  We investigate the single-atom transport in a two-leg ladder with only two
rungs, which together with the legs, enclose an artificial magnetic flux. Here,
the atoms on the two legs possess opposite onsite energies that produce an
energy offeset. We find that the atom incoming from the left leg can experience
from blockade to tranparency via modifying the onsite energy, hopping strength,
or magnetic flux, which can be potentially used for a quantum switcher.
Furthermore, the atom incoming from the left leg can also be perfectly routed
into the right leg, when, intriguingly, the outgoing atom in the R channel
possesses a wavevector that can be modulated by the magnetic flux. The result
may be potentially used for the interface that controls the communication
between two individual quantum devices of cold atoms. The method can also be
generalized to other artificial quantum systems, such as superconducting
quantum circuit system, optomechanical system, etc.
