We don't know how big the proton is. There is a persistent 7-standard-deviation discrepancy between two sets of measurements of the proton's charge radius: the radius measured using spectroscopy of muonic hydrogen, and the radius measured using regular hydrogen. Despite many attempted explanations, there are no obvious ways to resolve this discrepancy. If it holds up, it could be a sign of surprising new physics, such as violation of muon-electron universality.

We have built an experiment to accurately re-measure the proton's radius, using a precision measurement of the Lamb shift in hydrogen atoms. This transition needs to be measured to better than 1 part in 10^4 of its natural linewidth to shed some light on the proton radius puzzle. I will tell you about a new technique that we developed, which allows us to make measurements at a small fraction of the natural linewidth. I will present some recent results and describe on-going studies of systematic effects, towards a new determination of the radius of the proton.