  We introduce a new statistical technique for extracting the inhomogeneous
reionization signal from future high-sensitivity measurements of the cosmic
microwave background temperature and polarization fields. If reionization is
inhomogeneous, then the optical depth to recombination will be a function
tau(n) of position on the sky. Anisotropies in tau(n) alter the statistics of
the observed CMB via several physical mechanisms: screening of the surface of
last scattering, generation of new polarization via Thomson scattering from
reionization bubbles, and the kinetic Sunyaev-Zel'dovich effect. We construct a
quadratic estimator for the modes of the tau field. This estimator separates
the patchy reionization signal from the CMB in the form of a noisy map, which
can be cross-correlated with other probes of reionization or used as a
standalone probe. A future satellite experiment with sufficient sensitivity and
resolution to measure the lensed B-mode on most of the sky can constrain key
parameters of patchy reionization, such as the duration of the patchy epoch or
the mean bubble radius, at the ~10% level.
