sufficient optical depth to CMB photons due to free electrons in the IGM after reionization, some of the angular anisotropy in the unpolarized intensity can be converted to polarized anisotropy. This produces a peak in the polarization power spectrum at the angular scale size equivalent to the horizon at reionization with an amplitude proportional to the optical depth [@1997ApJ...488....1Z]. Measurements by the WMAP satellite of these effects indicate that the redshift of reionization is $z_r\approx11\pm4$ [@2007ApJS..170..377S], assuming an instantaneous transition.

Lyman-$\alpha$ absorption by neutral hydrogen is visible in the spectra of many high-redshift quasars and, thus, offers the second currently feasible probe of the ionization history of the IGM. Continuum emission from quasars is redshifted as it travels through the expanding Universe to the observer. Neutral hydrogen along the line of sight creates absorption features in the continuum at wavelengths corresponding to the local rest-frame wavelength of the Lyman-$\alpha$ line. Whereas CMB measurements place an integrated constraint on reionization, quasar absorption line studies are capable of probing the ionization history in detail along the sight-lines. There is a significant limitation to this approach, however. The Lyman-$\alpha$ absorption saturates at very low fractions of neutral hydrogen (of order $x_{HI} \approx 10^{-4}$). 