Transform-limited photon emission from quantum emitters is essential for
high-fidelity entanglement generation. In this study, we report the coherent
optical property of a single negatively-charged lead-vacancy (PbV) center in
diamond. Photoluminescence excitation measurements reveal stable fluorescence
with a linewidth of 39 MHz at 6 K, close to the transform-limit estimated from
the lifetime measurement. We observe four orders of magnitude different
linewidths of the two zero-phonon-lines, and find that that the phonon-induced
relaxation in the ground state contributes to this huge difference in the
linewidth. Due to the suppressed phonon absorption in the PbV center, we
observe nearly transform-limited photon emission up to 16 K, demonstrating its
high temperature robustness compared to other color centers in diamond.Comment: 13 pages,4 figure