While the theoretical case for observing polarization is strong, it is a difficult experimental task to observe signals of the low level of several K and below. Nonetheless, polarization experiments have one potential advantage over temperature anisotropy experiments. They can reduce atmospheric emission effects by differencing the polarization states on the same patch of sky instead of physically chopping between different angles on the sky since atmospheric emission is thought to be nearly unpolarized (see §5.2). However, to be successful an experiment must overcome a number of systematic effects, many of which are discussed in [Keating et al.] (1997). It must at least balance the sensitivity of the instrument to the orthogonal polarization channels (including the far side lobes) to nearly 8 orders of magnitude. Multiple levels of switching and a very careful design are minimum requirements.
To date the experimental upper limits on polarization of the CMB have been at least an order of magnitude larger than the theoretical expectations. The original polarization limits go back to Penzias & Wilson (1965) who set a limit of 10% on the polarization of the CMB. There have been several subsequent upper limits which have now reached the level of K (see Tab. 1), about a factor of 5-10 above the predicted levels for popular models (see Tab. 2).