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 deg2 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<ℓ<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)=5.8×10−71, corresponding to a
18.1σ detection of power. An upper limit is set on the tensor-to-scalar
ratio, r<0.44 at 95% confidence (the expected 1σ constraint on r
given the measurement uncertainties is 0.22). We find the measured B-mode power
is consistent with the Planck best-fit ΛCDM model predictions. Scaling
the predicted lensing B-mode power in this model by a factor Alens, the data
prefer Alens = 1.17±0.13. These data are currently the most precise
measurements of B-mode power at ℓ>320.Comment: 16 pages, 4 figures, Submitted to PR