Molecule quits tumbling when laser cools it

"In our quantum world, every type of motion has only certain allowed energies," says Brian Odom. "If I want to slow down a molecule, quantum mechanics tells me that it happens in steps. And there is a very lowest step that we can get the molecule down to, which is what we've done." (Credit: Ninaada Bellippady/Flickr)

Scientists have used a single laser to instantly cool a trapped molecule to the temperature of outer space and to stop its rotation.

The method stops a molecule from tumbling so that its potential for new applications can be harnessed.

“It’s counterintuitive that the molecule gets colder, not hotter, when we shine intense laser light on it,” says study leader Brian Odom, assistant professor of physics and astronomy at Northwestern University.

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“We modify the spectrum of a broadband laser, such that nearly all the rotational energy is removed from the illuminated molecules. We are the first to stop molecular tumbling in such a powerful yet simple way.”

It is not very difficult to trap many types of molecules and hold them precisely in place, Odom says, but they stubbornly persist in rotating just as much as if they were not trapped at all.

Using their customized laser, he and his colleagues cooled singly charged aluminum monohydride molecules from room temperature to 4 degrees Kelvin (minus 452 degrees Fahrenheit) in a fraction of a second.

The abrupt temperature drop stopped the molecules’ normally persistent tumbling motion in its tracks. Such control of molecules, of their rotational and vibrational states, is essential to using molecules in the construction of superfast quantum computers—machines whose processing power would be exponentially faster than today’s computers.

The new technique is faster, easier, and more practical and efficient than techniques developed thus far to control molecules. Details appear in Nature Communications.

Filter the spectrum

Researchers once assumed that far too many lasers would be required to cool molecular rotations. However, broadband laser light contains many different frequency components, and the Northwestern researchers used those components to custom design a laser for their task.

They filtered out the part of the spectrum that causes molecules to start spinning faster (and become hotter) while leaving in the useful frequency components that slows the molecules down (and also cools them).

Also noteworthy, Odom says, is that they cooled the molecule to its very lowest quantum rotational state using a room-temperature apparatus, not the cumbersome liquid helium cryostats some other researchers have used.

“In our quantum world, every type of motion has only certain allowed energies,” says Odom, an atomic physicist. “If I want to slow down a molecule, quantum mechanics tells me that it happens in steps. And there is a very lowest step that we can get the molecule down to, which is what we’ve done.”

The right molecule

Odom and his team chose to work with singly charged aluminum monohydride molecules because the molecule does not vibrate when it interacts with a laser. “By choosing the right molecule we were able to stop the molecules from rotating without worrying about the vibrations,” Odom says.

Aluminum monohydride molecules are inexpensive and could be used in wide range of applications, beyond quantum computing.

“There is a lot you can do if you get one species of molecule under control, such as we’ve done in this study,” Odom says.

In addition to quantum information processing, applications that could springboard off this new ability to control molecular rotors include ultracold quantum-controlled chemistry and tests of whether fundamental constants are truly static or if they vary in time.

The Air Force Office of Scientific Research, the National Science Foundation, and the David and Lucile Packard Foundation supported this research.

Source: Northwestern University