  The abundances of metals in the intergalactic medium (IGM) can be used to
constrain the amount of star formation at high redshift and the spectral shape
of the ionizing background radiation. For both purposes it is essential to
measure the abundances in regions of low density, away from local sources of
metals and ionizing photons. Here we report the first detection of OVI in the
low-density IGM at high redshift. We perform a pixel-by-pixel search for OVI
absorption in eight high quality quasar spectra spanning the redshift range z =
2.0-4.5. At 2 ~< z ~< 3, we clearly detect OVI in the form of a positive
correlation between the HI Ly-alpha optical depth and the optical depth in the
corresponding OVI pixel, down to an HI optical depth of 0.1. This is an order
of magnitude lower in HI optical depth than the best CIV measurements can probe
and constitutes the first clear detection of metals in underdense gas. The
non-detection of OVI at z > 3 is consistent with the enhanced photoionization
from a hardening of the UV background below z ~ 3 but could also be caused by
the high level of contamination from Ly series lines.
