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The mean free path of ionizing photons, $\lambda_{\rm mfp}$, is a key factorin the photoionization of the intergalactic medium (IGM). At $z \gtrsim 5$,however, $\lambda_{\rm mfp}$ may be short enough that measurements towards QSOsare biased by the QSO proximity effect. We present new direct measurements of$\lambda_{\rm mfp}$ that address this bias and extend up to $z \sim 6$ for thefirst time. Our measurements at $z \sim 5$ are based on data from the GiantGemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs. At $z\sim 6$ we use QSO spectra from Keck ESI and VLT X-Shooter. We measure$\lambda_{\rm mfp} = 9.09^{+1.62}_{-1.28}$ proper Mpc and$0.75^{+0.65}_{-0.45}$ proper Mpc (68% confidence) at $z = 5.1$ and 6.0,respectively. The results at $z = 5.1$ are consistent with existingmeasurements, suggesting that bias from the proximity effect is minor at thisredshift. At $z = 6.0$, however, we find that neglecting the proximity effectbiases the result high by a factor of two or more. Our measurement at $z = 6.0$falls well below extrapolations from lower redshifts, indicating rapidevolution in $\lambda_{\rm mfp}$ over $5 < z < 6$. This evolution disfavorsmodels in which reionization ended early enough that the IGM had time to fullyrelax hydrodynamically by $z = 6$, but is qualitatively consistent with modelswherein reionization completed at $z = 6$ or even significantly later. Our meanfree path results are most consistent with late reionization models wherein theIGM is still 20% neutral at $z=6$, although our measurement at $z = 6.0$ iseven lower than these models prefer.

The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization