  Fractional photon-assisted tunnelling is investigated both numerically and
analytically in a double-well lattice. While integer photon-assisted tunnelling
is a single-particle effect, fractional photon-assisted tunnelling is an
interaction-induced many-body effect. Double-well lattices with few particles
in each double well are ideal to study this effect far from the mean-field
effects. It is predicted that the 1/4-resonance is observable in such systems.
Fractional photon-assisted tunnelling provides a physically relevant model for
which N-th order time-dependent perturbation theory can be large although all
previous orders are small.
