• high-redshift quasar
• early Universe
• dark ages
• reionization

From CMB measurements, cosmic reionization is expected to happen at redshift around 7-8.
However, the details of the process are still very uncertain.
Quasar absorption spectra are one of the main probes of the reionization epoch, and in particular of its final phases.
The main observables are the evolution of the neutral hydrogen optical depth with redshifts, the properties of the completely absorbed regions and also the ionization status of the metal absorption
lines.
The XQR-30 survey collects intermediate resolution and high signal-to-noise X-Shooter spectra of 30 quasars at 5.8 < z < 6.6.
In this thesis, we will present new upper limits on the neutral hydrogen fraction obtained with the model independent method of the “dark pixel fraction” applied to a sample of more than 40 quasar spectra at high signal-to-noise ratio comprising XQR-30 and other objects from the literature.