Inflationary perturbations are fundamentally quantum. In practice, though, it is customary to treat them as classical stochastic fields when computing cosmological observables. This replacement relies on a "quantum-to-classical" transition which occurs on superhorizon scales during slow-roll inflation. At the closed-system level, the success of this transition relies on the suppression of the decaying mode (or equivalently, strong squeezing in the Schrödinger picture).
Of course, many well-motivated inflationary scenarios deviate from slow roll. In particular, models seeking to enhance small-scale power (e.g. for primordial black hole formation) generically rely on a revival of the decaying mode. Near the transition point where growing and decaying modes exchange dominance, interference effects can become important, potentially invalidating a stochastic description.
Employing the EFT of inflation and working to all orders in perturbation theory, we analyze the dynamics of quantum fluctuations in a background featuring a transient deviation from slow roll. In the far superhorizon limit, we construct the Wigner function W, whose positivity can be used to diagnose the availability of a stochastic description. We find that W becomes highly oscillatory and negative in certain regions of phase space --- a hallmark of quantum interference and non-classicality. Remarkably, these effects persist even upon the return to slow roll, indicating a kind of "classical-to-quantum" transition for the affected modes. We conclude that squeezing, or equivalently the size of linear theory growing and decaying modes, is not a reliable indicator of classicality.