  Quantum kinetically constrained models have recently attracted significant
attention due to their anomalous dynamics and thermalization. In this work, we
introduce a hitherto unexplored family of kinetically constrained models
featuring a conserved particle number and strong inversion-symmetry breaking
due to facilitated hopping. We demonstrate that these models provide a generic
example of so-called quantum Hilbert space fragmentation, that is manifested in
disconnected sectors in the Hilbert space that are not apparent in the
computational basis. Quantum Hilbert space fragmentation leads to an
exponential in system size number of eigenstates with exactly zero entanglement
entropy across several bipartite cuts. These eigenstates can be probed
dynamically using quenches from simple initial product states. In addition, we
study the particle spreading under unitary dynamics launched from the domain
wall state, and find faster than diffusive dynamics at high particle densities,
that crosses over into logarithmically slow relaxation at smaller densities.
Using a classically simulable cellular automaton, we reproduce the logarithmic
dynamics observed in the quantum case. Our work suggests that particle
conserving constrained models with inversion symmetry breaking realize so far
unexplored universality classes of dynamics and invite their further
theoretical and experimental studies.
