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 |θ0, φ0) in a generalized one-axis twisting model, where the detuning δ is taken into account. We show explicitly that regardless of δ and φ0, previous results of the ideal one-axis twisting are recovered as long as θ0=π/2. For a small departure of θ0 from π/2, however, the achievable variance (V -)min∼N2/3, which is larger than the ideal case N1/3. We also find that the maximal squeezing time tmin scales as N-5/6. Analytic expressions of (V-)min and tmin are presented and they agree with numerical simulations.