• B-modes
• CMB
• LSPE
• multiplexing
• SQUID
• TES

The cosmic microwave background (CMB) is a black-body radiation with T ≈ 2.72 K, permeating our universe isotropically and homogeneously to a level of ΔT/T = 10 μK, originated at the time of recombination of matter. The CMB has non-zero polarization because of Thomson scattering of photons with the electrons of the primordial plasma before recombination. CMB polarization can be decomposed into a curl-free component (called E-mode) and a divergence-free component (called B-mode). Inflation causes scalar and tensor perturbations to the metric. Scalar (density) perturbations produce only E-modes in the CMB, while tensor perturbations (gravity waves) produce both E-modes and B-modes. The amplitude of tensor perturbations is parametrized by the tensor-to-scalar ratio r. B-modes due to primordial gravity waves have not been detected so far. A detection of B-modes would therefore be a smocking gun for inflation and would give a tremendous hint of the possible GUT energy scale.
The Large Scale Polarization Explorer (LSPE) is a balloon-borne experiment which aims at measuring CMB polarization at large angular scales, and in particular the B-modes of CMB polarization. The Short Wavelength Instrument of the Polarization Explorer (SWIPE) will exploit an array of ≈ 300 TES bolometers to measure the CMB polarization.
TES bolometers are the most competitive CMB detector technology available. I describe their principle of operation and use simulated data to show that they are able to measure temperature variations down to the order of 100 nK, which is necessary to detect B-modes. I also show with a simulation how the spider-web geometry of the TES minimizes the interaction with cosmic rays, facilitating the removal of cosmic ray events from the data stream. The readout of small current signals produced by TES bolometers is realized using a SQUID, which amplifies the current signal and converts it into a voltage signal. I describe how I measured the V-Φ and V-I characteristic of the SQUID and its transimpedance. To increase the noise performance and achieve the necessary sensitivity to detect such small cosmological signals, the sensors are operated at sub-Kelvin temperatures and multiplexed. Frequency domain multiplexing is obtained placing in series with each sensor a superconducting LC filter. I describe in detail the design and production processes of the superconducting filters realized at the INFN Laboratories of Genoa and at the NEST facility in Pisa, as well as the preliminary tests performed at the INFN Laboratories of Pisa.