  We present a measurement of the cosmic microwave background (CMB) lensing
potential using 500 deg$^2$ of 150 GHz data from the SPTpol receiver on the
South Pole Telescope. The lensing potential is reconstructed with
signal-to-noise per mode greater than unity at lensing multipoles $L \lesssim
250$, using a quadratic estimator on a combination of CMB temperature and
polarization maps. We report measurements of the lensing potential power
spectrum in the multipole range of $100< L < 2000$ from sets of
temperature-only, polarization-only, and minimum-variance estimators. We
measure the lensing amplitude by taking the ratio of the measured spectrum to
the expected spectrum from the best-fit $\Lambda$CDM model to the
$\textit{Planck}$ 2015 TT+lowP+lensing dataset. For the minimum-variance
estimator, we find $A_{\rm{MV}} = 0.944 \pm 0.058{\rm (Stat.)}\pm0.025{\rm
(Sys.)}$; restricting to only polarization data, we find $A_{\rm{POL}} = 0.906
\pm 0.090 {\rm (Stat.)} \pm 0.040 {\rm (Sys.)}$. Considering statistical
uncertainties alone, this is the most precise polarization-only lensing
amplitude constraint to date (10.1 $\sigma$), and is more precise than our
temperature-only constraint. We perform null tests and consistency checks and
find no evidence for significant contamination.
