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We describe an on-sky demonstration of a microwave-multiplexing readout system in one of the receivers of the Keck Array, a polarimetry experiment observing the cosmic microwave background at the South Pole. During the austral summer of 2018–2019, we replaced the time-division multiplexing readout system with microwave-multiplexing components including superconducting microwave resonators coupled to radio frequency superconducting quantum interference devices at the sub-Kelvin focal plane, coaxial-cable plumbing and amplification between room temperature and the cold stages, and a SLAC Microresonator Radio Frequency system for the warm electronics. In the range 5–6 GHz, a single coaxial cable reads out 528 channels. The readout system is coupled to transition-edge sensors, which are in turn coupled to 150-GHz slot-dipole phased-array antennas. Observations began in April 2019, and we report here on an initial characterization of the system performance.

In this paper, we describe an on-sky demonstration of a microwave-multiplexing readout system in one of the receivers of the Keck Array, a polarimetry experiment observing the cosmic microwave background at the South Pole. During the austral summer of 2018–2019, we replaced the time-division multiplexing readout system with microwave-multiplexing components including superconducting microwave resonators coupled to radio frequency superconducting quantum interference devices at the sub-Kelvin focal plane, coaxial-cable plumbing and amplification between room temperature and the cold stages, and a SLAC Microresonator Radio Frequency system for the warm electronics. In the range 5–6 GHz, a single coaxial cable reads out 528 channels. The readout system is coupled to transition-edge sensors, which are in turn coupled to 150-GHz slot-dipole phased-array antennas. Observations began in April 2019, and we report here on an initial characterization of the system performance.

The BICEP/ Keck experiment (BK) is a series of small-aperture refracting telescopes observing degree-scale cosmic microwave background (CMB) polarization from the South Pole in search of a primordial B -mode signature. This B -mode signal arises from primordial gravitational waves interacting with the CMB and has amplitude parametrized by the tensor-to-scalar ratio r . Since 2016, BICEP3 and the Keck Array have been observing with 4800 total antenna-coupled transition-edge sensor detectors, with frequency bands spanning 95, 150, 220, and 270 GHz. Here we present the optical performance of these receivers from 2016 to 2019, including far-field beams measured in situ with an improved chopped thermal source and instrument spectral response measured with a field-deployable Fourier transform spectrometer. As a pair differencing experiment, an important systematic that must be controlled is the differential beam response between the co-located, orthogonally polarized detectors. We generate per-detector far-field beam maps and the corresponding differential beam mismatch that is used to estimate the temperature-to-polarization leakage in our CMB maps and to give feedback on detector and optics fabrication. The differential beam parameters presented here were estimated using improved low-level beam map analysis techniques, including efficient removal of non-Gaussian noise as well as improved spatial masking. These techniques help minimize systematic uncertainty in the beam analysis, with the goal of constraining the bias on r induced by temperature-to-polarization leakage to be subdominant to the statistical uncertainty. This is essential as we progress to higher detector counts in the next generation of CMB experiments.

Cosmic inflation is our current best theory for what occurred in the universe in the first instances of time. It postulates a brief period of exponential expansion in which quantum fluctuations are magnified to cosmic size and become the seeds for the growth of all structure in the Universe. Inflation makes a number of predictions, the most unique of which is the production of primordial gravitational waves (PGWs). Most of the predictions have since been tested, but the discovery of PGWs has eluded us to this day. The polarized Cosmic Microwave Background (CMB) is a powerful probe of these predictions, including the exciting possible existence of PGWs. This thesis provides a detailed account of the development of an optimal multi-component spectral-based likelihood analysis framework for joint analyses of heterogeneous multi-frequency CMB datasets, and its subsequent use for joint analysis of BICEP/Keck, Planck and WMAP CMB polarization data to derive the tightest constraints available on PGWs, parametrized by the tensor-to-scalar ratio. The manuscript also details the development of a spectral-based Fisher projection framework, specifically targeted towards optimizing tensor-to-scalar parameter constraints in the presence of galactic foregrounds and gravitational lensing of the CMB, that directly uses information from current BICEP/Keck achieved performances, to robustly forecast the science reach of upcoming CMB-polarization endeavors. This methodology allows for rapid iteration over experimental configurations and offers a flexible way to optimize the design of future experiments given a scientific goal. We document the use of this framework to perform forecasts for the next iteration of BICEP/Keck instrument -- BICEP-Array, as well as the next generation ground-based CMB experiment -- CMB-S4.