Fig. 4. Quasidistribution Q-function and spin-squeezing parameter.

(A) Experimental and numerical data of Q(θ, ϕ) in spherical coordinates, when the minimum values of the spin-squeezing parameters are achieved during the time evolutions with the strengths of the transverse fields h^x/2π ≃ 3 and 6 MHz, respectively. (B) Time evolution of the spin-squeezing parameters with h^x/2π ≃ 3 and 6 MHz, respectively. (C) Minimum spin-squeezing parameter ${\mathrm{\xi }}_{\text{min}}^2$ as a function of h^x. The solid lines in (B) are the numerical results using the Hamiltonian of our experimental system without considering decoherence. The blue shaded area in (B) is only accessible for entangled states. The dotted line in (C) is the piecewise linear fit, whose minimum point is close to the theoretically predicted critical point $h_c^x/2\mathrm{\pi }\simeq 5.7\text{MHz}$ (dashed line).