  The search for Population III (Pop III) stars has fascinated and eluded
astrophysicists for decades. One promising place for capturing evidence of
their presence must be high-redshift objects; signatures should be recorded in
their characteristic chemical abundances. We deduce the Fe and Mg abundances of
the broad-line region (BLR) from the intensities of ultraviolet Mg II and Fe II
emission lines in the near-infrared spectrum of UKIDSS Large Area Survey (ULAS)
J1342+0928 at $z = 7.54$, by advancing our novel flux-to-abundance conversion
method developed for $z\sim 1$ quasars. We find that the BLR of this quasar is
extremely enriched, by a factor of 20 relative to the solar Fe abundance,
together with a very low Mg/Fe abundance ratio: $[\mathrm{Fe/H}]=+1.36\pm0.19$
and $[\mathrm{Mg/Fe}]=-1.11\pm0.12$, only 700 million years after the Big Bang.
We conclude that such an unusual abundance feature cannot be explained by the
standard view of chemical evolution that considers only the contributions from
canonical supernovae. While there remains uncertainty in the high-mass end of
the Pop III IMF, here we propose that the larger amount of iron in ULAS
J1342+0928 was supplied by a pair-instability supernova (PISN) caused by the
explosion of a massive Pop III star in the high-mass end of the possible range
of 150-300 $M_\odot$. Chemical-evolution models based on initial PISN
enrichment well explain the trend in [Fe/Mg]-$z$ all the way from $z < 3$ to $z
= 7.54$. We predict that stars with very low [Mg/Fe] at all metallicities are
hidden in the Galaxy, and they will be efficiently discovered by ongoing
new-generation photometric surveys.
