Beating standard quantum limit via two-axis magnetic susceptibility measurement

We report a metrology scheme which measures magnetic susceptibility of an atomic spin ensemble along the $x$ and $z$ direction produces parameter estimation with precision beating standard quantum limit. The is initialized via one-axis squeezing optimized time $\phi$ to be estimated assumed as uniformly distributed between 0 $2\pi$. One can produced every two data measured axis respectively, has imprecision scaling $(1.43\pm{}0.02)/N^{0.687\pm0.003}$ respect number N spins. measurement easy implement thus one step towards practical application metrology.