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Understanding the cosmic reionization is one of the key goals of the modern observational cosmology. High‐redshift quasi‐stellar object (QSO) spectra can be used as background light sources to measure the absorption by the intervening neutral hydrogen. We investigate the neutral hydrogen absorption in a deep, moderate‐resolution Keck/Deimos spectrum of the QSO CFHQS J2329−0301 at  z  = 6.4. This QSO is one of the highest redshift QSOs presently known at  z  = 6.4 but is 2.5 mag fainter than a previously well‐studied QSO SDSS J1148+5251 at  z  = 6.4. Therefore, it has a smaller Stromgren sphere and allows us to probe the highest redshift hydrogen absorption to date. The average transmitted flux at 5.915 &lt;  z  abs  &lt; 6.365 (200 comoving Mpc) is consistent with zero in Lyα, Lyβ and Lyγ absorption measurements. This corresponds to the lower limit of the optical depth,  τ α  eff  &gt; 4.9 . These results are consistent with the strong evolution of the optical depth at  z  &gt; 5.7.

A Gunn–Peterson test with a QSO at z = 6.4