  Two-leg bosonic ladders with flux harbor a remarkable vortex-hole duality
between the weak-coupling vortex lattice superfluids and strong-coupling
charge-density-wave crystals. The strong-coupling crystalline states, which are
realized in the vicinity of $\pi$-flux, are independent of particle statistics,
and are related with the incompressible fractional quantum Hall states in the
thin-cylinder limit. These fully gapped ground states, away of $\pi$-flux,
develop nonzero chiral (spin) currents. Contact-interacting quantum gases
permit exploration of this vortex-hole duality in experiments.
