Fig. 1. Wigner functions of quantum states. (a) Vacuum state; (b) coherent state; (c)(d) amplitude quadrature and phase quadrature squeezed vacuum states; (e)(f) amplitude quadrature and phase quadrature squeezed coherent states (insets: corresponding edge distribution of quadrature amplitude and quadrature phase components of Wigner function) ^[31]

Fig. 2. Wavefunctions of two-mode states in the position (left) and momentum (right) bases. (a) Two-mode vacuum state is uncorrelated in both bases; (b) two-mode squeezed state with amplitude quadrature correlated, and phase quadrature observables anticorrelated; (c) two-mode squeezed state with amplitude quadrature anticorrelated, and phase quadrature correlated ^[31]

Fig. 3. Optical parametric amplifier. (a) Degenerate optical parametric amplifier; (b) polarization non-degenerate optical parametric amplifier

Fig. 4. Four-wave mixing process in atomic ensemble. (a) Single-mode squeezed state generated from degenerate case; (b) two-mode squeezed state generated from non-degenerate case ^[34]

Fig. 5. Principle of balanced homodyne detection ^[31]

Fig. 6. Principle and schematic of the test principle for error-tradeoff relation by using a EPR entangled state^[54]

Fig. 7. Schematic of connecting two local quantum networks by quantum entanglement swapping ^[25]

Fig. 8. Schematic of the gate sequence based on continuous-variable cluster state ^[10]

Fig. 9. Schematic of using amplitude-squeezed state of light to enhance the sensitivity of LIGO. (a) Schematic of experimental setup; (b) Wigner function projections of coherent state and squeezed coherent state in phase space; (c) photocurrents of coherent state and squeezed coherent state ^[31]

Fig. 10. Schematic of quantum-enhanced stimulated Raman scattering microscopy. (a) Schematic of experimental setup; (b) vibration spectra of different molecules ^[93]