Performance of the QUBIC CMB telescope's detectors

The Theory of Inflation postulates that, immediately after the Big Bang, the Universe expanded exponentially. If this theory is true, the resulting gravitational waves must have left traces in the CMB in the form of an extremely faint B-mode polarisation pattern. The Q and U Bolometric Interferometer for Cosmology (QUBIC) seeks to detect this polarisation pattern using the novel technique of bolometric interferometry. QUBIC’s Technological Demonstrator (TD) has been commissioned in Argentina and has been taking sky data regularly since March 2026. An array of back-to-back corrugated feed horns at the instrument’s aperture separates incident radiation into quasi-Gaussian beams, which are then recombined in QUBIC’s cold optical combiner. The resulting interference pattern is measured in QUBIC’s focal plane by transition edge sensor (TES) bolometers kept at sub-Kelvin temperatures. The phonon noise equivalent power (NEP) of each detector is estimated by measuring the current response of the detector across a range of bias voltages at various bath temperatures. We conduct a long-term analysis of phonon NEP results to give an indication of the TES array’s performance over time and the reliability of each detector. We find a mean phonon NEP of 4.0×10−17W/√Hz and a mean of (52.6±0.8)% successful phonon NEP estimations per dataset. The mean phonon NEP has improved in comparison to previous laboratory results, likely due to improved cryogenic system performance and reduced thermal background loading. The percentage of successful phonon NEP estimations has decreased, indicative of possible suboptimal configurations in the detector chain requiring further investigation.