Quantum signatures of breather-breather interactions

The spectrum of the quantum discrete nonlinear Schro¨dinger equation, or boson Hubbard Hamiltonian, on a periodic 1D lattice shows some interesting detailed band structure, which may be interpreted as the quantum signature of a two-breather interaction in the classical case. This fine structure is studied using degenerate perturbation theory.We also present a modification to this model, which increases the mobility of bound states.