  A recent paper reported elliptically polarized high-order harmonics from
aligned N$_2$ using a linearly polarized driving field [X. Zhou \emph{et al.},
Phys. Rev. Lett. \textbf{102}, 073902 (2009)]. This observation cannot be
explained in the standard treatment of the Lewenstein model and has been
ascribed to many-electron effects or the influence of the Coulomb force on the
continuum electron. We show that non-vanishing ellipticity naturally appears
within the Lewenstein model when using a multi-center stationary phase method
for treating the dynamics of the continuum electron. The reason for this is the
appearance of additional contributions, that can be interpreted as quantum
orbits in which the active electron is ionized at one atomic center within the
molecule and recombines at another. The associated exchange harmonics are
responsible for the non-vanishing ellipticity and result from a correlation
between the ionization site and the recombination site in high-order harmonic
generation.
