Measurements of the cosmic microwave background (CMB) can be used to map the distribution of matter in the universe through gravitational lensing deflections as the CMB photons travel through the ...large-scale structures of the universe. Gravitational lensing induces correlations between CMB temperature and polarization modes at different angular scales. These correlations can be extracted by lensing quadratic estimators to reconstruct the projected 2D lensing potential and matter distribution. The power spectrum of the lensing potential is a powerful probe of the growth of structure. It can constrain the sum of neutrino masses, dark energy, and the amplitude of matter density fluctuations. Furthermore, the matter distribution from lensing provides a promising source for cross-correlation with surveys at other wavelengths and offers a template for removing lensing-induced contamination in searches for inflationary gravitational waves. The third-generation camera for the South Pole Telescope (SPT-3G) is an excellent instrument for CMB lensing measurements with high-sensitivity from a new multichroic receiver with 16,000 polarization-sensitive detectors, and fine arcminute-scale resolution provided by the ten-meter dish of SPT. A compact Fourier-Transform spectrometer (FTS) was designed and constructed for detector characterization. I review the SPT-3G instrument with particular focus on the detectors and FTS and present measurements of the lensing potential and lensing power spectrum from the 2018 data of the SPT-3G survey.
The complex conductivity of a superconducting thin film is related to the quasiparticle density, which depends on the physical temperature and can also be modified by external pair breaking with ...photons and phonons. This relationship forms the underlying operating principle of Kinetic Inductance Detectors (KIDs), where the detection threshold is governed by the superconducting energy gap. We investigate the electromagnetic properties of thin-film aluminum that is proximitized with either a normal metal layer of copper or a superconducting layer with a lower <inline-formula><tex-math notation="LaTeX">T_{C}</tex-math></inline-formula>, such as iridium, in order to extend the operating range of KIDs. Using the Usadel equations along with the Nam expressions for complex conductivity, we calculate the density of states and the complex conductivity of the resulting bilayers to understand the dependence of the pair breaking threshold, surface impedance, and intrinsic quality factor of superconducting bilayers on the relative film thicknesses. The calculations and analyses provide theoretical insights in designing aluminum-based bilayer kinetic inductance detectors for detection of microwave photons and athermal phonons at the frequencies well below the pair breaking threshold of a pure aluminum film.
We present an updated design of the 220 GHz microwave kinetic inductance detector (MKID) pixel for SPT-3G+, the next-generation camera for the South Pole Telescope. We show results of the dark ...testing of a 63-pixel array with mean inductor quality factor \(Q_i = 4.8 \times 10^5\), aluminum inductor transition temperature \(T_c = 1.19\) K, and kinetic inductance fraction \(\alpha_k = 0.32\). We optically characterize both the microstrip-coupled and CPW-coupled resonators, and find both have a spectral response close to prediction with an optical efficiency of \(\eta \sim 70\%\). However, we find slightly lower optical response on the lower edge of the band than predicted, with neighboring dark detectors showing more response in this region, though at level consistent with less than 5\% frequency shift relative to the optical detectors. The detectors show polarized response consistent with expectations, with a cross-polar response of \(\sim 10\%\) for both detector orientations.
The Rayleigh scattering of cosmic microwave background (CMB) photons off the neutral hydrogen produced during recombination effectively creates an additional scattering surface after recombination ...that encodes new cosmological information, including the expansion and ionization history of the universe. A first detection of Rayleigh scattering is a tantalizing target for next-generation CMB experiments. We have developed a Rayleigh scattering forecasting pipeline that includes instrumental effects, atmospheric noise, and astrophysical foregrounds (e.g., Galactic dust, cosmic infrared background, or CIB, and the thermal Sunyaev-Zel'dovich effect). We forecast the Rayleigh scattering detection significance for several upcoming ground-based experiments, including SPT-3G+, Simons Observatory, CCAT-prime, and CMB-S4, and examine the limitations from atmospheric and astrophysical foregrounds as well as potential mitigation strategies. When combined with Planck data, we estimate that the ground-based experiments will detect Rayleigh scattering with a significance between 1.6 and 3.7, primarily limited by atmospheric noise and the CIB.
We present details of the design, simulation, and initial test results of prototype detectors for the fourth-generation receiver of the South Pole Telescope (SPT). Optimized for the detection of key ...secondary anisotropies of the cosmic microwave background (CMB), SPT-3G+ will measure the temperature and polarization of the mm/sub-mm sky at 220, 285, and 345 GHz, beyond the peak of the CMB blackbody spectrum. The SPT-3G+ focal plane will be populated with microwave kinetic inductance detectors (MKIDs), allowing for significantly increased detector density with reduced cryogenic complexity. We present simulation-backed designs for single-color dual-polarization MKID pixels at each SPT-3G+ observation frequency. We further describe design choices made to promote resonator quality and uniformity, enabling us to maximize the available readout bandwidth. We also discuss aspects of the fabrication process that enable rapid production of these devices and present an initial dark characterization of a series of prototype devices.
We have constructed a Fourier-transform spectrometer (FTS) operating between 50 and 330 GHz with minimum volume (355 x260 x64 mm) and weight (13 lbs) while maximizing optical throughput (100 ...\(\mathrm{mm}^2\) sr) and optimizing the spectral resolution (4 GHz). This FTS is designed as a polarizing Martin-Puplett interferometer with unobstructed input and output in which both input polarizations undergo interference. The instrument construction is simple with mirrors milled on the box walls and one motorized stage as the single moving element. We characterize the performance of the FTS, compare the measurements to an optical simulation, and discuss features that relate to details of the FTS design. The simulation is also used to determine the tolerance of optical alignments for the required specifications. We detail the FTS mechanical design and provide the control software as well as the analysis code online.
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