Development of the polarization modulator and multi-mode receivers for the search of CMB polarization

The most ambitious challenge in Experimental Cosmology today is the accurate measurement of the polarized signal of the Cosmic Microwave Background (CMB). This paper is focused on two of the critical instrumental challenges in the polarization measurement which forthcoming experiments like LSPE/SWIPE and LiteBIRD will deal with: the receivers and the polarization modulation system. SWIPE is one of the two instruments of the Large Scale Polarization Explorer (LSPE) which aims to measure the CMB polarization at large angular scale with a goal of tensor to scalar ratio r = 0.01. The detection of this tiny signal requires a very large array of polarization-sensitive detectors coupled to an imaging optical system to obtain a wide field of view. The development and testing status of a multi-mode pixel assembly is presented. In the second part of the paper we present the status of the polarization modulator. This system offers a solution to disentangle the polarized signal of interest from these unpolarized foregrounds and a rapidly-rotating half-wave plate is the most promising scheme. The LSPE/SWIPE polarization modulator unit operates at 1.6 K and is based on magnetic levitation which can significantly reduce the friction. A large number of tests were performed on a room temperature mockup and the optimized configuration is presented. A polarization modulator similar to the LSPE/SWIPE one will be used in the LiteBIRD mission, the next generation satellite mission planned to be operative in ∼ 10 years, which will map CMB polarization 20 times deeper than Planck, with a total error of δr < 0.001. The development of the two polarization modulator units is more challenging than for SWIPE due to the spacecraft requirements and the Technology Readiness Level (TRL) required.