Precise limits on cosmological variability of the fine-structure
  constant with zinc and chromium quasar absorption lines
  The strongest transitions of Zn and CrII are the most sensitive to relative
variations in the fine-structure constant ($\Delta\alpha/\alpha$) among the
transitions commonly observed in quasar absorption spectra. They also lie
within just 40\AA\ of each other (rest frame), so they are resistant to the
main systematic error affecting most previous measurements of
$\Delta\alpha/\alpha$: long-range distortions of the wavelength calibration.
While Zn and CrII absorption is normally very weak in quasar spectra, we
obtained high signal-to-noise, high-resolution echelle spectra from the Keck
and Very Large Telescopes of 9 rare systems where it is strong enough to
constrain $\Delta\alpha/\alpha$ from these species alone. These provide 12
independent measurements (3 quasars were observed with both telescopes) at
redshifts 1.0--2.4, 11 of which pass stringent reliability criteria. These 11
are all consistent with $\Delta\alpha/\alpha=0$ within their individual
uncertainties of 3.5--13 parts per million (ppm), with a weighted mean
$\Delta\alpha/\alpha = 1.2\pm1.7_{\rm stat}\pm0.9_{\rm sys}$ ppm (1$\sigma$
statistical and systematic uncertainties), indicating no significant
cosmological variations in $\alpha$. This is the first statistical sample of
absorbers that is resistant to long-range calibration distortions (at the $<$1
ppm level), with a precision comparable to previous large samples of $\sim$150
(distortion-affected) absorbers. Our systematic error budget is instead
dominated by much shorter-range distortions repeated across echelle orders of
individual spectra.
