QUBIC IV: Performance of TES Bolometers and Readout Electronics

Abstract

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 in Paris. The detection chain is currently made of 256 NbSi Transition Edge Sensors cooled to 320 mK. The readout system is a 128:1 Time Domain Multiplexing scheme based on 128 SQUIDs cooled at 1K that are controlled and amplified by an SiGe Application Specific Integrated Circuit at 40 K. We report the performance of this readout chain and the characterization of the TESs. 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/sqrt(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. While still limited by micro-phonics from the pulse tubes and noise aliasing from readout system, the Noise Equivalent Power is about 2 × 10 −16 W/ √ Hz, enough for the demonstration of bolometric interferometry. 18 4.8 Noise characterizations 21 4.8.1 Noise in normal and superconducting states 23 4.8.2 Noise in the transition 23 5 Conclusion 29

QUBIC I: Overview and science program

“…A single polarization is then selected thanks to a polarizing grid. Although reflecting half of the incoming photons may appear as a regrettable loss, it is in fact one of the key features of QUBIC for handling instrumental systematics, especially polarization-related ones: a single polarization is selected just after polarization modulation by a wire-grid, while our bolometers are not sensitive to polarization due to the XY symmetry of the absorbing grid (see Piat et al [35]). As a result, any crosspolarization occurring after the polarizing grid (horns, uncontrolled reflections inside the optical combiner) are negligible.…”

“…The backhorns directly illuminate the two-mirrors off-axis Gregorian optical combiner (described in detail in O'Sullivan et al [39]) that focuses the signal onto the two perpendicular focal planes, separated by a dichroic filter that splits the incoming waves into two wide bands centered at 150 GHz for the on-axis focal plane and 220 GHz for the off-axis one. The focal planes are each equipped with 992 NbSi Transition-Edge-Sensors (the detection chain is described in detail in Piat et al [35] from this series of articles) cooled down to 300 mK using a sorption fridge. A realistic view of the cryostat can be seen in the right panel of figure 3.…”

“…• The intrinsic detector noise has been measured to be 4.7×10 −17 W/ √ Hz but due to noise aliasing with the TD readout chain, the effective detector+readout noise is 2.05×10 −16 W/ √ Hz. This noise aliasing will be resolved for the FI using Nyquist inductance in order to reduce the noise bandwidth of the TES (see Piat et al [35]).…”

“…Note that detailed quasi-optical simulations have shown that the 150 GHz optimization of the back-short of the TES is also nearly optimal at 220 GHz allowing us to use identical detector designs for both frequencies [54,55]. A new readout electronics will avoid the TD noise aliasing through the addition of Nyquist inductors [35]. The additional focal plane also requires the installation of a dichroic filter at 45 degrees in between the two focal planes (light-blue in figure 3-left, and in red in figure 3-right).…”

“…The scientific overview and expected performance of the instrument are addressed here. The other papers in the special issue address the following topics: the ability of bolometric interferometry to do spectral imaging [33], the calibration and performance in the laboratory of the TD [34], the performance of the detector array and readout electronics [35], the cryogenic system performance [36], the Half Wave Plate rotator system [37], the back-to-back feedhorn-switch system [38], and the optical design and performance [39].…”