Measurements of Sub-degree B-mode Polarization in the Cosmic Microwave
  Background from 100 Square Degrees of SPTpol Data
  We present a measurement of the $B$-mode polarization power spectrum (the
$BB$ spectrum) from 100 $\mathrm{deg}^2$ of sky observed with SPTpol, a
polarization-sensitive receiver currently installed on the South Pole
Telescope. The observations used in this work were taken during 2012 and early
2013 and include data in spectral bands centered at 95 and 150 GHz. We report
the $BB$ spectrum in five bins in multipole space, spanning the range $300 \le
\ell \le 2300$, and for three spectral combinations: 95 GHz $\times$ 95 GHz, 95
GHz $\times$ 150 GHz, and 150 GHz $\times$ 150 GHz. We subtract small ($< 0.5
\sigma$ in units of statistical uncertainty) biases from these spectra and
account for the uncertainty in those biases. The resulting power spectra are
inconsistent with zero power but consistent with predictions for the $BB$
spectrum arising from the gravitational lensing of $E$-mode polarization. If we
assume no other source of $BB$ power besides lensed $B$ modes, we determine a
preference for lensed $B$ modes of $4.9 \sigma$. After marginalizing over
tensor power and foregrounds, namely polarized emission from galactic dust and
extragalactic sources, this significance is $4.3 \sigma$. Fitting for a single
parameter, $A_\mathrm{lens}$, that multiplies the predicted lensed $B$-mode
spectrum, and marginalizing over tensor power and foregrounds, we find
$A_\mathrm{lens} = 1.08 \pm 0.26$, indicating that our measured spectra are
consistent with the signal expected from gravitational lensing. The data
presented here provide the best measurement to date of the $B$-mode power
spectrum on these angular scales.
