Constraints of $\Delta\mu/\mu$ based on H$_2$ observations in QSO
  spectra at high redshifts
  This article summarizes the latest results on the proton-to-electron mass
ratio $\mu$ derived from H$_2$ observations at high redshift in the light of
possible variations of fundamental physical constants. The focus lies on UVES
observations of the past years as enormous progress was achieved since the
first positive results on $\Delta\mu/\mu$ were published. With the better
understanding of systematics, dedicated observation runs, and numerous
approaches to improve wavelength calibration accuracy, all current findings are
in reasonable good agreement with no variation and provide an upper limit of
$\Delta\mu/\mu < 1\times 10^{-5}$ for the redshift range of $2 <$ z $< 3$.
