A single laser could stop molecules immediately in their tracks, bringing scientists a step closer to creating a quantum computer. 

In quantum physics manipulating molecules to behave a certain way can be tricky, but Northwestern University scientists have found an "elegant" way to stop a molecule from tumbling so other applications can be harnessed.               

"It's counterintuitive that the molecule gets colder, not hotter when we shine intense laser light on it," said lead researchers Brian Odom, an assistant professor of physics and astronomy in the Weinberg College of Arts and Sciences. "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."

Many types of molecules are easy to capture and hold in place, but they also insist on rotating as if they were not actually trapped. Using the unique laser the researchers cooled singly charged aluminum monohydride molecules from room temperature to four degrees Kelvin in a fraction of a second. The dramatic temperature change proved to stop the molecules from rotating.

Harnessing control of a molecule's rotation is essential in the construction of superfast quantum computers, which are much faster and more powerful than modern computers.

In the past researchers thought it would take an impossible amount of lasers to stop a molecule's rotation using the cooling method; to solve this problem the scientists made a custom laser using different components of broadband light. The team filtered out the parts of the spectrum that cause the molecules to spin faster and get hotter while leaving in the parts that slow them down and cool them off.

"In our quantum world, every type of motion has only certain allowed energies," Odom said. "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 team chose aluminum monohydride molecules because they do not vibrate when interacting with a laser; they are also inexpensive and could be used in a wide range of applications past quantum computing including "ultracold quantum-controlled chemistry and tests of whether fundamental constants are truly static or if they vary in time."

"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 said.

The findings were published Sept. 2 by the journal Nature Communications.