Ultracold Atoms Used To Develop Quantum Simulator

By Shweta Iyer on May 15, 2014 4:18 PM EDT

Ultracold Atom Vacuum System
Pictured is a vacuum system that isolates ultracold atoms from room temperature and pressure. Atoms are suspended in this vacuum with laser beams, and manipulated with magnetic fields. (Photo: Denzil Green / Canadian Instit)

 Although they are vastly complex, simulators have been designed to improve our understanding of quantum materials. Now, CIFAR (Canadian Institute for Advanced Research) Fellow Joseph Thywissen (University of Toronto) has created quantum simulations with ultracold atoms, according to a press release Thursday..

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Minimal surfaces have long been studied using bubbles and soap film geometry. In fact according to Thywissen the solution to finding the minimal surfaces of fixed edges is to simply dip a wire hanger in soap solution. Just like soap bubbles answer several questions on minimal surfaces and architecture, Thywissen and his team hope to use simulations to solve several questions relating to quantum materials. Since the electrons within quantum materials, such as superconductors, zoom far too quickly for careful observation, Thywissen's team uses ultracold gases instead, in this way simulating one quantum system with another, more easily studied, quantum system at the University of Toronto's Ultracold Atoms Lab,.

"Simulation gives you the answers but not the theory behind them," says Thywissen.

Thywissen and his team talk about magnetism and diffusion of atoms in ultracold gases, in a report published in the journal ScienceThe researchers optically trapped a cloud of gas a billion times colder than air in a very low-pressure vacuum. By doing this the ultracold atoms behaved like a chain of little magnets. The scientists then oriented the atoms to point in the same direction in space, then manipulated the spins with an effect that's regularly used in hospitals for MRIs, called a spin echo.

They then twisted up the atoms in a corkscrew pattern and then untwisted it to measure the strength of interactions between atoms. They observed that first the atoms did not interact, but just a millisecond later they were strongly interacting and correlated.The scientists concluded that something effected the atom's magnetism, which caused the change.

"The Pauli Principle forbids identical ultracold atoms from interacting, so we knew something was scrambling the spins at a microscopic level," Thywissen says.The reason for this behaviour the scientists concluded was diffusion, the same process that takes place when the smell of perfume fills the air of a room, for example. "If I open a bottle of perfume in the front of the room, it takes a little while for those particles to diffuse to the back of the room," Thywissen says. "They bump into other particles on the way, but eventually get there. You can imagine that the more particles bump into each other, the slower diffusion occurs."

The scientists then tried to gauge the effect of slow diffusion by lowering the temperature below a millionth of a degree above absolute zero. But the speed of diffusion did not reach zero as expected, instead the experiment found a lower limit to diffusion."Whereas cars on the freeway need to drive below the speed limit, strongly interacting spins need to diffuse above a quantum speed limit," Thywissen says.Ultracold atoms provide a promising new medium for exploration and design of engineered quantum systems. They can be used to study the properties of complex systems like high-temperature superconductors and novel magnetic materials that have potential applications in data and energy storage. Thywissen concludes, "Our measurements imply a diffusivity bound whose mathematical simplicity is exciting: it hints at a universal principle about spin transport, waiting to be uncovered."

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