  Two interacting particles (TIP) in a disordered chain propagate beyond the
single particle localization length $\xi_1$ up to a scale $\xi_2 > \xi_1$. An
initially strongly localized TIP state expands almost ballistically up to
$\xi_1$. The expansion of the TIP wave function beyond the distance $\xi_1 \gg
1$ is governed by highly connected Fock states in the space of noninteracting
eigenfunctions. The resulting dynamics is subdiffusive, and the second moment
grows as $m_2 \sim t^{1/2}$, precisely as in the strong chaos regime for
corresponding nonlinear wave equations. This surprising outcome stems from the
huge Fock connectivity and resulting quantum chaos. The TIP expansion finally
slows down towards a complete halt -- in contrast to the nonlinear case.
