The South Pole Telescope is a 10m millimeter-wavelength telescope for finding galaxy clusters via the thermal Sunyaev-Zel'dovich (tSZ) effect. This thesis is divided into two parts. The first part describes the development of the kilopixel SPT-SZ receiver and the frequency-domain multiplexor (fMUX). The second part describes the first SPT power spectrum measurement and the first detection of the tSZ power spectrum.

The SPT-SZ focal plane consists of 960 spiderweb coupled transition-edge sensors. Due to strong electro-thermal feedback, these devices have good sensitivity and linearity, though risk spontaneous oscillations. Adding heat capacity to these devices can ensure stability, so long as the loopgain, [special characters omitted], is less than G_int/G ₀, the ratio between the thermal conductances linking the TES to the heat capacity and linking the heat capacity to the bath. I describe as experimental technique for measuring the internal thermal structure of these devices, allowing for rapid sensor evaluation.

The fMUX readout system reduces wiring complexity in this receiver by AC-biasing each sensor at a unique frequency and sending signals from multiple bolometers along one pair of wires. The Series SQUID Arrays (SSAs) used to read changes in bolometer current are notably non-linear and extremely sensitive to ambient magnetic fields. The SSAs are housed in compact magnetic shielding modules which reduces their effective area to 80 mΦ₀/gauss. The SSA are fedback with a flux-locked loop to improve their linearity and dynamic range, and decrease their input reactance. The FLL is bandwidth of 1 MHz with a measured loopgain of 10. In the current implementation, this bandwidth is limited between the SQUID input coil and other reactances, which I study in Chapter 4.

In the second part of the thesis I present power spectrum measurements for the first 100 deg² field observed by the SPT. On angular scales where the primary CMB anisotropy is dominant, ℓ [special characters omitted] 3000, the SPT power spectrum is consistent with the standard ΛCDM cosmology. On smaller scales, we see strong evidence for a point source contribution, consistent with a population of dusty, star-forming galaxies. I combine the 150 and 220 GHz data to remove the majority of the point source power, and use the point source subtracted spectrum to detect Sunyaev-Zel'dovich (SZ) power at 2.6 σ. At ℓ = 3000, the SZ power in the subtracted bandpowers is 4.2±1.5 μK², which is significantly lower than the power predicted by a fiducial model using WMAP5 cosmological parameters.