The millimeter-wave bolometric interferometer

The Millimeter-Wave Bolometric Interferometer (MBI) is designed for sensitive measurements of the polarization of the cosmic microwave background (CMB). MBI combines the differencing capabilities of an interferometer with the high sensitivity of bolometers at millimeter wavelengths. It views the sky directly through corrugated horn antennas with low sidelobes and nearly symmetric beam patterns to avoid spurious instrumental polarization from reflective optics. The design of the first version of the instrument with four 7-degree-FOV corrugated horns (MBI-4) is discussed. The MBI-4 optical band is defined by filters with a central frequency of 90 GHz. The set of baselines determined by the antenna separation makes the instrument sensitive to CMB polarization fluctuations over the multipole range l=150-270. In MBI- 4, the signals from antennas are combined with a Fizeau beam combiner and interference fringes are detected by an array of spider-web bolometers with NTD germanium thermistors. In order to separate the visibility signals from the total power detected by each bolometer, the phase of the signal from each antenna is modulated by a ferrite-based waveguide phase shifter. Observations are planned from the Pine Bluff Observatory outside Madison, WI

BRAIN/MBI: a bolometric interferometer dedicated to the CMB polarization

In this paper we present a new experiment dedicated to the study of the Cosmic Microwave Background (CMB) polarization. BRAIN/MBI, the result of the merging of two formerly distinct experiments, MBI (see [1], and references therein) and BRAIN (see [2], and references therein), both based on a Bolometric Interferometry (BI in the following), will be called henceforth QUBIC (Q and U Bolometric Interferometer for Cosmology). This ground-based experiment will be one of the next-generation CMB polarimeters and will fill a technological gap, being the only adding interferometer proposed in the field of CMB research, and with a sensitivity needed to target B-modes. Among proposed and/or running experiments, there are fully integrated coherent polarimeters (QUIET [3]), imagers (ClOVER [4], BICEP [5], QUaD [6]) and broadband heterodyne interferometers (AMiBA [7]). QUBIC will explore a different experimental approach, allowing cross-checks with other experimental techniques, and the final validation of BI at mm-waves. This is of crucial importance, since the detection of B-modes (if any) will be achieved by an experiment reaching the best balance between sensitivity and accuracy (control of systematics). The structure of the paper is the following. We introduce in brief the science case driving this experiment; we outline the basic principles of BI, mostly developed by people within this collaboration; we present the architecture and some of the main characteristics foreseen for QUBIC. Then we concentrate on subsystems which have a unique role in BI: the phase shifter and the beam combiner. For these subsystems we present a variety of possible technological choices, some of them now under study