Measurements of B-mode Polarization of the Cosmic Microwave Background
  from 500 Square Degrees of SPTpol Data
  We report a B-mode power spectrum measurement from the cosmic microwave
background (CMB) polarization anisotropy observations made using the SPTpol
instrument on the South Pole Telescope. This work uses 500 deg$^2$ of SPTpol
data, a five-fold increase over the last SPTpol B-mode release. As a result,
the bandpower uncertainties have been reduced by more than a factor of two, and
the measurement extends to lower multipoles: $52 < \ell < 2301$. Data from both
95 and 150 GHz are used, allowing for three cross-spectra: 95 GHz x 95 GHz, 95
GHz x 150 GHz, and 150 GHz x 150 GHz. B-mode power is detected at very high
significance; we find $P(BB < 0) = 5.8 \times 10^{-71}$, corresponding to a
$18.1 \sigma$ detection of power. An upper limit is set on the tensor-to-scalar
ratio, $r < 0.44$ at 95% confidence (the expected $1 \sigma$ constraint on $r$
given the measurement uncertainties is 0.22). We find the measured B-mode power
is consistent with the Planck best-fit $\Lambda$CDM model predictions. Scaling
the predicted lensing B-mode power in this model by a factor Alens, the data
prefer Alens = $1.17 \pm 0.13$. These data are currently the most precise
measurements of B-mode power at $\ell > 320$.
