  We present the design and characterization of the POLARBEAR experiment.
POLARBEAR will measure the polarization of the cosmic microwave background
(CMB) on angular scales ranging from the experiment's 3.5 arcminute beam size
to several degrees. The experiment utilizes a unique focal plane of 1,274
antenna-coupled, polarization sensitive TES bolometers cooled to 250
milliKelvin. Employing this focal plane along with stringent control over
systematic errors, POLARBEAR has the sensitivity to detect the expected small
scale B-mode signal due to gravitational lensing and search for the large scale
B-mode signal from inflationary gravitational waves.
  POLARBEAR was assembled for an engineering run in the Inyo Mountains of
California in 2010 and was deployed in late 2011 to the Atacama Desert in
Chile. An overview of the instrument is presented along with characterization
results from observations in Chile.
