  Silicon-vacancy (SiV) centers in diamond are promising systems for quantum
information applications due to their bright single photon emission and
optically accessible spin states. Furthermore, SiV centers in low-strain
diamond are insensitive to pertubations of the dielectric environment, i.e.
they show very weak spectral diffusion. This property renders ensembles of SiV
centers interesting for sensing applications. We here report on
photoluminescence excitation (PLE) spectroscopy on an SiV ensemble in a low
strain, CVD-grown high quality diamond layer, where we measure the fine
structure with high resolution and obtain the linewidths and splittings of the
SiV centers. We investigate the temperature dependence of the width and
position of the fine structure peaks. Our measurements reveal linewidths of
about 10 GHz as compared to a lifetime limited width on the order of 0.1 GHz.
This difference arises from the inhomogeneous broadening of the transitions
caused by residual strain. To overcome inhomogeneous broadening we use spectral
hole burning spectroscopy which enables us to measure a nearly lifetime limited
homogeneous linewidth of 279 MHz. Furthermore, we demonstrate evidence of
coherent interaction in the system by driving a $\Lambda$-scheme. Additional
measurements on single emitters created by ion implantation confirm the
homogeneous linewidths seen in the spectral hole burning experiments and relate
the ground state splitting to the decoherence rate.
