Spin squeezing in a generalized one-axis twisting model

We investigate the dependence of spin squeezing on the polar angle of the initial coherent spin state |θ 0 , ϕ 0 ⟩ in a generalized one-axis twisting model, where the detuning δ is taken into account. We show explicitly that regardless of δ and ϕ 0 , previous results of the ideal one-axis twisting are recovered as long as θ 0 =π/2. For a small departure of θ 0 from π/2, however, the achievable variance ( V − ) min ∼ N 2/3 , which is larger than the ideal case N 1/3 . We also find that the maximal squeezing time t min scales as N −5/6 . Analytic expressions of ( V − ) min and t min are presented and they agree with numerical simulations.