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We present a measurement of the $B$-mode polarization power spectrum (the$BB$ spectrum) from 100 $\mathrm{deg}^2$ of sky observed with SPTpol, apolarization-sensitive receiver currently installed on the South PoleTelescope. The observations used in this work were taken during 2012 and early2013 and include data in spectral bands centered at 95 and 150 GHz. We reportthe $BB$ 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, 95GHz $\times$ 150 GHz, and 150 GHz $\times$ 150 GHz. We subtract small ($< 0.5\sigma$ in units of statistical uncertainty) biases from these spectra andaccount for the uncertainty in those biases. The resulting power spectra areinconsistent with zero power but consistent with predictions for the $BB$spectrum arising from the gravitational lensing of $E$-mode polarization. If weassume no other source of $BB$ power besides lensed $B$ modes, we determine apreference for lensed $B$ modes of $4.9 \sigma$. After marginalizing overtensor power and foregrounds, namely polarized emission from galactic dust andextragalactic sources, this significance is $4.3 \sigma$. Fitting for a singleparameter, $A_\mathrm{lens}$, that multiplies the predicted lensed $B$-modespectrum, and marginalizing over tensor power and foregrounds, we find$A_\mathrm{lens} = 1.08 \pm 0.26$, indicating that our measured spectra areconsistent with the signal expected from gravitational lensing. The datapresented here provide the best measurement to date of the $B$-mode powerspectrum on these angular scales.

Measurements of Sub-degree B-mode Polarization in the Cosmic Microwave Background from 100 Square Degrees of SPTpol Data