  The GBAR experiment will time the free fall of cold antihydrogen atoms
dropped onto an annihilation plate to test the universality of free fall on
antimatter. In this contribution, we study the quantum reflection of the
anti-atom resulting from the Casimir-Polder attraction to the plate. We
evaluate the Casimir-Polder potential and the associated quantum reflection
amplitudes and find that reflection is enhanced for weaker potentials. A
Liouville transformation of the Schr\"odinger equation is used to map the
quantum reflection problem onto an equivalent problem of scattering on a
barrier, leading to an intuitive understanding of the phenomenon.
