  We theoretically demonstrate that an external magnetic field can be used to
control quantum reflection of matter waves in graphene due to its extraordinary
magneto-optical properties. We calculate the quantum reflection probabilities
in graphene for three experimentally relevant atomic species (He, Na, and Rb)
using the full Casimir-Polder potential computed by Lifshitz formula valid at
all distance regimes, going beyond the traditional approach to quantum
reflection, based on power law potentials, which are known to be valid only in
the short distance (non-retarded van der Waals) or in the large distance
(retarded) regimes. We predict the energy range for which quantum reflection is
more likely to occur as a function of the magnetic field, and show that the
quantum reflection probabilities exhibit discontinuities that reflect the
structure of Landau levels in graphene. Altogether our findings suggest an
alternative way to control quantum reflection at the nanoscale, and pave the
way for the design of alternative, magnetically tuned reflective diffraction
elements for matter waves.
