  The prospect of studying topological matter with the precision and control of
atomic physics has driven the development of many techniques for engineering
artificial magnetic fields and spin-orbit interactions. Recently, the idea of
introducing nontrivial topology through the use of internal (or external)
atomic states as effective "synthetic dimensions" has garnered attraction for
its versatility and possible immunity from heating. Here, we directly engineer
tunable artificial gauge fields through the local control of tunneling phases
in an effectively two-dimensional manifold of discrete atomic momentum states.
We demonstrate the ability to create homogeneous gauge fields of arbitrary
value, directly imaging the site-resolved dynamics of induced chiral currents.
We furthermore engineer the first inhomogeneous artificial gauge fields for
cold atoms, observing the magnetic reflection of atoms incident upon a
step-like variation of an artificial vector potential. These results open up
new possibilities for the study of topological phases and localization
phenomena in atomic gases.
