  Quantum emitters hosted in crystalline lattices are highly attractive
candidates for quantum information processing, secure networks and nanosensing.
For many of these applications it is necessary to have control over single
emitters with long spin coherence times. Such single quantum systems have been
realized using quantum dots, colour centres in diamond, dopants in
nanostructures and molecules . More recently, ensemble emitters with spin
dephasing times on the order of microseconds and room-temperature optically
detectable magnetic resonance have been identified in silicon carbide (SiC), a
compound being highly compatible to up-to-date semiconductor device technology.
So far however, the engineering of such spin centres in SiC on single-emitter
level has remained elusive. Here, we demonstrate the control of spin centre
density in ultrapure SiC over 8 orders of magnitude, from below $10^{9}$ to
above $10^{16} \,$cm$^{-3}$ using neutron irradiation. For a low irradiation
dose, a fully photostable, room-temperature, near infrared (NIR) single photon
emitter can clearly be isolated, demonstrating no bleaching even after
$10^{14}$ excitation cycles. Based on their spectroscopic fingerprints, these
centres are identified as silicon vacancies, which can potentially be used as
qubits, spin sensors and maser amplifiers.
