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CHIPS Articles: Atoms that hum

Atoms that hum
An expanding cloud of atoms could offer insight into unanswered questions about the universe
By CHIPS Magazine - May 3, 2018
Scientists from the Joint Quantum Institute (JQI), working with a cloud of ultra-cold atoms, discovered behavior that is strikingly similar to the universe in microcosm. Their work covers new connections between atomic physics and the sudden expansion of the early universe, according to a paper published April 19 in Physical Review X and featured in Physics, according to a release by the JQI.

"From the atomic physics perspective, the experiment is beautifully described by existing theory," says Stephen Eckel, an atomic physicist at the National Institute of Standards and Technology (NIST) and the lead author of the new paper. "But even more striking is how that theory connects with cosmology."

In several sets of experiments, Eckel and his colleagues rapidly expanded the size of a doughnut-shaped cloud of atoms, taking snapshots during the process. “The growth happens so quickly that the cloud is left humming, and a related hum may have appeared on cosmic scales during the rapid expansion of the early universe — an epoch that cosmologists refer to as the period of inflation,” the JQI said.

The work brought together experts in atomic physics and gravity, and the authors say “it is a testament to the versatility of the Bose-Einstein condensate (BEC) — an ultra-cold cloud of atoms that can be described as a single quantum object — as a platform for testing ideas from other areas of physics.”

Britannica.com describes the BEC as a state of matter in which separate atoms or subatomic particles that are cooled to near absolute zero (0 K, - 273.15 °C, or - 459.67 °F; K = kelvin) coalesce into a single quantum mechanical entity that can be described by a wave function on a near-macroscopic scale.

"Maybe this will one day inform future models of cosmology," Eckel says. "Or vice versa. Maybe there will be a model of cosmology that’s difficult to solve but that you could simulate using a cold atomic gas."

It’s not the first time that researchers have connected BECs and cosmology. Previous studies mimicked black holes and searched for analogs of the radiation predicted to emit from their shadowy boundaries. The new experiments focus instead on the BEC’s response to a rapid expansion, a process that suggests several analogies to what may have happened during the period of inflation.

The first and most direct analogy involves the way that waves travel through an expanding medium. Such a situation doesn’t arise often in physics, but it happened during inflation on a grand scale, according to the physicists. During that expansion, space itself stretched any waves to much larger sizes and stole energy from them through a process known as Hubble friction, according to the scientific research revealed in the new paper.

In one set of experiments, researchers spotted analogous features in their cloud of atoms. They imprinted a soundwave onto their cloud — alternating regions of more atoms and fewer atoms around the ring, like a wave in the early universe — and watched it disperse during expansion. Unsurprisingly, the soundwave stretched out, but its amplitude also decreased. The math revealed that this hindering looked just like Hubble friction, and the behavior was explicitly detailed by calculations and numerical simulations.

"It's like we're hitting the BEC with a hammer," says Gretchen Campbell, the NIST co-director of the Joint Quantum Institute and a coauthor of the paper, "and it’s sort of shocking to me that these simulations so nicely replicate what's going on."

In a second set of experiments, the team uncovered another, more speculative analogy. For these tests they left the BEC free of any soundwaves but provoked the same expansion, watching the BEC slosh back and forth until it relaxed.

In a way, that relaxation also resembled inflation. Some of the energy that drove the expansion of the universe ultimately ended up creating all of the matter and light around us. And although there are many theories for how this happened, cosmologists aren’t exactly sure how that leftover energy got converted into all the matter we see and live among today.

In the BEC, the energy of the expansion was quickly transferred to things like soundwaves traveling around the ring. Some early guesses for why this was happening looked promising, but they fell short of predicting the energy transfer accurately. So the team turned to numerical simulations that could capture a more complete picture of the physics.

What emerged was a complicated account of the energy conversion the JQI described this way: “After the expansion stopped, atoms at the outer edge of the ring hit their new, expanded boundary and got reflected back toward the center of the cloud. There, they interfered with atoms still traveling outward, creating a zone in the middle where almost no atoms could live. Atoms on either side of this inhospitable area had mismatched quantum properties, like two neighboring clocks that are out of sync.”

The state resulting was highly unstable and eventually collapsed, leading to the creation of little quantum whirlpools (or vortices) throughout the cloud the authors said. These vortices would then break apart and generate soundwaves that ran around the ring, like the particles and radiation left over after inflation. Some vortices even escaped from the edge of the BEC, creating an imbalance that left the cloud rotating.

Unlike the analogy to Hubble friction, the complex story of how splashing atoms can create dozens of quantum whirlpools may not show any resemblance to what goes on during and after inflation. But Ted Jacobson, a coauthor of the new paper and a physics professor at the University of Maryland specializing in black holes, says that his discussions with atomic physicists yielded benefits outside these technical results.

"What I learned from them, and from thinking so much about an experiment like that, are new ways to think about the cosmology problem," Jacobson says. "And they learned to think about aspects of the BEC that they would never have thought about before. Whether those are useful or important remains to be seen, but it was certainly stimulating."

Scientists agreed that looking at the research with a different perspective provides a better chance of solving the problem.

The JQI announced that future experiments may study the complicated transfer of energy during expansion more closely, or even search for further cosmological analogies. "The nice thing is that from these results, we now know how to design experiments in the future to target the different effects that we hope to see," Campbell says. "And as theorists come up with models, it does give us a testbed where we could actually study those models and see what happens."

The new paper included contributions from two coauthors not mentioned in the text: Avinash Kumar, a graduate student at JQI; and Ian Spielman, a JQI Fellow and NIST physicist.

Reference Publication
"A Rapidly Expanding Bose-Einstein Condensate: An Expanding Universe in the Lab," S Eckel, A Kumar, T Jacobson, I B. Spielman, G K. Campbell, Phys. Rev. X, 8, 021021 (2018)

The Joint Quantum Institute (JQI) serves as a world-class research institute, conducting fundamental investigations of coherent quantum phenomena and laying the foundation for engineering and controlling complex quantum systems capable of using the coherence and entanglement of quantum mechanics; pursuing that goal through the work of leading quantum scientists from the Department of Physics of the University of Maryland (UMD), the National Institute of Standards and Technology (NIST) and the Laboratory for Physical Sciences (LPS). Each institution brings to JQI major experimental and theoretical research programs that are dedicated to the goals of controlling and exploiting quantum systems.

The JQI provides a unique, interdisciplinary center for the interchange of ideas among atomic physics, condensed matter and quantum information scientists.

At the same time, the JQI is expected to train scientists and engineers for future industrial opportunities and provide U.S. industry with cutting-edge research results.

An expanding, ring-shaped cloud of atoms shares several striking features with the early universe. Credit: E. Edwards/JQI
An expanding, ring-shaped cloud of atoms shares several striking features with the early universe. Credit: E. Edwards/JQI
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