Speaker
Description
I discuss the thermalization of closed quantum systems. This refers to their ability to relax toward steady states described by only a few quantities, such as mean energy or particle number. I introduce the eigenstate thermalization hypothesis, which underpins our current understanding of this process. I then present notable exceptions to thermalization, focusing on Hilbert space fragmentation, where the Hamiltonian breaks into exponentially many (in system size) dynamically disconnected blocks. In the final part, I show that a suitably chosen perturbation can induce a gradual merging of these fragmented subspaces. This slow restoration of ergodicity gives rise to an extended critical regime, marked by multiple peaks in the fidelity susceptibility. Each peak signals a change in the number of blocks and corresponds to ultra-slow relaxation of local observables.
