Multipartite nonlocality in one-dimensional quantum spin chains at finite temperatures

Abstract

Multipartite nonlocality is an important measure of multipartite quantum correlations. In this paper, we show that the nonlocal $n$-site Mermin-Klyshko operator ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{M}}_{n}$ can be exactly expressed as a matrix product operator with a bond dimension $D=2$, and then the calculation of nonlocality measure $\mathcal{S}$ can be simplified into standard one-dimensional (1D) tensor networks. With the help of this technique, we analyze finite-temperature multipartite nonlocality in several typical 1D spin chains, including an $XX$ model, an $XXZ$ model, and a Kitaev model. For the $XX$ model and the $XXZ$ model, in a finite-temperature region, the logarithm measure of nonlocality (${log}_{2}\mathcal{S}$) is a linear function of the temperature $T$, i.e., ${log}_{2}\mathcal{S}\ensuremath{\sim}\ensuremath{-}aT+b$. It provides us with an intuitive picture about how thermodynamic fluctuations destroy multipartite nonlocality in 1D quantum chains. Moreover, in the $XX$ model $\mathcal{S}$ presents a magnetic-field-induced oscillation at low temperatures. This behavior has a nonlocal nature and cannot be captured by local properties such as the magnetization. Finally, for the Kitaev model, we find that in the limit $T\ensuremath{\rightarrow}0$ and $N\ensuremath{\rightarrow}\ensuremath{\infty}$ the nonlocality measure may be used as an alternative order parameter for the topological-type quantum phase transition in the model.