  Spin-photon interfaces are strong candidates for building blocks of future
quantum networks and quantum computers. Several systems currently under
examination present promising features, but none of them yet fulfil all
requirements for these aims. A particularly attractive strategy for the
realization of these applications is the creation of strings of entangled
photons, where quantum correlations among the photons are mediated by
operations on the spin of the emitter. Here, we demonstrate for the first time
the creation of spin-photon entanglement within the fundamental unit of a
novel, scalable protocol based on time-to-polarization conversion. This
principle allows us to bypass many of the imperfections of currently available
photon sources and can therefore be utilized with a large variety of emitters.
We execute the protocol using a nitrogen-vacancy centre in diamond, which
possesses a long coherence lifetime and multiple spin degrees of freedom,
thereby offering an outlook towards the creation of large entangled states.
