Enhanced Lieb-Robinson bounds for a class of Bose-Hubbard type Hamiltonians

Abstract: Several recent works have considered Lieb-Robinson bounds (LRBs) for Bose-Hubbard-type Hamiltonians. For certain special classes of initial states (e.g., states with particle-free regions or perturbations of stationary states), the velocity of information propagation was bounded by a constant in time, $v\leq C$, similarly to quantum spin systems. However, for the more general class of bounded-density initial states, the first-named author together with Vu and Saito derived the velocity bound $v\leq C t^{D-1}$, where $D$ is the spatial lattice dimension. For $D\geq 2$, this bound allows for accelerated information propagation. It has been known since the work of Eisert and Gross that some systems of lattice bosons are capable of accelerated information propagation. It is therefore a central question to understand under what conditions the bound $v\leq C t^{D-1}$ can be enhanced. Here, we prove that additional physical constraints, translation-invariance and a $p$-body repulsion of the form $n_x^p$ with $p>D+1$, lead to a LRB with $v\leq C t^{\frac{D}{p-D-1}}$ for any initial state of bounded energy density. We also identify examples of quantum states which show that no further enhancement is possible without using additional dynamical constraints.