  Spin-active quantum emitters have emerged as a leading platform for quantum
technologies. However, one of their major limitations is the large spread in
optical emission frequencies, which typically extends over tens of GHz. Here,
we investigate single V4+ vanadium centres in 4H-SiC, which feature
telecom-wavelength emission and a coherent S=1/2 spin state. We perform
spectroscopy on single emitters and report the observation of spin-dependent
optical transitions, a key requirement for spin-photon interfaces. By
engineering the isotopic composition of the SiC matrix, we reduce the
inhomogeneous spectral distribution of different emitters down to 100 MHz,
significantly smaller than any other single quantum emitter. Additionally, we
tailor the dopant concentration to stabilise the telecom-wavelength V4+ charge
state, thereby extending its lifetime by at least two orders of magnitude.
These results bolster the prospects for single V emitters in SiC as material
nodes in scalable telecom quantum networks.

Quantum Physics;Materials Science;Optics