Inflation -- the leading model for the earliest moments of the time, in which the Universe undergoes a period of rapid, accelerating expansion -- generically predicts a background of primordial gravitational waves, which generate a B-mode component in the polarization of the cosmic microwave background (CMB). The measurement of such a B-mode signature would lend significant support to the paradigm of inflation. However, observed B modes also contain a component from the gravitational lensing of primordial E modes, which can obscure the measurement of the primordial B modes. We reduce the uncertainties in the B-mode measurement contributed from this lensing component by a technique called 'delensing.' In this talk, I will give an update on the current delensing effort on the BICEP/Keck data, using data from the South Pole Telescope (SPT) and the Planck satellite. This analysis will tighten the constraint on the amplitude of primordial gravitational waves, parameterized through the tensor-to-scalar ratio r that is related to the energy scale of inflation. For upcoming analyses, efficient delensing relies on high signal-to-noise measurements of the CMB lensing mass map. I will show the current state-of-the-art measurement of CMB lensing using SPTpol data, its inferred cosmological constraints, and its relevance for delensing. I will then discuss on-going efforts and novel methods in making lensing mass maps using new data/simulations from current and next-generation CMB experiments. After discussing the related challenges and opportunities, I will finish with an outlook on constraining r in this decade.
