Measurements of the cosmic microwave background (CMB) can be used to map the distribution of matter in the universe through gravitational lensing deflections as the CMB photons travel through the large-scale structures of the universe. Gravitational lensing induces correlations between CMB temperature and polarization modes at different angular scales. These correlations can be extracted by lensing quadratic estimators to reconstruct the projected 2D lensing potential and matter distribution. The power spectrum of the lensing potential is a powerful probe of the growth of structure. It can constrain the sum of neutrino masses, dark energy, and the amplitude of matter density fluctuations. Furthermore, the matter distribution from lensing provides a promising source for cross-correlation with surveys at other wavelengths and offers a template for removing lensing-induced contamination in searches for inflationary gravitational waves. The third-generation camera for the South Pole Telescope (SPT-3G) is an excellent instrument for CMB lensing measurements with high-sensitivity from a new multichroic receiver with 16,000 polarization-sensitive detectors, and fine arcminute-scale resolution provided by the ten-meter dish of SPT. A compact Fourier-Transform spectrometer (FTS) was designed and constructed for detector characterization. I review the SPT-3G instrument with particular focus on the detectors and FTS and present measurements of the lensing potential and lensing power spectrum from the 2018 data of the SPT-3G survey.




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