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We have obtained spectra of 163 quasars at $z_\mathrm{em}>4.4$ with theGemini Multi Object Spectrometers on the Gemini North and South telescopes, thelargest publicly available sample of high-quality, low-resolution spectra atthese redshifts. From this homogeneous data set, we generated stacked quasarspectra in three redshift intervals at $z\sim 5$. We have modelled the fluxbelow the rest-frame Lyman limit ($\lambda_\mathrm{r}<912$\AA) to assess themean free path $\lambda_\mathrm{mfp}^{912}$ of the intergalactic medium toHI-ionizing radiation. At mean redshifts $z_\mathrm{q}=4.56$, 4.86 and 5.16, wemeasure $\lambda_\mathrm{mfp}^{912}=(22.2\pm 2.3, 15.1\pm 1.8, 10.3\pm1.6)h_{70}^{-1}$ proper Mpc with uncertainties dominated by sample variance.Combining our results with $\lambda_\mathrm{mfp}^{912}$ measurements from lowerredshifts, the data are well modelled by a simple power-law$\lambda_\mathrm{mfp}^{912}=A[(1+z)/5]^\eta$ with $A=(37\pm 2)h_{70}^{-1}$ Mpcand $\eta = -5.4\pm 0.4$ between $z=2.3$ and $z=5.5$. This rapid evolutionrequires a physical mechanism -- beyond cosmological expansion -- which reducesthe cosmic effective Lyman limit opacity. We speculate that the majority of HILyman limit opacity manifests in gas outside galactic dark matter haloes,tracing large-scale structures (e.g. filaments) whose average density (andconsequently neutral fraction) decreases with cosmic time. Our measurements ofthe strongly redshift-dependent mean free path shortly after the completion ofHI reionization serve as a valuable boundary condition for numerical modelsthereof. Having measured $\lambda_\mathrm{mfp}^{912}\approx 10$ Mpc at $z=5.2$,we confirm that the intergalactic medium is highly ionized by that epoch andthat the redshift evolution of the mean free path does not show a break thatwould indicate a recent end to HI reionization.

The Giant Gemini GMOS survey of z>4.4 quasars - I. Measuring the mean free path across cosmic time