Fig. 1. Bloch sphere representations for spin-coherent state and spin-squeezed state. (a) Spin-coherent state; (b) spin-squeezed state

Fig. 2. Schematic diagrams of the preparation of spin-squeezed state in different physical platforms

Fig. 3. Gain of phase sensitivity over the standard quantum limit achieved experimentally^[9]

Fig. 4. Schematic diagrams of entanglement-enhanced magnetometers. (a) EPR entanglement-enhanced magnetometry^[31]; (b) preparation of conditional spin-squeezed states based on flash frequency technology^[33]

Fig. 5. Schematic diagram of entanglement-enhanced matter-wave interferometric measurement^［35］