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Physicists use classical concepts to decipher strange quantum behaviors in an ultracold gas



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There they are, in all their weird quantum glory: ultra-cold lithium atoms in the optical trap run by UC Santa Barbara undergraduate student Alec Cao and his colleagues from David Weld̵

7;s atomic physics group. Held by lasers in a regular lattice formation and “driven” by pulses of energy, these atoms were doing crazy things.

“It was a little weird,” Weld said. “The atoms were pumped in one direction. Sometimes they were pumped in another direction. Sometimes they tore apart and created these structures that looked like DNA.”

These new and unexpected behaviors were the results of an experiment conducted by Cao, Weld and colleagues to push the boundaries of our knowledge of the quantum world. The results? New directions in the field of dynamic quantum engineering and a tempting path towards a link between classical and quantum physics.

Their research is published in the journal Physical Review Research.

“A lot of fun things happen when you shake a quantum system,” said Weld, whose lab creates “artificial solids” – low-dimensional lattices of light and ultra-cold atoms – to simulate the behavior of quantum particles in real, more densely packed solids when subjected to driving forces. The recent experiments were the latest in a line of reasoning that dates back to 1929, when physicist and Nobel laureate Felix Bloch first predicted that within the confines of a periodic quantum structure, a quantum particle will oscillate under a constant force.

“They actually move back and forth, which is a consequence of the wave nature of matter,” Weld said. Although these position-space Bloch oscillations were predicted nearly a century ago, they have only been directly observed relatively recently; in fact, Weld’s group was the first to see them in 2018, with a method that made these swashings infinitesimal, often fast, wide and slow and easy to see.

A decade ago, other experiments added a time dependence to Bloch’s oscillating system by subjecting it to additional periodic force and discovered even more intense activity. The swings above the swings, the Super Bloch swings, were discovered.

For this study, the researchers took the system one step further by changing the space in which these atoms interact.

“We are actually changing the lattice,” Weld said, varying the laser intensities and external magnetic forces that not only added a time dependence, but also curved the lattice, creating an inhomogeneous force field. Their method of creating large, slow oscillations, he added, “gave us the opportunity to see what happens when you have a Bloch oscillating system in a heterogeneous environment.”

This is when things get weird. The atoms moved back and forth, sometimes expanding, other times creating patterns in response to the pulses of energy that pushed the lattice in various ways.

“We could track their progress in numbers if we worked hard,” Weld said. “But it was a bit difficult to understand why they do one thing and not the other.”

It was the intuition of Cao, the lead author of the article, that led to a way to decipher the strange behavior.

“When we studied dynamics for all times at once, we just saw a mess because there was no underlying symmetry, making physics difficult to interpret,” said Cao, who is starting his fourth year at the College of Creative. Studies of the UCSB.

To draw the symmetry, the researchers simplified this seemingly chaotic behavior by eliminating one dimension (in this case, time) using a mathematical technique initially developed to observe classical nonlinear dynamics called the Poincaré section.

“In our experiment, a time interval is fixed by how we periodically change the lattice over time,” Cao said. “When we threw out all the ‘in-between’ times and looked at the behavior once each period, structure and beauty emerged in the shapes of the trajectories because we adequately respected the symmetry of the physical system.” Observing the system only in periods based on this time interval has produced something like a stop-motion representation of the complicated but cyclical movements of these atoms.

“What Alec understood is that these paths – these Poincaré orbits – tell us exactly why in some driving regimes the atoms are pumped, while in other driving regimes the atoms expand and break the wave function,” he said. added Weld. One direction researchers could take from here, he said, is to use this knowledge to design quantum systems to have new behaviors through driving, with applications in growing fields such as topological quantum computing.

“But another direction we can take is to see if we can study the emergence of quantum chaos when we start doing things like adding interactions to a driven system like this,” Weld said.

This is no small feat. Physicists have been trying for decades to find links between classical and quantum physics, a common mathematics that could explain concepts in one field that seem to have no analogues in the other, such as classical chaos, the language of which does not exist in quantum mechanics.

“You’ve probably heard of the butterfly effect: A butterfly flapping its wings in the Caribbean can cause a typhoon somewhere around the world,” Weld said. “This is actually a characteristic of classical chaotic systems, which have a sensitive dependence on initial conditions. This characteristic is actually very difficult to reproduce in quantum systems: it is puzzling to find the same explanation in quantum systems. So this is perhaps a small one. piece of that body of research. ”


Ultra-cool quantum particles break classical symmetry


More information:
Alec Cao et al. Transport controlled by the topology of the Poincaré orbit in a guided inhomogeneous reticular gas, Physical Review Research (2020). DOI: 10.1103 / PhysRevResearch.2.032032

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University of California – Santa Barbara




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Physicists use classical concepts to decipher strange quantum behaviors in an ultracold gas (2020, September 9)
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