This paper presents a key experiment demonstrating long-distance quantum entanglement between two solid-state spin qubits separated by 3 meters. The qubits are nitrogen-vacancy (NV) centers in diamond, which are well-suited for quantum information processing due to their long-lived electronic spin and robust optical interface. The researchers achieved entanglement by creating spin-photon entanglement through a resonant laser pulse and subsequent joint measurement of the photons. The detection of the photons heralds the projection of the spin qubits onto an entangled state. The experiment verified the non-local quantum correlations by performing single-shot readout on the qubits in different bases. The entanglement protocol involved preparing both NV centers in a superposition state, exciting them with a laser pulse, and detecting the emitted photons. The photons were then directed to a beamsplitter and detected in the two output ports. The detection of one photon in each excitation round heralds the entanglement and triggers individual spin readout. The protocol was robust against photon loss and insensitive to optical path length differences. The experiment also addressed challenges such as photon indistinguishability and background suppression. The researchers used a cross-polarized excitation-detection scheme and a detection time filter to minimize the contribution of scattered laser photons. They also used microfabricated solid-immersion lenses to enhance collection efficiency and spectral filtering to suppress non-resonant NV emission. The results demonstrated the generation of entangled states with high fidelity, as verified by measuring the spin-spin correlations. The fidelity was calculated using a strict lower bound based on measurement results from different bases. The results showed that the experiment yielded the two desired entangled states, with a fidelity of (69±5)% for ψ⁻ and (58±6)% for ψ⁺. The success of the experiment was attributed to the high-quality NV centers and the robust protocol. The researchers also discussed potential improvements for future experiments, such as further improving photon indistinguishability and working at lower temperatures to reduce phonon-mediated excited-state mixing. The results demonstrate the potential of solid-state qubits for quantum information processing and the realization of quantum networks.This paper presents a key experiment demonstrating long-distance quantum entanglement between two solid-state spin qubits separated by 3 meters. The qubits are nitrogen-vacancy (NV) centers in diamond, which are well-suited for quantum information processing due to their long-lived electronic spin and robust optical interface. The researchers achieved entanglement by creating spin-photon entanglement through a resonant laser pulse and subsequent joint measurement of the photons. The detection of the photons heralds the projection of the spin qubits onto an entangled state. The experiment verified the non-local quantum correlations by performing single-shot readout on the qubits in different bases. The entanglement protocol involved preparing both NV centers in a superposition state, exciting them with a laser pulse, and detecting the emitted photons. The photons were then directed to a beamsplitter and detected in the two output ports. The detection of one photon in each excitation round heralds the entanglement and triggers individual spin readout. The protocol was robust against photon loss and insensitive to optical path length differences. The experiment also addressed challenges such as photon indistinguishability and background suppression. The researchers used a cross-polarized excitation-detection scheme and a detection time filter to minimize the contribution of scattered laser photons. They also used microfabricated solid-immersion lenses to enhance collection efficiency and spectral filtering to suppress non-resonant NV emission. The results demonstrated the generation of entangled states with high fidelity, as verified by measuring the spin-spin correlations. The fidelity was calculated using a strict lower bound based on measurement results from different bases. The results showed that the experiment yielded the two desired entangled states, with a fidelity of (69±5)% for ψ⁻ and (58±6)% for ψ⁺. The success of the experiment was attributed to the high-quality NV centers and the robust protocol. The researchers also discussed potential improvements for future experiments, such as further improving photon indistinguishability and working at lower temperatures to reduce phonon-mediated excited-state mixing. The results demonstrate the potential of solid-state qubits for quantum information processing and the realization of quantum networks.