SPTpol: an instrument for CMB polarization measurements with the South
  Pole Telescope
  SPTpol is a dual-frequency polarization-sensitive camera that was deployed on
the 10-meter South Pole Telescope in January 2012. SPTpol will measure the
polarization anisotropy of the cosmic microwave background (CMB) on angular
scales spanning an arcminute to several degrees. The polarization sensitivity
of SPTpol will enable a detection of the CMB "B-mode" polarization from the
detection of the gravitational lensing of the CMB by large scale structure, and
a detection or improved upper limit on a primordial signal due to inflationary
gravity waves. The two measurements can be used to constrain the sum of the
neutrino masses and the energy scale of inflation. These science goals can be
achieved through the polarization sensitivity of the SPTpol camera and careful
control of systematics. The SPTpol camera consists of 768 pixels, each
containing two transition-edge sensor (TES) bolometers coupled to orthogonal
polarizations, and a total of 1536 bolometers. The pixels are sensitive to
light in one of two frequency bands centered at 90 and 150 GHz, with 180 pixels
at 90 GHz and 588 pixels at 150 GHz. The SPTpol design has several features
designed to control polarization systematics, including: single-moded feedhorns
with low cross-polarization, bolometer pairs well-matched to difference
atmospheric signals, an improved ground shield design based on far-sidelobe
measurements of the SPT, and a small beam to reduce temperature to polarization
leakage. We present an overview of the SPTpol instrument design, project
status, and science projections.
