We investigate the behavior of persistent currents for a fixed number of
noninteracting fermions in a periodic quantum ladder threaded by Aharonov-Bohm
and transverse magnetic fluxes Φ and χ. We show that the coupling
between ladder legs provides a way to effectively change the ground-state
fermion-number parity, by varying χ. Specifically, we demonstrate that
varying χ by 2π (one flux quantum) leads to an apparent fermion-number
parity switch. We find that persistent currents exhibit a robust 4Ï€
periodicity as a function of χ, despite the fact that χ→χ+2π leads to modifications of order 1/N of the energy spectrum, where N
is the number of sites in each ladder leg. We show that these parity-switch and
4Ï€ periodicity effects are robust with respect to temperature and disorder,
and outline potential physical realizations using cold atomic gases and, for
bosonic analogs of the effects, photonic lattices.Comment: 5 pages, 4 figures + Supplemental Materia