Physicists work to create exotic quantum state

Cooled Matter in Molecular Bose-Einstein Condensates: A Physicist’s Journey into Strange Worlds

Quantum physicists predicted in the 1920s that, when cooled down to close to absolute zero, matter would start to behave in strange ways. The more you know about a particle’s momentum, the more uncertain its position is. If matter is cooled so much that is almost stationary, uncertainty in its position balloons. Once uncertainty becomes greater than the distance between the particles, they become indistinguishable and overlap to occupy a single, lowest-energy quantum state — a Bose–Einstein condensate.

Molecules are much more complicated than atoms, says Sebastian Will, a physicist at Columbia University in New York City, who led the latest work. They can rotate and vibrate in ways that are impossible for atoms, and polar molecules — which have positively and negatively charged ends — can interact over long ranges through electromagnetic forces. A molecule would allow physicists to understand a wider array of phenomena because they are able to see long-range interactions.

Researchers can use the system to test out phenomena like magnetism and the emission of Hawking radiation from a model black hole, because it displays quantum behavior that is well controlled and at a macroscopic scale. Condensates have been used as quantum sensors and atomic clocks, and even gone into space.

But there’s a catch. “Compared to atoms … molecules are more difficult to control and cool,” says Zoe Yan, a physicist at the University of Chicago in Illinois.

Feshbach structures have been damaged before. But in stable molecules, the final stage of cooling, to turn clouds of them into a condensate, has been scuppered by chemical reactions between colliding molecules. The interactions cause the molecule to escape and leave too many to work with.

Will and his team found a way to prevent these collisions in a cloud of polar molecules, each made from one sodium and one caesium atom. The team used microwave fields to make the cloud rotates and make the molecules oscillate. Together these fields oriented the molecules such that they always repelled each other. Will says that this turned out to be crucial.

Physicists will also be able to test predictions about how this strange matter will behave. Will says that tuning the microwave fields will allow the team to separate the system into quantum droplets, a new phase of matter. By confining the condensate in two dimensions using lasers, the team also hopes to watch while the molecules arrange themselves, under a microscope, to form a kind of crystal. It has never been possible, says Will.

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