Resonant Dipole-Dipole Energy Transfer in a Nearly Frozen Rydberg Gas

In a room temperature vapor of Rb Rydberg atoms, resonant dipole-dipole energy transfer occurs via binary collisions. In contrast, in the $300\mu \mathrm{K}$ vapor produced by a magneto-optical trap, the atoms are nearly frozen in place and many atoms interact simultaneously, as in an amorphous solid. As a result of the simultaneous multiple atom interactions, the energy transfer resonances are broadened substantially in frequency.