Quantum Fisher Information in Two-Qubit Pure States.

In terms of quantum Fisher information (QFI), a quantity χ was introduced by Pezzé and Smerzi (Phys. Rev. Lett. 102 100401, ). They pointed out that the inequality χ<1 was a sufficient condition for multiparticle entanglement. For the two-qubit symmetric states, we found that the inequality χ<1 is a necessary and sufficient condition for entanglement and spin squeezing, and that χ is equal to the second kind of spin squeezing parameter $\xi _{2}^{2}$. For the two-qubit asymmetric states, it is only a sufficient condition. In order to make it a necessary and sufficient condition, we extend the concept of the QFI and χ, and generalize the relations among the entanglement measurement, the spin squeezing parameters and χ in symmetric pure states to arbitrary pure states. [ABSTRACT FROM AUTHOR]