Fig. 1. A simplified diagram of quadripartite entanglement and an energy level diagram of rubidium-85: (a) C₀ and C₂ are vacuum states, Pr₀ and Pr₂ are coherent states; C₁ and are the twin beams generated by the first four-wave mixing process, C₃ and are the twin beams generated by the second four-wave mixing process; Pr₁ and Pr₃ are produced by mixing beams and through a linear beam splitter; (b) the double Λ energy level structure of D1 line in rubidium-85, Δ and δ represent one-photon detuning and two-photon detuning respectively.

Fig. 2. The smallest symplectic eigenvalue v of all 1 × 3 scenarios, as a function of the power gains G₁ and G₂: (a) C₁ is partially transposed; (b) Pr₁ is partially transposed; (c) Pr₃ is partially transposed; (d) C₃ is partially transposed.

Fig. 3. The smallest symplectic eigenvalues v of all 2 × 2 scenarios, as a function of the power gains G₁ and G₂: (a) C₁ and Pr₁ arepartially transposed; (b) C₁ and Pr₃ are partially transposed; (c) C₁ and C₃ are partially transposed.

Fig. 4. Effect of the transmissivity of the linear beam splitter on the quadripartite entanglement of the system: (a) C₁ is partially transposed; (b) Pr₁ is partially transposed; (c) Pr₃ is partially transposed; (d) C₃ is partially transposed; (e) C₁ and Pr₁ are partially transposed; (f) C₁ and Pr₃ are partially transposed; (g) C₁ and C₃ are partially transposed.

Fig. 5. The smallest symplectic eigenvalues v of all tripartite states as a function of power gains G₁ and G₂: (a)−(c) The smallest symplectic eigenvaluesv of tripartite state composed of C₁, Pr₁ and Pr₃; (d)−(f) the smallest symplectic eigenvaluesv of tripartite state composed of C₁, Pr₁ and C₃; (g)−(i) the smallest symplectic eigenvaluesv of tripartite state composed of C₁, Pr₃ and C₃; (j)−(l) the smallest symplectic eigenvaluesv of tripartite state composed of Pr₁, Pr₃ and C₃.

Fig. 6. The smallest symplectic eigenvalues v of all bipartite states as a function of power gains G₁ and G₂: (a) The smallest symplectic eigenvalues v of bipartite state composed of C₁ and Pr₁; (b) the smallest symplectic eigenvalues v of bipartite state composed of C₁ and Pr₃; (c) the smallest symplectic eigenvalues v of bipartite state composed of C₁ and C₃; (d) the smallest symplectic eigenvalues v of bipartite state composed of Pr₁ and Pr₃; (e) the smallest symplectic eigenvalues v of bipartite state composed of Pr₁ and C₃; (f) the smallest symplectic eigenvalues v of bipartite state composed of Pr₃ and C₃.