  We introduce and study three-dimensional quantum dimer models with positive
resonance terms. We demonstrate that their ground state wave functions exhibit
a nonlocal sign structure that can be exactly formulated in terms of loops, and
as a direct consequence, monomer excitations obey Fermi statistics. The sign
structure and Fermi statistics in these "signful" quantum dimer models can be
naturally described by a parton construction, which becomes exact at the
solvable point.
