Research Highlights

New theory describing the spin behavior of ultracold polar molecules is opening the door to explorations of exciting, new physics in JILA’s cold molecular lab, operated by the Jin and Ye groups. According to the Rey theory group and its collaborators, ultracold dipolar molecules can do even more interesting things than swapping spins.

Symmetries described by SU(N) group theory made it possible for physicists in the 1950s to explain how quarks combine to make protons and neutrons and JILA theorists in 2013 to model the behavior of atoms inside a laser. Now, the Ye group has observed a manifestation of SU(N≤10) symmetry in the magnetic behavior of strontium-87 (⁸⁷Sr) atoms trapped in crystals of light created by intersecting laser beams inside a quantum simulator (originally developed as an optical atomic clock).

There’s exciting news from JILA’s ultracold molecule collaboration. The Jin, Ye, Holland, and Rey groups have come up with new theory (verified by experiment) that explains the suppression of chemical reactions between potassium-rubidium (KRb) molecules in the KRb quantum simulator. The main reason the molecules do not collide and react is continuous measurement of molecule loss from the simulator.

Research associate Bo Yan and his colleagues recently observed spin exchanges in ultracold potassium-rubidium (KRb) molecules inside an optical lattice (a crystal of light formed by interacting laser beams). In solid materials, such spin exchanges are the building blocks of advanced materials and exotic behavior.

Because quantum mechanics is crucial to understanding the behavior of everything in the Universe, one can understand key elements of the behavior of a neutron star by investigating the behavior of an atomic system in the laboratory. This is the promise of the new quantum simulator in the Ye labs. It is a fully controllable quantum system that is being used as a laboratory to study the behavior of other less controllable and more poorly understood quantum systems.

Ana Maria Rey’s group is devising new theoretical methods to help experimentalists use ultracold atoms, ions, and molecules to model quantum magnetism in solids. Research associate Kaden Hazzard, former research associate Salvatore Manmana, newly minted Ph.D. Michael Foss-Feig, and Fellow Rey are working on developing new tools to understand these models, which describe both solids and ultracold particles. The theorists are collaborating with three experimental teams at JILA and the National Institute of Standards and Technology (NIST).

When experimental physicists at Penn State were unable to observe some of the predicted behaviors of ultracold rubidium (Rb) atoms expanding inside a two-dimensional crystal of light, they turned to their theorist colleagues at the City University of New York and JILA for an explanation. Graduate student Shuming Li and Fellow Ana Maria Rey were happy to oblige.

Most scientists think it is really hard to correlate, or entangle, the quantum spin states of many particles in an ultracold gas of fermions. Fermions are particles like electrons (and some atoms and molecules) whose quantum spin states prevent them from occupying the same lowest-energy state and forming a Bose-Einstein condensate. Entanglement means that two or more particles interact and retain a connection. Once particles are entangled, if something changes in one of them, all linked partners respond.

News Flash!  The Rey group has discovered another good reason for using alkaline-earth atoms, such as strontium (Sr) or Ytterbium (Yb), in experimental quantum simulators. Quantum simulators are systems that mimic interesting materials or mathematical models in a very controlled way. The new reason for using alkaline earth atoms in such systems comes from the fact that their nuclei come in as many as 10 different magnetic flavors, i.e., their spins can be in 10 different quantum states.

Incredibly sensitive measurements can be made using particles that are correlated in a special way (called entanglement.)  Entanglement is one of the spooky properties of quantum mechanics – two particles interact and retain a connection, even if separated by huge distances.  If you do something to one of the particles, its linked partners will also respond.

Physicists would very much like to understand the physics underlying high-temperature superconductors. Such an understanding may lead to the design of room temperature superconductors for use in highly efficient and much lower-cost transmission networks for electricity. A technological breakthrough like this would drastically reduce world energy costs. However, this breakthrough requires a detailed understanding of the physics of high-temperature superconductivity.

Theoretical physicists recently combined two powerful tools for exploring ultracold atomic gases: Optical lattices and Feshbach resonances. Optical lattices are crystals of light formed by interacting laser beams. Feshbach resonances in an ultracold atom gas occur at a particular magnetic field strength and cause ultracold atoms to form very large, loosely associated molecules. However, because lattice atoms interact strongly at a Feshbach resonance, the physics of Feshbach resonances in an optical lattice is quite complicated.

In 2008-2009, much to their amazement,researchers working on the Jun Ye group’s neutral Sr optical atomic clock discovered tiny frequency shifts caused by colliding fermions! They figured out that the clock laser was interacting slightly differently with the Sr atoms inside a one-dimensional (pancake-shaped) trap. The light-atom interactions resulted in the atoms no longer being identical. And, once they were distinguishable, formerly unneighborly atoms were able to run into each other, compromising clock performance.

“Nature is built quantum mechanically,” says Fellow Jun Ye, who wants to understand the connections between atoms and molecules in complex systems such as liquids and solids (aka condensed matter). He says that the whole Universe is made of countless interacting particles, and it would be impossible to figure out the myriad connections between them one particle at a time, either theoretically or experimentally.

Fellows Ana Maria Rey and Jun Ye have come up with a clever idea that should make it much easier to design a quantum computer based on alkaline-earth atoms such as strontium (Sr). In this work, they collaborated with former research associate Marty Boyd, former JILA Fellow Peter Zoller (University of Innsbruck), and colleagues from Harvard University and the University of Innsbruck.