  Quantum backflow refers to the counterintuitive fact that the probability can
flow in the direction opposite to the momentum of a quantum particle. This
phenomenon has been seen to be small and fragile for one-dimensional systems,
in which the maximal amount of backflow has been found to be bounded. Quantum
backflow exhibits dramatically different features in two-dimensional systems
that, contrary to the one-dimensional case, allow for degenerate energy
eigenstates. Here we investigate the case of a charged particle that is
confined to move on a finite disk punctured at the center and that is pierced
through the center, and normally to the disk, by a magnetic flux line. We
demonstrate that quantum backflow can be unbounded (in a certain sense), which
makes this system a promising physical platform regarding the
yet-to-be-performed experimental observation of this fundamental quantum
phenomenon.
