  We use archival WISE and Spitzer photometry to derive optical line fluxes for
a sample of distant quasars at z ~6. We find evidence for exceptionally high
equivalent width [OIII] emission (rest-frame EW ~400 {\AA}) similar to that
inferred for star-forming galaxies at similar redshifts. The median Halpha and
Hbeta equivalent widths are derived to be ~400{\AA} and 100~{\AA},
respectively, and are consistent with values seen among quasars in the local
Universe, and at z ~2. After accounting for the contribution of photoionization
in the broad line regions of quasars, we suggest that the OIII emission
corresponds to strong, narrow line emission likely arising from feedback due to
massive star-formation in the quasar host. The high [OIII]/Hbeta line ratios
can uniquely be interpreted with radiative shock models, and translate to
magnetic field strengths of ~8 microGauss with shock velocities of ~400km/s.
Our measurement implies that strong, coherent magnetic fields were present in
the interstellar medium at a time when the universe was < 1 billion years old.
Comparing our estimated magnetic field strengths with models for the evolution
of galaxy-scale fields, favors high seed field strengths exceeding 0.1
microGauss, the first observational constraint on such fields. This high value
favors scenarios where seed magnetic fields were produced by turbulence in the
early stages of galaxy formation. Forthcoming mid-infrared spectroscopy with
the James Webb Space Telescope will help constrain the physical conditions in
quasar hosts further.
