Detector Development for Polarization Studies of the Cosmic Microwave Background

Among the most pressing questions remaining in cosmology today are the issues of the true nature of dark energy and the mechanism of inflation. Studies of the polarization anisotropies of the Cosmic Microwave Background provide one of the most promising methods of obtaining insight into these key gaps in our understanding of the origins and evolution of the universe. The ACTPol instrument, a polarization sensitive receiver for the Atacama Cosmology Telescope, aims to directly explore both of these unanswered questions by characterizing the polarization and temperature anisotropies of the CMB at small angular scales. ACTPol also serves as an important test of the developing polarimeter technology that will be needed for future instruments designed to probe the energy scale of inflation. The ACTPol instrument has been fully deployed with three kilo-TES arrays of well-characterized, highly sensitive detectors and has collected three seasons of polarized observations to-date. The measurements from the first season along have enabled measurements of the CMB power spectra [11], the lensing of the CMB through cross-correlation with CIB measurements [12], and the lensing of the CMB by dark matter halos [13]. The large volume of additional data collected in the next two observing seasons, including multi-frequency data from the most recent observations, will enable a wide range of further scientific results. The experience gained from the process of designing, fabricating, and characterizing the ACTPol detectors has both validated the polarimeter technology and provided significant insight into improvements for future CMB cameras. For example, the next-generation ACT receiver, Advanced ACTPol [14], will switch from the bilayer TES technology to a AlMn alloy superconductor to improve TES uniformity and parameter control. Achieving increased sensitivity for future polarized CMB observations will require large arrays of background limited detectors. ACTPol has demonstrated a number of techniques for rapid assembly, characterization, and optimization of such arrays.