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Quantum Simulation: A New Frontier

 

JILA Researchers are exploring quantum simulation with two systems: The Sr-lattice optical atomic clock in the Ye labs and the KRb ultracold molecules in the Jin-Ye lab. Both simulators rely on JILA’s exquisite precision measurement capabilities.

The Sr-Lattice Clock Simulator

With a few technical modifications, the Jun Ye group’s Sr-lattice optical atomic clock has been transformed into a quantum simulator. The Ye group plans to use the new simulator to investigate the quantum behavior of Sr atoms, including the Ana Maria Rey group’s theory of quantum magnetism. The group also wants to use the simulator to explore novel quantum states engineered with lattice-confined atoms and new phases of matter.

The simulator will allow researchers to study naturally occurring magnetism by using atomic collisions and dipolar interactions between the clock electronic states as proxies for the interactions in solids causing magnetic properties. Under current operating conditions, interactions can be very weak in the clock experiment, but given that other noise sources have been largely suppressed, interactions can dominate the dynamics and give rise to strongly correlated quantum states. These states exhibit novel dynamics dictated by a magnetic spin model.  

The Sr optical lattice clock is currently the most precise clock in the world. However, to fully resolve quantum correlations, an even better level of precision and stability is required. The researchers expect to be able to reach this level in the near future and use it to discover all sorts of new ways in which the Sr atoms interact. They also expect to gain an understanding of the complex quantum dynamics that take place during the operation of the clock. This understanding, in turn, will open up the possibility of using novel quantum states to push the frontier of measurement science.

Since precision measurement tools have already allowed researchers to observe quantum interactions inside the Sr-lattice clock, the clock has become a laboratory for exploring spin behavior in the quantum world. Spins interact with each other in a complex manner that is nearly impossible to model with classical physics. Plus, the interactions are so low in energy, they can only be detected in the laboratory with the very best precision measurement tools. These tools allow the researchers to remove irrelevant classical noise and ensure that quantum phenomena stand out in the new simulator.

With precision measurement, it's possible to observe the evolution of a quantum spin system. The clock laser can place all the atoms in a superposition, in which all the spins point up and down at the same time. Over time, the collective superposition of all the atoms will evolve. In watching this evolution, the researchers have concluded that the atoms must be "talking" to each other, or perhaps even "hooking up" in a quantum process known as entanglement.

The experimental signature of such an event include the progressive distortion of the clock transition spectrum as spin interactions increase, a shift of the atomic transition frequency because of spin interactions, distinctive changes in behavior when spins lose their coherence, and a modification in the quantum spin-noise distribution.

In the long term, the researchers want to operate their simulator at ultralow temperatures with the goal of mimicking quantum magnetism in a very controlled way. The Rey and Ye groups will work together on this project.

The KRb Quantum Simulator

Ultracold KRb molecules offer many advantages for quantum simulation. These polar molecules can rotate and vibrate as well interact with each other even if they are separated by long distances. For instance, the quantum state of a molecule at one site in an optical lattice (a crystal of light formed by interacting laser beams) can influence how another molecule relatively far away evolves. Such long-range interactions make it possible for researchers to observe quantum behavior at relatively “warm” temperatures of a few hundred billionths of a degree above absolute zero (0° K, or –459.67° F).

At such low temperatures, the molecules are frozen, but they can still see each other and interact from a distance. This interaction means researchers can observe interesting phenomena such as spin exchange interactions, which are the building blocks of advanced materials and exotic behavior in solid materials. In 2013, the Deborah Jin–Jun Ye collaboration observed spin exchange interactions (predicted by the Ana Maria Rey theory group) for the first time ever in ultracold KRb molecules inside an optical lattice.

The spin exchanges occurred when a rotationally excited KRb molecule interacted with a nonrotating KRb molecule in its ground state. The interactions resulted in the molecules swapping their quantum spin states. The spin swap caused the rotating molecule to stop spinning and enter its ground state, while the second molecule started spinning and became excited. While all this was happening, the molecules in the experiment all remained in their original positions inside the optical lattice.

The ability to observe spin swapping will impact future research in such diverse areas as high-temperature superconductivity, energy transport through biomolecules and in chemical reactions, spintronics (a new kind of microelectronics) as well as the physics of liquids and solids. Spin exchange interactions are critical to an in-depth understanding of these systems, and the KRb quantum simulator will allow JILA researchers to investigate spin exchange on the quantum level.

The quantum simulator will also make it possible to explore how spin exchange interactions are affected by disorder inside the optical lattice. Disorder may prevent spin swapping, localizing the range of spin exchanges. The competition between spin exchange interactions and disorder is one of today’s hottest research topics.

Upcoming investigations with the KRb quantum simulator will focus on the interplay between rotation and motion driven by dipolar interactions. The interplay can be thought of as a type of spin-orbit coupling (i.e., the interaction of a particle’s spin with its motion). Researchers also plan to study spin diffusion. The experimental study of spin diffusion is particularly interesting because it’s difficult to describe theoretically.

The researchers also want to figure out how to cool KRb molecules to near absolute zero. Such an ultracold system will make it possible to explore the relationship between tunneling and spin flips, which, in turn, may open the door to an understanding of high-temperature superconductivity.