Relative entropy for locally squeezed states

Abstract

Relative entropy serves as a fundamental measure of state distinguishability in both quantum information theory and relativistic quantum field theory. Despite its conceptual importance, however, explicit computations of relative entropy remain notoriously difficult. Thus far, results in closed form have only been obtained for ground states, coherent states, and, more recently, single-mode squeezed states. In this work, we extend the analysis to multi-mode squeezed states, imposing that the squeezing generators be local either in space or in spacetime, which results in a continuum of squeezed modes. We provide a detailed and self-contained analysis of such states for a free scalar quantum field on Minkowski spacetime, connecting also with older results on the essential self-adjointness of the Wick square, and showing that they lie in the folium of the Minkowski vacuum representation. Although the local squeezing is natural from a foundational standpoint, we uncover a severe incompatibility between locality and squeezing: the relative entropy between a locally squeezed state and the vacuum generally diverges, however small the squeezing is. This shows that while locally squeezed states are well-defined elements of the state space of a free quantum field, they are infinitely different from the vacuum, in contrast to coherent states whose relative entropy with respect to the vacuum is finite.