  We present the experimental generation of large effective magnetic fields for
ultracold atoms using photon-assisted tunneling in an optical superlattice. The
underlying method does not rely on the internal structure of the atoms and
therefore constitutes a general approach to realize widely tunable artificial
gauge fields without the drawbacks of near-resonant optical potentials. When
hopping in the lattice, the accumulated phase shift by an atom is equivalent to
the Aharonov-Bohm phase of a charged particle exposed to a staggered magnetic
field of large magnitude, on the order of one flux quantum per plaquette. We
study the ground state of this system and observe that the frustration induced
by the magnetic field can lead to a degenerate ground state for non-interacting
particles. We provide a local measurement of the phase acquired by single
particles due to photon-assisted tunneling. Furthermore, the quantum cyclotron
orbit of single atoms in the lattice exposed to the effective magnetic field is
directly revealed.
