QUBIC: the Q and U bolometric interferometer for cosmology

O'SULLIVAN, C. ET AL.; C. G. SCOCCOLA

Texas

Conferencia; SPIE Astronomical Telescopes & Instrumentation coference; 2018

QUBIC, the Q & U Bolometric Interferometer for Cosmology, is a novelground-based instrument that has been designed to measure the extremelyfaint B-mode polarisation anisotropy of the cosmic microwave backgroundat intermediate angular scales (multipoles of 𝑙 = 30 - 200).Primordial B-modes are a key prediction of Inflation as they can only beproduced by gravitational waves in the very early universe. To achievethis goal, QUBIC will use bolometric interferometry, a technique thatcombines the sensitivity of an imager with the systematic error controlof an interferometer. It will directly observe the sky through an arrayof 400 back-to-back entry horns whose signals will be superimposed usinga quasi-optical beam combiner. The resulting interference fringes willbe imaged at 150 and 220 GHz on two focal planes, each tiled with NbSiTransition Edge Sensors, cooled to 320 mK and read out with time-domainmultiplexing. A dichroic filter placed between the optical combiner andthe focal planes will select the two frequency bands. A very largereceiver cryostat will cool the optical and detector stages to 40 K, 4K, 1 K and 320 mK using two pulse tube coolers, a novel 4He sorptioncooler and a double-stage 3He/4He sorption cooler. Polarisationmodulation and selection will be achieved using a cold stepped half-waveplate (HWP) and polariser, respectively, in front of the sky-facinghorns. A key feature of QUBIC's ability to control systematic effects isits `self-calibration' mode where fringe patterns from individualequivalent baselines can be compared. When observing, however, all thehorns will be open simultaneously and we will recover a synthetic imageof the sky in the I, Q and U Stokes' parameters. The synthesised beampattern has a central peak of approximately 0.5 degrees in width, withsecondary peaks further out that are damped by the 13-degree primarybeam of the horns. This is Module 1 of QUBIC which will be installed inArgentina, near the city of San Antonio de los Cobres, at the AltoChorrillos site (4869 m a.s.l.), Salta Province. Simulations have shownthat this first module could constrain the tensor-to-scalar ratio downto σ(r) = 0.01 after a two-year survey. We aim to add furthermodules in the future to increase the angular sensitivity and resolutionof the instrument. The QUBIC project is proceeding through a sequence ofsteps. After an initial successful characterisation of the detectionchain, a technological demonstrator is being assembled to validate thefull instrument design and to test it electrically, thermally andoptically. The technical demonstrator is a scaled-down version ofModule 1 in terms of the number of detectors, input horns and pulsetubes and a reduction in the diameter of the combiner mirrors andfilters, but is otherwise similar. The demonstrator will be upgraded tothe full module in 2019. In this paper we give an overview of the QUBICproject and instrument.