Wehrl entropy production rate across a dynamical quantum phase transition

Abstract

The quench dynamics of many-body quantum systems may exhibit nonanalyticities in the Loschmidt echo, a phenomenon known as dynamical phase transition (DPT). Despite considerable research into the underlying mechanisms behind this phenomenon, several open questions still remain. Motivated by this, we put forth a detailed study of DPTs from the perspective of quantum phase space and entropy production, a key concept in thermodynamics. We focus on the Lipkin-Meshkov-Glick model and use spin-coherent states to construct the corresponding Husimi-Q quasiprobability distribution. The entropy of the Q function, known as Wehrl entropy, provides a measure of the coarse-grained dynamics of the system and, therefore, evolves nontrivially even for closed systems. We show that critical quenches lead to a quasimonotonic growth of the Wehrl entropy in time, combined with small oscillations. The former reflects the information scrambling characteristic of these transitions and serves as a measure of entropy production. On the other hand, the small oscillations imply negative entropy production rates and therefore signal the recurrences of the Loschmidt echo. Finally, we also study a Gaussification of the model based on a modified Holstein-Primakoff approximation. This allows us to identify the relative contribution of the low-energy sector to the emergence of DPTs. The results presented in this article are relevant not only from the dynamical quantum phase transition perspective but also for the field of quantum thermodynamics, since they point out that the Wehrl entropy can be used as a viable measure of entropy production.