  Upcoming cosmic microwave background (CMB) experiments will measure
temperature fluctuations on small angular scales with unprecedented precision.
Small-scale CMB fluctuations are a mixture of late-time effects: gravitational
lensing, Doppler shifting of CMB photons by moving electrons (the kSZ effect),
and residual foregrounds. We propose a new statistic which separates the kSZ
signal from the others, and also allows the kSZ signal to be decomposed in
redshift bins. The decomposition extends to high redshift, and does not require
external datasets such as galaxy surveys. In particular, the high-redshift
signal from patchy reionization can be cleanly isolated, enabling future CMB
experiments to make high-significance and qualitatively new measurements of the
reionization era.
