Optimal symmetry operators

Abstract

We present a constructive method to maximize the expectation value of operators that implement a symmetry on a subsystem, making use of modular tools. More generally, we study the positive cones associated with a von Neumann algebra, as defined by Araki. Given a reference vector, an algebra, and a state on the algebra, the purification of the state in the cone $α= 0$, associated with the reference vector and the algebra, yields the unique vector whose overlap with the reference vector is maximal among all possible purifications. This establishes that the supremum in Uhlmann's theorem is uniquely attained by this vector, thereby providing the fidelity between the given state and the state obtained by restricting the reference vector to the algebra. Moreover, this purification can be explicitly constructed using modular tools. In addition, given an automorphism of the algebra, we show how to construct isometries implementing the automorphism using the positive cones. We prove that the isometry constructed from the cone $α= 0$ is the one with maximal expectation value among all possible isometries implementing the automorphism. We illustrate these ideas with two simple examples: one involving a system of two spins, and the other in the theory of the massless scalar field in 3+1 dimensions.