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.

We present the design, manufacturing and performance of the horn-switch system developed for the technological demonstrator of QUBIC (the Q&U Bolometric Interferometer for Cosmology). This system consists of 64 back-to-back dual-band (150 GHz and 220 GHz) corrugated feed-horns interposed with mechanical switches used to select desired baselines during the instrument self-calibration. We manufactured the horns in aluminum platelets milled by photo-chemical etching and mechanically tightened with screws. The switches are based on steel blades that open and close the waveguide between the back-to-back horns and are operated by miniaturized electromagnets. The measured electromagnetic performance of the feedhorns agrees with simulations. In particular we obtained a return loss around –20 dB up to 230 GHz and beam patterns in agreement with single-mode simulations down to –30 dB. The switches for this prototype were designed and built for the 150 GHz band. In this frequency range we find return and insertion losses consistent with expectations (< –25 dB and ∼ –0.1 dB, respectively) and an isolation larger than 70 dB. In this paper we also show the current development status of the feedhorn-switch system for the QUBIC full instrument, based on an array of 400 horn-switch assemblies.

Current experiments aimed at measuring the polarization of the Cosmic Microwave Background (CMB) use cryogenic detector arrays with cold optical systems to boost their mapping speed. For this reason, large volume cryogenic systems with large optical windows, working continuously for years, are needed. The cryogenic system of the QUBIC (Q & U Bolometric Interferometer for Cosmology) experiment solves a combination of simultaneous requirements: very large optical throughput (∼40 cm2sr), large volume (∼1 m3) and large mass (∼165 kg) of the cryogenic instrument. Here we describe its design, fabrication, experimental optimization and validation in the Technological Demonstrator configuration. The QUBIC cryogenic system is based on a large volume cryostat that uses two pulse-tube refrigerators to cool the instrument to ∼3 K. The instrument includes the cryogenic polarization modulator, the corrugated feedhorn array, and the lower temperature stages: a 4He evaporator cooling the interferometer beam combiner to ∼1 K and a 3He evaporator cooling the focal-plane detector arrays to ∼0.3 K. The cryogenic system has been tested and validated for more than 6 months of continuous operation. The detector arrays have reached a stable operating temperature of 0.33 K, while the polarization modulator has operated at a ∼10 K base temperature. The system has been tilted to cover the boresight elevation range 20°-90° without significant temperature variations. The instrument is now ready for deployment to the high Argentinean Andes.

Setting an upper limit or detection of B-mode polarization imprinted by gravitational waves from Inflation is one goal of modern large angular scale cosmic microwave background (CMB) experiments around the world. A great effort is being made in the deployment of many ground-based, balloon-borne and satellite experiments, using different methods to separate this faint polarized component from the incoming radiation. QUBIC exploits one of the most widely-used techniques to extract the input Stokes parameters, consisting in a rotating half-wave plate (HWP) and a linear polarizer to separate and modulate polarization components. QUBIC uses a step-by-step rotating HWP, with 15° steps, combined with a 0.4°s-1 azimuth sky scan speed. The rotation is driven by a stepper motor mounted on the cryostat outer shell to avoid heat load at internal cryogenic stages. The design of this optical element is an engineering challenge due to its large 370 mm diameter and the 8 K operation temperature that are unique features of the QUBIC experiment. We present the design for a modulator mechanism for up to 370 mm, and the first optical tests by using the prototype of QUBIC HWP (180 mm diameter). The tests and results presented in this work show that the QUBIC HWP rotator can achieve a precision of 0.15° in position by using the stepper motor and custom-made optical encoder. The rotation induces <5.0 mW (95% C.L) of power load on the 4 K stage, resulting in no thermal issues on this stage during measurements. We measure a temperature settle-down characteristic time of 28 s after a rotation through a 15° step, compatible with the scanning strategy, and we estimate a maximum temperature gradient within the HWP of ≤ 10 mK. This was calculated by setting up finite element thermal simulations that include the temperature profiles measured during the rotator operations. We report polarization modulation measurements performed at 150 GHz, showing a polarization efficiency >99% (68% C.L.) and a median cross-polarization χPol  of 0.12%, with 71% of detectors showing a χPol + 2σ upper limit <1%, measured using selected detectors that had the best signal-to-noise ratio.