– is the subject of this paper.

The polarization field can be decomposed uniquely on the full sky into a gradient part, with electric ($E$) parity, and a curl part with magnetic ($B$) parity [@kamionkowski97; @seljak97]. The cosmological importance of this decomposition is that linear density perturbations cannot produce magnetic polarization, and so, in standard scenarios and at linear order, large-angle $B$-mode polarization is produced solely by gravitational waves. Second-order effects, most notably weak gravitational lensing by large-scale structure, can generate $B$ modes from primary $E$ modes [@zaldarriaga98]. The lens-induced $B$ modes have a blue spectrum, and should dominate any primary signal from gravitational waves on sub-degree scales. On larger scales, they may still dominate if the gravitational wave amplitude is sufficiently low. Either way, the variance of the $B$ modes is expected to be much smaller than that of the $E$ modes. Given the different parity properties of the $E$ and $B$ fields, CMB polarization in a statistically-isotropic and parity-respecting universe is characterized by two power spectra, $C_l^E$ and $C_l^B$, and also the cross-power $C_l^{TE}$ between $E$ and the temperature anisotropies.

A number of different methods have been developed to estimate these CMB polarization power spectra. The optimal method, 