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The BICEP/Keck series of experiments target the Cosmic Microwave Background at degree-scale resolution from the South Pole. Over the next few years, the \"Stage-3\" BICEP Array (BA) telescope will improve the program's frequency coverage and sensitivity to primordial B-mode polarization by an order of magnitude. The first receiver in the array, BA1, began observing at 30/40 GHz in early 2020. The next two receivers, BA2 and BA3, are currently being assembled and will map the southern sky at frequencies ranging from 95 GHz to 150 GHz. Common to all BA receivers is a refractive, on-axis, cryogenic optical design that focuses microwave radiation onto a focal plane populated with antenna-coupled bolometers. High-performance antireflective coatings up to 760 mm in aperture are needed for each element in the optical chain, and must withstand repeated thermal cycles down to 4 K. Here we present the design and fabrication of the 30/40 GHz anti-reflection coatings for the recently deployed BA1 receiver, then discuss laboratory measurements of their reflectance. We review the lamination method for these single- and dual-layer plastic coatings with indices matched to various polyethylene, nylon and alumina optics. We also describe ongoing efforts to optimize coatings for the next BA cryostats, which may inform technological choices for future Small-Aperture Telescopes of the CMB \"Stage 4\" experiment.

We present the first constraints on multipole-dependent cosmic birefringence using CMB polarization data from the BK18 dataset, which combines observations from BICEP2, Keck Array, and BICEP3 at frequencies of 95, 150, and 220 GHz. Photon coupling to an axion-like field leads to the rotation of CMB polarization, inducing non-zero EB cross-correlations. We show that a multipole-dependent rotation beta(l) imprints a distinct signature in the polarization spectra that can be constrained. Specifically, we consider an Early Dark Energy (EDE) scenario in which a pseudoscalar field couples to photons through a Chern-Simons interaction, generating a polarization rotation with multipole dependence. We introduce a phenomenological beta(l) as a step function, obtaining constraints on the step function size consistent with zero, with uncertainties less than 0.15 degrees (68% CL). In addition, using multi-frequency EE, BB, and EB cross-spectra, along with robust BICEP/Keck foreground treatment and likelihood framework, we derive constraints on the axion-photon coupling amplitude g for several choices of EDE parameters. For the baseline best-fit value f_EDE = 0.087 from the Planck 2018 analysis, we obtain g = 0.11 +/- 0.37 (68% CL), consistent with previous limits.

Inflation, an extension to the standard ΛCDM model which posits a brief, accelerated expansion early in the Universe, naturally solves the horizon and flatness problems with the standard model and provides a source of the initial perturbations that seed large-scale structure. Most inflationary models predict a stochastic background of gravitational waves which would imprint a unique B-mode pattern in the polarization of the Cosmic Microwave Background (CMB) that peaks at degree angular scales. The strength of this signature is parametrized by the tensor-to-scalar ratio r. A nonzero measurement of r would provide direct evidence for inflation. The BICEP/Keck series of CMB experiments has been observing the polarized CMB from the Amundsen-Scott South Pole Station since 2006, using small-aperture, on-axis refractors. Continuous integration of a low-foreground, ~1-2% patch of the sky has produced maps over multiple frequencies that lead to world-leading constraints on r from B-mode measurements: r_0.05 < 0.034 and σ_r = 0.009 using data through the 2018 observing season. The BICEP/Keck telescopes measure polarization by taking the difference between two co-located, orthogonally polarized detectors. A prominent systematic that must be controlled is the leakage from the bright CMB temperature sky into the polarization measurement due to mismatch in the angular response patterns (beams) of the two detectors within a pair. In this dissertation, we use high-fidelity in-situ measurements of the per-detector beam response in conjunction with specialized simulations to quantify the level of temperature-to-polarization (T-P) leakage expected in the BK18 data set, and the associated impact on the r constraint. We also use Fourier transform spectrometer measurements of the spectral response to constrain the bias on r from band center uncertainties. We introduce simple metrics evaluated only from the beam maps (without simulations) that estimate the T-P leakage and other optical properties, and discuss progress of optical modeling of small-aperture telescope beams, including their ability to predict T-P leakage. Finally, we assess the utility of these simulations, metrics, and optical models as we move forward to the next generation of CMB experiments with hundreds of thousands of detectors.

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 BICEP3 and BICEP Array polarimeters are small-aperture refracting telescopes located at the South Pole designed to measure primordial gravitational wave signatures in the Cosmic Microwave Background (CMB) polarization, predicted by inflation. Constraining the inflationary signal requires not only excellent sensitivity, but also careful control of instrumental systematics. Both instruments use antenna-coupled orthogonally polarized detector pairs, and the polarized sky signal is reconstructed by taking the difference in each detector pair. As a result, the differential response between detectors within a pair becomes an important systematic effect we must control. Additionally, mapping the intensity and polarization response in regions away from the main beam can inform how sidelobe levels affect CMB measurements. Extensive calibration measurements are taken in situ every austral summer for control of instrumental systematics and instrument characterisation. In this work, we detail the set of beam calibration measurements that we conduct on the BICEP receivers, from deep measurements of main beam response to polarized beam response and sidelobe mapping. We discuss the impact of these measurements for instrumental systematics studies and design choices for future CMB receivers.