Sebastian Will Is on a Quest for Ultracold Quantum Control

Sebastian Will, a professor of physics at Columbia, has received a $1.32 million grant from the Gordon and Betty Moore Foundation. The grant will support Will’s efforts to create and study a molecular version of a Bose-Einstein Condensate (BEC). 

Originally predicted by the physicists Satyendra Nath Bose and Albert Einstein, BECs are unique states of quantum matter that occur at extremely cold temperatures, just fractions of a degree above absolute zero. When temperatures are this low, gaseous particles normally in motion will come to a near standstill and start acting as a single entity. Now a macroscopic whole, researchers can study the effects of quantum mechanics more easily than if they were dealing with many individual quantum particles.

BECs made from clouds of single types of atoms were first created in 1995. Will and his lab are poised to make a much richer molecular version; work published this summer in Nature Physics brings them a step closer to that goal. “We are at a juncture where we can potentially open a whole new research field,” Will said. “We are extremely grateful for the support of the Moore Foundation as we push toward creating and investigating molecular BECs.”

The Path to Ultracold

Will’s journey toward ultracold science started at the University of Mainz in Germany with a class on atomic physics taught by Immanuel Bloch. “Today, we often say we are in the second quantum revolution, but even as an undergraduate not even twenty years ago, quantum mechanics was taught as a tool to understand nature,” said Will. “But during Bloch’s course, I learned that we can actually control and manipulate quantum systems. This idea that we are not just observers but creators was very inspirational.”

Shifting away from his original plans to become a theorist, Will pursued an undergraduate research opportunity at MIT with Wolfgang Ketterle, who won the 2001 Nobel Prize in Physics for the creation of the first atomic BEC. Will returned to Germany for his PhD with Bloch. When his mentor moved from the University of Mainz to Ludwig Maximilian University of Munich, Will went with him, and completed his PhD there. With Bloch, Will was part of the first generation of students to use ultracold atoms as early quantum simulators, a form of quantum computers that don’t require quantum bits. 

Will returned to MIT as a postdoc in 2011. Now working with Martin Zwierlein, he shifted from working with ultracold atoms to cooling two-atom, or diatomic, molecules. “Some people joke that a diatomic molecule is a molecule with one atom too many. At the time we knew so little about molecules that we were bold enough to try,” Will recalled. It took over three years, but he and his colleagues eventually succeeded in creating one of the first stable ultracold diatomic molecules, sodium potassium.

He brought the techniques used to create ultracold molecules to his own lab at Columbia, which got up and running in 2018. His office features a beloved cryostat, a device that cools materials to ultra-low temperatures, that once belonged to Nobel Laureate and former Columbia professor Horst Störmer.

But the cryostat for liquid helium is more a memento than a piece of functioning equipment; Will doesn’t use cryogenic equipment in his lab “We need to go colder than liquid helium, to temperatures below a microkelvin above absolute zero,” said Will.  And for that, they use lasers.

With carefully calibrated arrays of laser and hundreds of optical elements, the Will lab is currently creating molecular gas clouds of sodium-cesium. The process is actually three experiments in one, he noted: There’s one experiment to cool sodium, another to cool cesium, and a molecule assembler system to put together the molecule, atom by ultracold atom. The resulting molecules are close to forming BECs, and the lab is perfecting the final steps to nudge them just a little bit colder. 

Science at a Distance

To laser-cool a particle, researchers blast it repeatedly with photons until it stops moving. Molecules are challenging to cool because they have, in scientists’ lingo, multiple quantum degrees of freedom. Whereas atoms simply have electrons that can jump between energy levels, molecules have interacting electrons, and the atoms that make them up also vibrate and rotate relative to each other. That all makes for a complicated quantum particle, Will said. Labs like his need to target each of these degrees of freedom in order to create ultracold molecules. 

But the effort to cool polar molecules is worth it to study long-range interactions, which, Will noted, is where the most interesting physics happens In a familiar example, long-range interactions between polar water molecules are what give rise to ice. But that’s a classical crystal. In the world of quantum mechanics, scientists suspect that long-range interactions will give rise to new types of self-organization, such as superfluidity, in which a material flows with zero resistance; new types of crystals; and even new states of matter like supersolids, which are simultaneously both a crystal and a superfluid.

Though Will has an eye toward eventually applying ultracold molecules in devices such as quantum simulators or computers, there are more fundamental questions that his lab hopes to answer first, like how things self-organize in the quantum regime and how that can be controlled. “We will be able to see each and every molecule,”  Will said.  All starting from a molecular BEC, in which thousands of molecules will exist in the same quantum state.

In the future, Will sees the lines between the subfield of atomic, molecular, and optical physics in which he operates blurring with condensed matter physics, which studies larger assemblages of atoms arranged into crystals. He hopes to someday create artificial crystals made of molecular BECs that could serve as simpler and easier-to-manipulate models of the physics that govern solids.

As Will thinks about his own students, he hopes to continue to convey the sense of wonder that captured his attention years ago in Bloch’s lecture hall. (Will remains in touch with Bloch, as well as Ketterle and Zwierlein.) “We can build complex quantum systems from the bottom up. We can control single quantum particles and do interesting things with them,” Will said. “That’s still a miracle, even though we do it every day in the lab.”