  We investigate the effect of coupling between translational and internal
degrees of freedom of composite quantum particles on their localization in a
random potential. We show that entanglement between the two degrees of freedom
weakens localization due to the upper bound imposed on the inverse
participation ratio by purity of a quantum state. We perform numerical
calculations for a two-particle system bound by a harmonic force in a 1D
disordered lattice and a rigid rotor in a 2D disordered lattice. We illustrate
that the coupling has a dramatic effect on localization properties, even with a
small number of internal states participating in quantum dynamics.
