Measuring CMB polarisation with the Planck HFI

The Planck High Frequency Instrument (HFI) is the most sensitive instrument currently being built for the measurement of Cosmic Microwave Background anisotropies. In addition to unprecendented sensitivity to CMB temperature fluctuations, the HFI has polarisation-sensitive detectors in 3 frequency channels (143, 217 and 353 GHz), which will constrain full-sky polarised emission of the CMB and foregrounds at these frequencies. The sensitivity of the instrument will allow a clear detection of CMB polarisation signals and should yield a precise measurement of its power spectrum at all angular scales between l= 50 and l= 1000, as well as constraints on the polarised emission at larger scales where a polarised signal from inflationary gravity waves or from reionisation is expected in many cosmological scenarios.