Fig. 1. Bloch sphere representation of the atomic spin vector. (a) Collective spin vector, the red arrows represent the product of each atomic spin vector, where θ and ϕ are the polar and azimuthal angles, respectively, the colorful disk represent its uncertainty distribution; (b) coherent spin state; (c) spin squeezed state

Fig. 2. Ramsey interferometers with (a) coherent spin states and (b) spin squeezed states ^{[27, 42]}

Fig. 3. Spin squeezing by QND measurement in free space. (a) Atom-light interaction in free space; (b) generation of spin squeezed states with QND measurement^[26]

Fig. 4. Cavity-based QND measurement for the generation of spin squeezed states^[52-54]. (a) Atom-cavity coupling system; (b) working in the dispersive regime; (c) working in the near resonant (normal mode splitting) regime

Fig. 5. Spin squeezing with one-axis twisting Hamiltonian

Fig. 6. Spin squeezed Yb optical lattice clock^[29]. (a) Experimental schematic; (b) experimental results

Fig. 7. Spin squeezed Rb atom interferometer^[29]. (a) Experimental schematic; (b) experimental results

Fig. 8. Spin squeezed Rb atom magnetometer^[32]. (a) Experimental schematic; (b) experimental results

Fig. 9. Entanglement-enhanced distributed quantum sensing network from the Kasevich group^[63]