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 $500{\mathrm{deg}}^2$ 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: $95\mathrm{GHz}\times 95\mathrm{GHz}$, $95\mathrm{GHz}\times 150\mathrm{GHz}$, and $150\mathrm{GHz}\times 150\mathrm{GHz}$. $B$-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. With a prior on the galactic dust from Planck, WMAP and BICEP2/Keck observations, the SPTpol $B$-mode data can be used to set an upper limit 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 $\mathrm{\Lambda }\mathrm{CDM}$ model predictions. Scaling the predicted lensing $B$-mode power in this model by a factor $A_{\mathrm{lens}}$, the data prefer $A_{\mathrm{lens}}=1.17\pm 0.13$. These data are currently the most precise measurements of $B$-mode power at $\ell >320$.

• Received 23 April 2020
• Accepted 3 June 2020

DOI:https://doi.org/10.1103/PhysRevD.101.122003