CMB Polarization Measurements: Cutting the trail to CMB-S4
The CMB community started the planning stages for the "ultimate" ground-based CMB polarization experiment, CMB-S4, in 2013. Since then, the current generation of ground-based experiments has published several detections of B-mode polarization from gravitational lensing, demonstrating the techniques that will be expanded to characterize the signal across the sky. Deep searches for primordial B-mode polarization have shown the challenges in foreground contamination and control of systematics for reaching high sensitivity at large angular scales. The CMB community is currently working on defining the science goals and instrument requirements for CMB-S4. Meanwhile, the next generation of even more powerful CMB polarization experiments will soon begin to produce results. I will give an overview of the current state of the field, and the ongoing planning for CMB-S4.