  Resonance fluorescence from a quantum emitter is an ideal source to extract
indistinguishable photons. By using the cross polarization to suppress the
laser scattering, we observed resonance fluorescence from GeV color centers in
diamond at cryogenic temperature. The Fourier-transform-limited linewidth
emission with $T_2/2T_1\sim0.86$ allows for two-photon interference based on
single GeV color center. Under pulsed excitation, the 24 ns separated photons
exhibit a Hong-Ou-Mandel visibility of $0.604\pm0.022$, while the
continuous-wave excitation leads to a coalescence time window of 1.05 radiative
lifetime. Together with single-shot readout of spin states, it paves the way
towards building a quantum network with GeV color centers in diamond.

Quantum Physics;Mesoscale and Nanoscale Physics