Hydrogen reionization ends by z = 5.3: Lyman-α optical depth measured by the XQR-30 sample

The presence of excess scatter in the Ly-alpha forest at z similar to 5.5, together with the existence of sporadic extended opaque Gunn-Peterson troughs, has started to provide robust evidence for a late end of hydrogen reionization. However, low data quality and systematic uncertainties complicate the use of Ly-alpha transmission as a precision probe of reionization's end stages. In this paper, we assemble a sample of 67 quasar sightlines at z > 5.5 with high signal-to-noise ratios of >10 per <= 15 km s(-1) spectral pixel, relying largely on the new XQR-30 quasar sample. XQR-30 is a large program on VLT/X-Shooter which obtained deep (SNR > 20 per pixel) spectra of 30 quasars at z > 5.7. We carefully account for systematics in continuum reconstruction, instrumentation, and contamination by damped Ly-alpha systems. We present improved measurements of the mean Ly-alpha transmission over 4.9 < z < 6.1. Using all known systematics in a forward modelling analysis, we find excellent agreement between the observed Ly-alpha transmission distributions and the homogeneous-UVB simulations Sherwood and Nyx up to z <= 5.2 (<1 sigma), and mild tension (similar to 2.5 sigma) at z = 5.3. Homogeneous UVB models are ruled out by excess Ly-alpha transmission scatter at z >= 5.4 with high confidence (>3.5 sigma). Our results indicate that reionization-related fluctuations, whether in the UVB, residual neutral hydrogen fraction, and/or IGM temperature, persist in the intergalactic medium until at least z = 5.3 (t = 1.1 Gyr after the big bang). This is further evidence for a late end to reionization.