Infinite Distance Limits and Information Theory

Abstract: The classical information metric provides a unique notion of distance on the space of probability distributions with a well-defined operational interpretation: two distributions are far apart if they are readily distinguishable from one another. The quantum information metric generalizes this to the space of quantum states, and thus defines a notion of distance on an arbitrary continuous family of quantum field theories via their vacua that is proportional to the metric on moduli space when restricted appropriately. In this paper, we study this metric and its operational interpretation in a variety of examples. We specifically focus on why and how infinite distance singularities appear. We argue that two theories are infinitely far apart if they are hyper-distinguishable: that is, if they can be distinguished from one another, with certainty, using only a few measurements. We explain why such singularities appear for the simple harmonic oscillator yet are absent for quantum field theories near a typical quantum critical point, and show how an infinite distance point can emerge when a tower of fields degenerates in mass. Finally, we use this perspective to provide a potential bottom-up motivation for the Swampland Distance Conjecture and indicate how we might extend it beyond current lampposts.