CMBPol Mission Concept Study: Reionization Science with the Cosmic   Microwave Background
  We summarize existing constraints on the epoch of reionization and discuss the observational probes that are sensitive to the process. We focus on the role large scale polarization can play. Polarization probes the integrated optical depth across the entire epoch of reionization. Future missions such as Planck and CMBPol will greatly enhance our knowledge of the reionization history, allowing us to measure the time evolution of the ionization fraction. As large scale polarization probes high redshift activity, it can best constrain models where the Universe was fully or partially ionized at early times. In fact, large scale polarization could be our only probe of the highest redshifts. 
Matias Zaldarriaga, Loris Colombo, Eiichiro Komatsu, Adam Lidz,   Michael Mortonson, S. Peng Oh, Elena Pierpaoli, Licia Verd...