  Deuterons are the most abundant secondary cosmic ray species in the Galaxy,
but their study has been severely limited due to experimental challenges. In an
era with new experiments and high-precision measurements in cosmic rays, having
a low-uncertainty deuteron flux in a wide energy range becomes possible. The
deuteron-over-helium ratio ($d$/$^4$He) is important to understand the
propagation of cosmic rays in the Galaxy and in the heliosphere, complementing
observations with heavier nuclei like the boron-to-carbon ratio. In this work,
the most up-to-date results of the deuteron flux and the $d$/$^4$He ratio at
the top of the atmosphere have been obtained using {\tt GALPROP} and a 3D solar
modulation model. It was found that the simulation describes the deuteron flux
and $d$/$^4$He data below 1\,GeV/$n$ within the uncertainties of the model.
However, the model underestimates the best-published measurements available at
high energy. This discrepancy suggests different effective diffusions have to
be considered between secondary light species like deuterons and heavier
nuclei. Either this is a consequence of a break in the universality of
propagation between light and heavier nuclei or a lack of precision
measurements, it is something AMS-02 will help to resolve in the near future.
