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The Magnificent Quantum Laboratory

Published: 08-08-2013

Inside the Ye group’s new quantum simulator, the electron spins of strontium atoms evolve over time and may become entangled (lower right). Once the particles are entangled, if something changes in one of them, all linked partners respond. Credit: Ye group and Brad Baxley, JILA

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.

Most people would imagine such a quantum simulator as being very different from a clean and deceptively simple experiment like the Ye group’s strontium (Sr) lattice optical atomic clock, which includes an exquisite precision measurement capability. But, they would be wrong. The precision measurement capability of the Sr-lattice clock offers researchers a unique ability to probe systems for long times. This ability is necessary for the study of quantum behavior in atomic systems, whose interactions (and energy) are tinier than other quantum systems such as liquids, solids, or neutron stars.

So, once newly minted Ph.D. Mike Martin and his colleagues had developed one of the world’s most accurate and precise atomic clocks, all they needed to transform the clock into a quantum simulator was a lot of patience and a powerful collaboration with the Rey theory group. The first use of the new simulator to probe collective atomic interactions was reported recently in Science.

The interactions studied by Martin and his colleagues can be used to model a magnetic spin system. A better understanding of the quantum behavior of magnetic spins will not only lead to a better understanding of magnetism in general, but also of superconductivity. It will also lead to a deeper appreciation of the mysteriously interconnected quantum world and its relationship to the world we know.

Even though the researchers discovered how to use the Sr-lattice clock as a quantum simulator, their initial goals were to make a better clock and improve advanced tools used to measure the clock’s accuracy and precision. To accomplish these goals, Martin built one of the world’s best, lowest-noise clock lasers. Pulses from this laser interacted with Sr atoms inside a stack of 100 “pancakes” of approximately 20 atoms each, causing the atoms to evolve between two energy levels. The energy-level evolutions are the ticks of the clock.

However, the Ye group already knew that with the new, quiet laser with its enhanced precision, atomic interactions would still appear to play a dominant role in causing the tiny variations in the ticks of the clock. As Martin and others probed the causes of these variations, they sought help from the Rey theory group to eliminate them. New theory helped the experimentalists eliminate the variations in the clock ticks due to atomic interactions.

In the process, Martin and his colleagues realized that since their precision measurement tools had allowed them to observe quantum interactions inside the clock, the clock would be a superb 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, such interactions are so low in energy, they can only be detected in the laboratory with the very best precision measurement tools.

“Precision measurement and control allow us to remove irrelevant classical noise and ensure that quantum phenomena stand out in the quantum laboratory, “ Ye explained. “And, what happens in the laboratory is helping us to expand the frontiers of measurement science because we can use our deeper understanding of the quantum system to design better tools.”

Precision measurement makes it possible to observe the evolution of a quantum spin system. The clock laser can place all the atoms in the simulator in a “superposition,” in which all the spins point up and down at the same time (as shown in the figure). Over time, the collective superposition of all these atoms will evolve.

In watching the evolution of spin states, the researchers have concluded that the atoms must be “talking” to each other inside the pancakes, or perhaps even “hooking up” in a spooky quantum process known as entanglement. Once particles are entangled, if something changes in one of them, all linked partners respond. The experimental signatures for these effects include the progressive distortion of the clock transition spectrum as spin interactions increase, a shift of the atomic transition frequency due to spin interactions, distinctive changes in behavior when spins lose their coherence, and a modification in the quantum spin-noise distribution.

The Rey group is working with the Ye group to explain observations and guide future experiments. The theorists are developing mathematical models to describe the behavior of Sr atoms in the clock. They have already shown that Sr atoms can simulate some aspects of the behavior of electrons in magnetic materials. They are now working to see whether the similarities hold for more complex situations.

Preliminary work suggests that an atomic clock simulator may behave similarly to real spins in magnetic materials. If so, the simulator may lead to better modeling and understanding of quantum magnetism, exotic materials, and superconductivity, which occurs in materials that can conduct electricity without resistance. — Julie Phillips

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