Witnessing nonclassical multipartite states
A. Saguia, C. C. Rulli, Thiago Rodrigues de Oliveira, M. S. Sarandy · Physical Review A · 2011
We investigate a witness for nonclassical multipartite states based on their disturbance under local measurements. The witness operator provides a sufficient condition for nonclassicality that coincides with a nonvanishing global quantum discord, but it does not demand an extremization procedure. Moreover, for the case of ${Z}_{2}$-symmetric systems, we rewrite the witness in terms of correlation functions so that classicality is found to necessarily require either vanishing magnetization in the invariant axis or isotropy of the two-point function in the transverse spin plane. We illustrate our results in quantum spin chains, where a characterization of factorized ground states (with spontaneously broken ${Z}_{2}$ symmetry) is achieved. As a by-product, the witness is also shown to indicate a second-order quantum phase transitions, which is illustrated for both the $XY$ and the Ashkin-Teller spin chains.