  Single defect centers in layered hexagonal boron nitride (hBN) are promising
candidates as single photon sources for quantum optics and nanophotonics
applications. However, until today spectral instability hinders many
applications. Here, we perform resonant excitation measurements and observe
Fourier-Transform limited (FL) linewidths down to $\approx 50$ MHz. We
investigate optical properties of more than 600 quantum emitters (QE) in hBN.
The QEs exhibit narrow zero-phonon lines (ZPL) distributed over a spectral
range from 580 nm to 800 nm and with dipole-like emission with high
polarization contrast. The emission frequencies can be divided into four main
regions indicating distinct families or crystallographic structures of the QEs,
in accord with ab-initio calculations. Finally, the emitters withstand transfer
to a foreign photonic platform - namely a silver mirror, which makes them
compatible with photonic devices such as optical resonators and paves the way
to quantum photonics applications including quantum commmunications and quantum
repeaters.
