Ultracold atoms exhibit a new type of quantum magnetic behavior

December 17, 2020

(Nanowerk News) A new study highlights an amazing dance among spinning atoms. In a paper appearing in the magazine Nature (“Spinning transport in a tunable Heisenberg model produced by ultracold atoms”), researchers from MIT and Harvard University show how magnetic forces at the quantum, atomic scale affect how atoms control the spins.

In experiments with ultracold lithium atoms, the researchers looked at different ways in which the spins of the atoms develop. Like tippy ballerinas pirouetting back to an upright position, the spinning atoms return to the symmetrical side in a way that depends on the magnetic forces between individual atoms. For example, the atoms can spin to equilibrium in ballistic very quickly, ?? fashion or in a slower, more diffused pattern. spins - basic units of magnetism MIT and Harvard researchers have studied how basic units of magnetism, called spins (the black arrows), move around and interact with other spins, in a series of single atoms (the colored areas). The background shows a true image of the spins, showing an occasional different change of the blue atoms (spinning up). (Image courtesy of researchers)

The researchers found that these behaviors, which have not been observed to date, could be mathematically explained by the Heisenberg model, a set of equations commonly used for predicting magnetic behavior. Their results address the fundamental nature of magnetism, reflecting the diversity of behavior in one of the simplest magnetic materials.

Maybe this improved understanding of magnetism will help design engineers ?? spintronic ?? machines, which transmit, process, and store information using quantum grain spinning rather than electric currents.

?? Exploring one of the simplest magnetic materials, we have advanced an understanding of magnetism, ?? said Wolfgang Ketterle, John D. Arthur professor of physics at MIT and team leader of MIT. ?? When you find new onions in one of the simplest models in physics for magnetism, you have the opportunity to fully explain and understand it. This is what gets me out of bed in the morning, and excites me. ??

The co-authors of Ketterle ?? MIT graduate student and lead author Paul Niklas Jepsen, along with Jesse-Amato Grill, Ivana Dimitrova, both MIT postdocs, Wen Wei Ho, postdoc at Harvard University and Stanford University, and Eugene Demler, a Harvard professor of physics . They are all researchers at the MIT-Harvard Center for Ultracold Atoms. The MIT team is attached to the Department of Physics and Electrical Research Laboratory.

Rows of spins

Quantum spin is thought to be the microscopic unit of magnetism. At the quantum scale, atoms can spin clockwise or counterclockwise, giving them direction, like a compass needle. In magnetic materials, spinning of many atoms can reveal several onions, including symmetrical states, where atomic spins are connected to each other, and dynamic behavior, where the spins over many atoms resemble a pattern. like a wave.

This is the last pattern that was studied by the researchers. The dinamics of the wavelike spinning pattern are well aware of the magnetic forces between atoms. The wave pattern erupted much faster for isotropic magnetic forces than for anisotropic forces. (Isotropic forces do not depend on how all the spins are directed in space).

Ketterle’s group aimed to study this phenomenon with an experiment in which they first used laser-based cooling techniques to bring lithium atoms down to about 50 nanokelvin ?? more than 10 million times colder than intersex space.

At such deep temperatures, frozen atoms are virtually stationary, so that researchers can accurately see the magnetic effects that would be hidden by the thermal movement of the atoms. The researchers then used a system of lasers to capture and arrange several layers with 40 atoms each, like beads on a string. In total, they created a surface of about 1,000 strings, containing about 40,000 atoms.

?? You can think of the flames as tweezers that capture the atoms, and if they are warmer they would escape, ?? Jepsen explains.

They then applied a pattern of radio waves and a pulsating magnetic force to the entire surface, which stimulated each atom across the string to turn its spin into a helical (or wavelike) pattern. The wave-like patterns of these layers together correspond to the periodic density change of the ?? spin up ?? atoms that are like a strip pattern, which the researchers could image on a detector. They then watched as the stripe patterns disappeared as the individual spins of the atoms approached their state of equilibrium.

Ketterle compares the experiment to plucking a guitar string. If the researchers looked at the spins of the atoms at equilibrium, this would not tell them much about the magnetic forces between the atoms, just as a stationary guitar string would not reveal much about its physical properties. . By plucking the string, taking it out of symmetry, and seeing how it vibrates and finally returns to its original state, one basic thing can be learned about its physical features. at the string.

?? What we are doing here is, are we ?? sort of spins the string of spins. We insert this helix pattern, and then observe how this pattern behaves as a time function, ?? Ketterle says. ?? This allows us to see the effect of different magnetic forces between the spins. ??

Ballistics and inc

In their experiment, the researchers changed the strength of the gravitational force applied, to change the width of the stripes in the atomic spinning patterns. They measured how fast, and in what ways, the patterns went down. Depending on the nature of magnetic forces between atoms, they observed very different behaviors in terms of how quantum spouts returned to equilibrium.

They found a transition between ballistic behavior, where the spins quickly fired back into a symmetrical state, and scattered behavior, where the spins move worse, and the overall stripe pattern spreads slowly. back to equilibrium, as a drop of ink spreads slowly in water.

Some of this behavior has been predicted theoretically, but has not been looked at in detail until now. Some other results were completely unexpected. In addition, the researchers found that their observations mathematically corresponded to what they measured with the Heisenberg model for their experimental parameters. They teamed up with theorists at Harvard, who did modern calculations of spinning dynamics.

It was interesting to see that there were buildings that were easy to measure, but difficult to work out, and other buildings could be measured, but not measured, ?? Ho says.

In addition to advancing an understanding of magnetism at a basic level, the team’s results can be used to study the properties of new materials, as a kind of quantum simulator. Such a platform could function as a special-purpose quantum computer that measures material behavior, in a way that transcends the capabilities of today’s most powerful computers.

?? With all the current excitement about the promise of quantum information science to solve practical problems in the future, it’s good to see work like this come to fruition today, “says John Gillaspy, program officer in the Department of Physics at the National Science Foundation, is the funder of the research.