Them's the Brakes

The Bohn group has just come up with an exciting, really complicated experiment for someone else to do. This is something theorists like graduate student Ryan Wilson, former research associate Shai Ronen, and Fellow John Bohn get a kick out of. In this case, they’re recommending an experiment to measure how fast a tiny blue laser would have to move through a dipolar Bose-Einstein condensate (BEC) to create ripples. Energy lost to the ripples would create a drag force on the laser, signaling the onset of friction inside what would otherwise be a frictionless superfl uid. And, of course, friction means putting the brakes on the superfluid — literally.

And that’s not all, according to Wilson and his colleagues: Such an experiment could also demonstrate the presence of a roton. A roton is the quantum of energy that may represent the link between superfluid helium and a yet-to-be-observed “supersolid” phase of helium. In helium, it is also the excitation, or quasi particle, that determines the fluid velocity at which friction sets in. The analog of the roton in a dipolar BEC has been often discussed, but never seen.

Wilson hopes this situation will change soon, once his new theoretical analysis becomes widely known. He believes that dipolar BECs provide an amazing opportunity to explore the relationship between roton behavior and superfluidity. This is the reason he decided to model an experiment with a dipolar BEC similar to an actual experiment done 15 years ago at MIT on a BEC of sodium atoms. This experiment, unfortunately, was somewhat inconclusive.

In the new model, a blue laser beam sweeps through a dipolar BEC of chromium (Cr) atoms at constant speed. As this speed reaches a critical velocity, it excites roton modes, friction develops, and the condensate begins to slosh. The onset of sloshing is linked to a critical velocity, which, in turn, depends on the density of the Cr atoms making up the BEC and their dipole moment. According to the model, the critical velocity is somewhat smaller than expected from studies of denser materials. However, Wilson and his colleagues believe this fi nding is related to the role that the roton plays in the mechanical stability of a dipolar BEC.

Wilson and his colleagues hope that their proposed experiment will soon come into reality. They’d like to see an unequivocal measurement of the critical velocity in a dipolar BEC and fi nd out whether dipolar BECs are similar to superfl uid helium. Plus, they’d like to prove there’s actually a roton hiding somewhere in a BEC of Cr atoms. Of course, the theorists already think the roton is real.

Finding a roton would offer clues to the behavior of systems in which the frequency of waves decreases as the wavelengths get shorter. It also may shed light on what occurs when a collection of atoms is trying to be a solid rather than a liquid. For instance, the spectrum of the roton in liquid helium correlates with the clustering of helium atoms, a fi rst step in the formation of a solid structure.

“The roton is really an oddball phenomenon,” says Bohn. “If you drag a stick (i.e., laser) slowly enough through a BEC, there’s no friction. But once you determine the critical velocity for introducing friction, you can measure the underlying structure of the fl uid and how it uses energy. Plus, it should be really handy to be able to turn friction on and off in a laboratory system.”

Bohn and Wilson see at least a thousand device applications for their new superfl uid BEC brakes. All they have to do is convince someone to do the diffi cult experiments needed to refi ne the control of quantum friction.   - Julie Phillips