recent years. The explanation of the problem is not known so far. There are proposals of new dedicated scattering experiments with electrons [@gasparyan; @ISR_A1] and muons [@Gilman:2013eiv]. On the theory side, the discrepancy was addressed in terms of effective non-relativistic QED interactions [@Hill:2011wy], dispersion relations  [@Lorenz:2012tm], exotically large hadronic effects [@Miller:2012ne], or of new physics affecting the muon and electron differently [@TuckerSmith:2010ra; @Batell:2011qq; @Carlson:2012pc].\
Further information can come from measuring the deuteron radius using the Lamb shift in muonic deuterium. The deuteron radius from electron based experiments is already known to good accuracy. The best results come from using the isotope shift, that is, measuring the $1S$-$2S$ splittings in electron-proton ($e$-$H$) and electron-deuteron ($e$-$D$) hydrogen and finding from residual corrections that $$r^2_E(d) - r^2_E(p) = 3.82007(65) {\rm\ fm^2}   \,,$$ as quoted in [@Parthey:2010; @huber], where the $r_E(p,d)$ are charge radii. The isotope shift number is so accurate that the uncertainty in the deuteron radius-squared becomes in practice the same as for the proton, and using the CODATA 2010 value for the proton radius one finds [@Mohr:2012tt], $$r_E(d) = 2.1424 (21) {\rm\ fm}  \,.$$ The uncertainty is $0.1\%$. In contrast, the current best direct electron-deuteron scattering results yield $r_E(d)=2.128(11)$ fm [@Sick:1996], 