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The Resonance Motel

Where molecules check in, but they don't check out
Published: 03-13-2014

The Resonance Motel at JILA, where molecules check in, but they don’t check out. Credit: The Bohn group and Brad Baxley, JILA

Quantum chaos just showed up in an ultracold gas of erbium atoms, and the Bohn theory group knows why. Theorists expect quantum chaos to appear when quantum mechanical objects get sufficiently complicated. But until now, scientists hadn’t realized that something as simple as a pair of colliding atoms could be complicated enough for quantum chaos to appear. For instance, the Bohn group has spent several years investigating the theoretical spectra of ultracold molecules, which contain a plethora of densely packed, randomly spaced Fano-Feshbach resonances. The researchers speculate that the spread of these resonances will be similar to patterns explained by quantum Chaos Theory.

It turns out that complicated atoms like erbium also have a boatload of resonances, as reported by Bohn and his experimental collaborators in Nature online March 12, 2014. Nearly 200 resonances showed up in a relatively narrow magnetic-field scan that would have captured only two or three Feshbach resonances in rubidium or other “simple” alkali atoms studied at JILA and all around the world. There are so many resonances in ultracold erbium that it may not be possible to investigate them individually. The best possible route to understanding them may be to ask what the resonances are like taken together.

Luckily, the well-known physicist Freeman Dyson came up with a theory in the 1960s to describe all sorts of bumping and shaking going on inside the nuclei of atoms. Dyson’s statistical analyses of atomic nuclei not only revealed that resonances tend to be spread out relatively evenly, but also that resonances seemed to follow the patterns explained by classical Chaos Theory. The Bohn group decided to look at classical Chaos Theory as a way to begin to understand chaos in the ultracold quantum world.

The group realized a while ago that a chaotic spread of resonances might not be good news for experimentalists planning to investigate ultracold molecules. That’s because molecules are attracted to the resonance motel. They like to check in. Ordinarily, if the molecules bump into each other at sufficiently low temperature, there isn’t usually enough energy to cause a chemical reaction,  and they eventually bounce harmlessly off one another. However, in a resonance, the molecules exploit their energy of attraction to get them rotating and vibrating, so there would be lots of jiggling and bumping around.

But the trouble is, when the molecules move into this resonant mode, it takes a long time before one of the molecules randomly acquires enough energy to escape rather than to just jiggle. In fact, most of the time, escape would take longer than an ultracold experiment lasts. This is a problem for scientists because when molecules are living in a resonance motel, they disappear from view in the experiment. No wonder on a recent occasion at a conference, researchers who investigate ultracold molecules referred to Bohn as the “Angel of Death.”

Bohn’s Innsbruck collaborators also approached him at a conference to ask if he thought they’d be able to see a slew of resonances in erbium since it was a complicated atom. Bohn told them it would be worth a look. They looked and found 190 resonances in one isotope and 189 of them in another isotope. (Isotopes have the same number of protons in their nuclei, but different numbers of neutrons.)

Thanks to the Innsbruck team, the world has seen the first experimental verification of chaotic behavior in the interactions between ultracold atoms—a stunning result that promises to entirely change the landscape of ultracold atomic and molecular physics.

In developing a theory of ultracold resonant behavior, Bohn worked with research associate James Croft, graduate student Brandon Ruzic, former research associate Michael Mayle, and former senior research associate Goulven Quéméner. His experimentalist colleagues included Albert Frisch, Michael Mark, Kiyotaka Aikawa, and Francesca Ferlaino of the Universität Innsbruck. Constantinos Makrides, Alexander Petrov, and Svetlana Kotochigova of Temple University contributed to the theoretical analysis of quantum chaos in ultracold collisions of erbium.—Julie Phillips


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