Operator growth and eigenstate entanglement in an interacting integrable Floquet system

We analyze a simple model of quantum dynamics, which is a discrete-time deterministic version of the Fredrickson-Andersen model. This model is integrable, with a quasiparticle description related to the classical hard-rod gas. Despite the integrability of the model, commutators of physical operators grow with a diffusively broadening front, in this respect resembling generic chaotic models. In addition, local operators behave consistently with the eigenstate thermalization hypothesis (ETH). However, large subsystems violate ETH; as a function of subsystem size, eigenstate entanglement first increases linearly and then saturates at a scale that is parametrically smaller than half the system size.