Global effects of quantum states induced by locally invariant measurements
Shunlong Luo, Shuangshuang Fu · Europhysics Letters (EPL) · 2010
In quantum mechanics, general measurements often cause disturbance which may be exploited to quantify entanglement, nonlocality or quantumness. Imagine a bipartite state ρ ab shared by two parties a and b , and a von Neumann measurement performed locally on party a which does not disturb the local state ρ a ≔tr b ρ ab , but nevertheless may disturb the global state ρ ab . This disturbance is an indication of some kind of correlations or global effect in ρ ab which cannot be accounted for locally. We propose to use the maximum disturbance on ρ ab caused by locally non-disturbing measurements as a figure of merit quantifying the global effect (nonlocality), and investigate its fundamental properties. For general two-qubit states and some higher-dimensional symmetric states, we present analytic formulas for their global effects.