Why are ytterbium atoms used for quantum computing? Ytterbium seems to be the favorite for element for trapped ion quantum computing experiments. Why is this?
 A: Ytterbium is one of a class of atoms that have a Hydrogen-like configuration of valence electrons when singly ionized. Beryllium, Magnesium, Strontium and Calcium are other examples. 
Hydrogen-like atoms and ions typically have more "cycling transitions" -- a pair of states where the only allowed spontaneous decay transition from the upper state is back to the same lower state. This makes laser cooling easier from a technical standpoint, since it usually only requires one or two lasers. 
I'm sure there are more detailed reasons to prefer Yb over Mg (for example) that an expert at quantum information can provide. I believe this is the "bare-bones" reason, however. 
Edit: After having a look at the NIST atomic spectra database information for Yb$^+$ and Mg$^+$, the next most likely reason seems to be the convenience of laser cooling transition wavelengths. I believe the most convenient transitions for Doppler cooling are either $^2{\rm S}_{1/2} \rightarrow ^2{\rm P}_{1/2}$ or ${}^2{\rm S}_{1/2} \rightarrow {}^2{\rm D}_{3/2}$. In Mg$^+$, both transitions are at UV wavelengths. In Yb$^+$ and Ca$^+$ they are more easily accessible. 
