useful within a memory-based
quantum repeater or relay.
