On the existence of an energy gap in one-dimensional Lesanovsky's model

We study the quantum lattice gas model in one dimension introduced by Lesanovsky, who showed that the exact ground state and a couple of excited states can be obtained analytically. The Hamiltonian of the model depends solely on the parameter $z$, the meaning of which is a fugacity in the corresponding classical lattice gas model. For small $z$ ($0<z<1$), we prove that there is an energy gap between the ground state and the excited states by applying Knabe's method.