  The silicon-vacancy (SiV) and nitrogen-vacancy (NV) centers in diamond are
commonly regarded as prototypical defects for solid-state quantum information
processing. Here we show that when silicon and nitrogen are simultaneously
introduced into the diamond lattice these defects can strongly interact and
form larger complexes. Nitrogen atoms strongly bind to Si and SiV centers and
complex formation can occur. Using a combination of hybrid density functional
theory (DFT) and group theory, we analyze the electronic structure and provide
various useful physical properties, such as hyperfine structure, quasi-local
vibrational modes, and zero-phonon line, to enable experimental identification
of these complexes. We demonstrate that the presence of substitutional silicon
adjacent to nitrogen significantly shifts the donor level toward the conduction
band, resulting in an activation energy for the SiN center that is comparable
to phosphorus. We also find that the neutral SiNV center is of particular
interest due to its photon emission at $\sim$1530 nm, which falls within the C
band of telecom wavelengths, and its paramagnetic nature. In addition, the
optical transition associated with the SiNV$^0$ color center exhibits very
small electron--phonon coupling (Huang--Rhys factor~=~0.78) resulting in high
quantum efficiency (Debye-Waller factor = 46\%) for single-photon emission.
These features render this new center very attractive for potential application
in scalable quantum telecommunication networks.
