Decoherence from isocurvature perturbations in inflation

We present a quantitative study of the quantum decoherence of curvature perturbations during inflation in the presence of isocurvature modes. If the latter cannot be observed directly, tracing them out effectively decoheres the curvature perturbation even in the absence of a direct coupling between the scalar fields involved. We then calculate the entanglement entropy and argue that it provides a quantitative measure for decoherence.