  In the diffraction-limited near-field propagation regime, free-space optical
quantum key distribution (QKD) systems can employ multiple spatial modes to
improve their key rate. Here, we analyze QKD using the non-orthogonal flat-top
focused beams. Although they suffer from a rate penalty, their ease of
implementation makes them an attractive alternative to the well-studied
orthonormal Laguerre-Gauss (LG) modes. Indeed, in the presence of turbulence,
the non-orthogonal modes may achieve higher QKD rate than the LG modes.
