The supplementary information provides detailed experimental methods and results for a study on the entanglement of nitrogen-vacancy (NV) centers in diamond. The setup includes a cryogenic confocal microscope for optical access to NV centers, with two excitation channels and two collection channels. The NV centers are excited using a doubled YAG laser and resonant optical pumping with external cavity lasers. The setup is controlled by an FPGA-based intelligent DAQ device, and data acquisition is performed using a time-tagged-single-photon-counting device. The NV center's electronic states and optical transitions are described, including the selection rules for transitions between the triplet excited state and the triplet ground state. The effect of strain on the optical transitions is discussed, and the polarization properties of the transitions are verified. Spin readout is achieved through resonant laser excitation, and the conditional readout technique is described to improve the accuracy of spin measurements. The fidelity of the entangled state is estimated using time bin optimization and maximum likelihood estimation, with the results indicating a high probability of entanglement.The supplementary information provides detailed experimental methods and results for a study on the entanglement of nitrogen-vacancy (NV) centers in diamond. The setup includes a cryogenic confocal microscope for optical access to NV centers, with two excitation channels and two collection channels. The NV centers are excited using a doubled YAG laser and resonant optical pumping with external cavity lasers. The setup is controlled by an FPGA-based intelligent DAQ device, and data acquisition is performed using a time-tagged-single-photon-counting device. The NV center's electronic states and optical transitions are described, including the selection rules for transitions between the triplet excited state and the triplet ground state. The effect of strain on the optical transitions is discussed, and the polarization properties of the transitions are verified. Spin readout is achieved through resonant laser excitation, and the conditional readout technique is described to improve the accuracy of spin measurements. The fidelity of the entangled state is estimated using time bin optimization and maximum likelihood estimation, with the results indicating a high probability of entanglement.