The race to measure the electron’s electric dipole moment (eEDM) is picking up speed across the world, thanks to graduate student Ed Meyer of JILA’s Lazy Bohn’s Ranch (i.e., John Bohn’s theory group). Meyer has identified more than a dozen horses, a.k.a. molecules and molecular ions, with strong enough internal electric fields to compete in the eEDM derby. Imperial College of London’s Ed Hinds is riding YbF (ytterbium fluoride) and leads by a nose.
JILA Fellow Eric Cornell is counting on HfF+ (hafnium fluoride) to carry him across the finish line ahead of the rest. Yale’s Dave DeMille has teamed up with Harvard’s John Doyle to ride ThO (thorium oxide) to victory. And, the University of Michigan’s Aaron Leanhardt (formerly a postdoc with Cornell) is closing to within a length with WC (tungsten carbide).
Meyer has just identified what is surely the coldest horse that will ever compete in the eEDM derby: YbSr+. This ultracold yearling has some important advantages: (1) it may be relatively easy to make lots of it from ultracold atoms of Yb and Sr, (2) being colder may endow it with much longer coherence times at ultracold temperatures than the other “ordinary” eEDM candidate molecular ions, and (3) it could be made in the JILA basement.
In addition, YbSr+ doesn’t have any hyperfine structure. Hyperfine structure is a bad thing to have in the eEDM derby. It’s like having a horse with six left legs. Remove the hyperfine structure, which is due to nuclear spin, and your horse is back to normal and running on four legs. The only spin in this baby is the spin of the valence electron — and that’s the spin that matters.
The new ion is not without drawbacks, however. Its two constituent atoms are both heavy elements, meaning that the valence electron can hang out near either nucleus. Thus in an eEDM experiment, one could conceivably get twice the signal or no signal at all. However, Yb has a stronger pull on the electron than Sr, which reduces the chances of no signal. On the positive side, the ion has a nice short bond length, and at ultracold temperatures, it’s in its ground state. All this fine young stallion needs now is a willing jockey. - Julie Phillips