QUBIC - Bolometric Interferometry for the search of the Cosmic Microwave Background B polarization – QUBIC

The QUBIC project is a novel instrumental concept designed for the search for B-polarization in the Cosmic Microwave Background radiation (CMB). This radiation is expected from tensor perturbations of the metric (primordial gravitational waves) generated at the epoch of inflation. Its discovery would be a major breackthrough in cosmology, opening a new window towards the primordial Universe. The B-mode level is directly related to the tensor-to-scalar ratio, which value would allow to distinguish between the many inflation models compatible with the data available today. The quest for B-modes is one of the main challenges of today's observational cosmology and a number of teams (especially in the USA) are proposing or running dedicated instruments (imagers). QUBIC is the only collaboration in Europe searching for B-modes.

QUBIC is the first bolometric interferometer, combining the advantages of bolometric detectors in terms of sensitivity (background limited noise) to those of interferometry in terms of control of systematic effects. QUBIC will comprise in its full version 6 modules of 400 primary horns each. A module consists in a set of 400 horns placed on a squared array behind the optical window of a cryostat. This array of horns selects the baselines (Fourier modes) of the interferometer, a milimeter-wave version of the Fizeau interferometer with 400 apertures.The signal of the input horns is re-emited towards an optical combiner inside the cryostat using back-horns. A rotating half-wave plate modulates the signal in polarization just after the back-horns. Finally, a polarizing grid splits the signal into X and Y polarizations, each being focused on a focal plane equipped with bolometers.

In traditional interferometry, the direct observables are the visibilties whose Inverse Fourier Transform is the synthesized image. Our observable is directly the synthesized image, obtained for each of the Q and U Stokes parameters after demodulation of the Half-wave plate rotation. Our instrument can therefore be used as a synthesis imager, allowing to scan the sky (and recover the I Stokes parameter), make maps using mapmaking techniques, and obtain angular power spectra from these maps using standard CMB data analysis techniques. Our beam is however significantly different from that of a direct imager (formed by the horn field of view and by the telescope) as it comes from the baselines (Fourier modes) selected by the array of input horns. The sensitivity of such an instrument is comparable to that of a direct imager while allowing to a better control of systematics thanks to its interferometric nature.

The synthesized beam can be known to extreme accuracy in our instrumental setup using tow different techniques. The first one is the traditional one, observing a point source (external calibrator) with the instrument and mapping the beam that way. The second is specific to our instrument and is completeley based on its interferometric nature: we observe the calibration source with each of the horn pairs available in the setup (using waveguide switches to alternatively close some of the horns) and the use of our maximal redundancy allows us to reconstruct the systematic effects (relative gains, polarization coupling, location of the horns) for each channel separately (a channel being a horn and a bolometer). These parameters are accounted for in the data analysis therefore allowing for an extra-level of control over the systematics with respect to traditional imagers.

The present application aims at funding the french contribution the first module of the QUBIC instrument (allowing to constrain a tensor/scalar ratio 0.05 at 95% Cl.L. in one year) comprising 400 primary horns at 150 GHz, two focal planes with 1024 TES sensors. France is leading the project and we are involved in the data analysis and simulation, detection chain, horns and waveguide switches fabrication and final integration.