Cooling molecules the optoelectric way

The ability to cool a trapped gas of atoms to a near standstill was key to opening up the ultracold regime of atomic physics, in which various quantum effects become important. Researchers would like to do the same thing with molecules, but the molecules' many additional degrees of freedom foil most attempts at cooling. Now, Gerhard Rempe and colleagues (Max Planck Institute for Quantum Optics, Garching, Germany) have demonstrated a cooling method that capitalizes on molecular complexity. In a gas of molecules confined to an electric trap, they choose a few rotational states with different responses to the electric field gradient, as shown in the figure. As a molecule in state B moves toward the edge of the trap, it must climb a steep potential hill and lose kinetic energy. When it reaches the point where states A and B differ by an energy equal to an applied RF field, the field stimulates a transition between the two states, and the molecule, now in state A, rolls down a shallower hill back to the center of the trap. Because the returning molecules gain less kinetic energy than they lost when they climbed, the gas temperature decreases. Then molecules are pumped from state A back into state B, and the process repeats. Rempe and colleagues used the technique to cool a fluoromethane (CH₃F) gas from 390 mK to 29 mK. That's orders of magnitude away from the quantum regime, but the researchers anticipate that they'll be able to achieve lower temperatures and cool other molecules. (M. Zeppenfeld et al., Nature, in press, doi:10.1038/nature11595.)—Johanna Miller