Resonant energy transfer among ultracold highly-excited atoms*

Ultracold highly-excited atoms can interact strongly through a field tuned dipole-dipole interaction.  We observe this interaction by measuring how population is redistributed among energy eigenstates as the atoms exchange energy.  Selective field ionization is often used to measure the population distribution in highly-excited atoms.  In this technique, a ramped electric field ionizes the atoms, with weakly bound electrons ionizing at low field (early in time) and tightly bound electrons ionizing at high field (later in time).  Unfortunately, it is often difficult to resolve the signals from states that are close in binding energy.  We have recently developed an improved selective ionization technique that allows us to separate the signals from closely spaced states.  Using this technique, we can precisely quantify the dipole-dipole interaction among atoms.