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

Insights into physical conditions and magnetic fields from high redshift quasars