The Q & U Bolometric Interferometer for Cosmology (QUBIC) is a novel kind of polarimeter optimized for the measurement of the B-mode polarization of the Cosmic Microwave Background (CMB), which is one of the major challenges of observational cosmology. The signal is expected to be of the order of a few tens of nK, prone to instrumental systematic effects and polluted by various astrophysical foregrounds which can only be controlled through multichroic observations. QUBIC is designed to address these observational issues with a novel approach that combines the advantages of interferometry in terms of control of instrumental systematic effects with those of bolometric detectors in terms of wide-band, background-limited sensitivity. The QUBIC synthesized beam has a frequency-dependent shape that results in the ability to produce maps of the CMB polarization in multiple sub-bands within the two physical bands of the instrument (150 and 220 GHz). These features make QUBIC complementary to other instruments and makes it particularly well suited to characterize and remove Galactic foreground contamination. In this article, first of a series of eight, we give an overview of the QUBIC instrument design, the main results of the calibration campaign, and present the scientific program of QUBIC including not only the measurement of primordial B-modes, but also the measurement of Galactic foregrounds. We give forecasts for typical observations and measurements: with three years of integration on the sky and assuming perfect foreground removal as well as stable atmospheric conditions from our site in Argentina, our simulations show that we can achieve a statistical sensitivity to the effective tensor-to-scalar ratio (including primordial and foreground B-modes) σ(r)=0.015.

Bolometric interferometry is a novel technique that has the ability to perform spectral imaging. A bolometric interferometer observes the sky in a wide frequency band and can reconstruct sky maps in several sub-bands within the physical band in post-processing of the data. This provides a powerful spectral method to discriminate between the cosmic microwave background (CMB) and astrophysical foregrounds. In this paper, the methodology is illustrated with examples based on the Q & U Bolometric Interferometer for Cosmology (QUBIC) which is a ground-based instrument designed to measure the B-mode polarization of the sky at millimeter wavelengths. We consider the specific cases of point source reconstruction and Galactic dust mapping and we characterize the point spread function as a function of frequency. We study the noise properties of spectral imaging, especially the correlations between sub-bands, using end-to-end simulations together with a fast noise simulator. We conclude showing that spectral imaging performance are nearly optimal up to five sub-bands in the case of QUBIC.

The Q and U Bolometric Interferometer for Cosmology (QUBIC) is a ground-based experiment that aims to detect B-mode polarization anisotropies [1] in the CMB at angular scales around the ℓ ≃100 recombination peak. Systematic errors make ground-based observations of B modes at millimetre wavelengths very challenging and QUBIC mitigates these problems in a somewhat complementary way to other existing or planned experiments using the novel technique of bolometric interferometry. This technique takes advantage of the sensitivity of an imager and the systematic error control of an interferometer. A cold reflective optical combiner superimposes the re-emitted beams from 400 aperture feedhorns on two focal planes. A shielding system composed of a fixed groundshield, and a forebaffle that moves with the instrument, limits the impact of local contaminants. The modelling, design, manufacturing and preliminary measurements of the optical components are described in this paper.

We report on an extensive test campaign of a prototype version of the QUBIC (Q & U Bolometric Interferometer for Cosmology) instrument, carried out at Astroparticle Physics and Cosmology (APC) in Paris. Exploiting the novel concept called bolometric interferometry, QUBIC is designed to measure the CMB polarization at 150 and 220 GHz from a high altitude site at Alto Chorillo, Argentina. The prototype model called QUBIC Technological Demonstrator (QUBIC-TD) operates in a single frequency band (150 GHz) and with a reduced number of baselines, but it contains all the elements of the QUBIC instrument in its final configuration. The test campaign included measurements of the synthesized beam and of the polarization performance, as well as a verification of the interference fringe pattern. A modulated, frequency-tunable millimetre-wave source was placed in the telescope far-field and was used to simulate a point source. The QUBIC-TD field of view was scanned across the source to produce beam maps. Our measurements confirm the frequency-dependent behaviour of the beam profile, which gives QUBIC the possibility to do spectral imaging. The measured polarization performance indicates a cross-polarization leakage less than 0.6%. We also successfully tested the polarization modulation system, which is provided by a rotating half wave plate. We demonstrate the full mapmaking pipeline using data from this measurement campaign, effectively giving an end-to-end checkout of the entire QUBIC system, including all hardware subsystems, their interfaces, and the software to operate the whole system and run the analysis. Our results confirm the viability of bolometric interferometry for measurements of the CMB polarization.

A prototype version of the Q & U bolometric interferometer for cosmology (QUBIC) underwent a campaign of testing in the laboratory at Astroparticle Physics and Cosmology laboratory in Paris (APC). The detection chain is currently made of 256 NbSi transition edge sensors (TES) cooled to 320 mK. The readout system is a 128:1 time domain multiplexing scheme based on 128 SQUIDs cooled at 1 K that are controlled and amplified by a SiGe application specific integrated circuit at 40 K. We report the performance of this readout chain and the characterization of the TES. The readout system has been functionally tested and characterized in the lab and in QUBIC. The low noise amplifier demonstrated a white noise level of 0.3 nV/√Hz. Characterizations of the QUBIC detectors and readout electronics includes the measurement of I-V curves, time constant and the noise equivalent power. The QUBIC TES bolometer array has approximately 80% detectors within operational parameters. It demonstrated a thermal decoupling compatible with a phonon noise of about 5 × 10-17 W/√Hz at 410 mK critical temperature. While still limited by microphonics from the pulse tubes and noise aliasing from readout system, the instrument noise equivalent power is about 2 × 10-16 W/√Hz, enough for the demonstration of bolometric interferometry.