Perspective on witnessing entanglement in hybrid quantum systems

Hybrid quantum systems aim at combining the advantages of different physical and producing innovative devices. In particular, hybrid combination superconducting circuits spins in solid-state crystals is a versatile platform to explore many electrodynamics problems. Recently, remote coupling nitrogen-vacancy center diamond via bus was demonstrated. However, rigorous experimental test nature this system and, entanglement still missing. We review theoretical ideas generate detect present our own scheme achieve this.