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Dr. Shohini Ghose
Dr. Shohini Ghose

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Laurier researcher published in world’s preeminent science journal

Communications, Public Affairs & Marketing

Oct 7/09

A Laurier researcher has broken into the international big leagues with the publication of her co-authored paper, “Quantum Signatures of Chaos in a Kicked Top,” in one of the world’s most prestigious journals, Nature.

Dr. Shohini Ghose, an assistant professor in the Physics and Computer Science department, says she and colleague Dr. Poul Jessen of the University of Arizona did some long-distance “yahooing!” when word came that their article, which was also co-authored by three graduate students, had been accepted.

Ghose, who received her PhD from the University of New Mexico in 2003, is an expert in quantum chaos. Jessen’s lab looks at cold atoms… “very cold” atoms, Ghose says.

Ghose and Jessen had talked of collaborating on something, and with the state-of-the-art equipment in Jessen’s labs they realized they had the tools to look for something nobody else had observed: direct evidence for a connection between the very different worlds of quantum mechanics and classical chaos, and at an unprecedented level of detail.

“It was ideally suited to do this very textbook experiment on quantum chaos,” Ghose says.

“Chaos – unpredictability – is present all around us,” she says. Most of us have heard of the “butterfly effect,” which hypothesizes that a butterfly flapping its wings in India can ultimately impact the weather halfway around the world.

But this unpredictable behaviour is possible only in the deterministic world described by classical physics, not in the quantum world where uncertainty rules.

“The question,” says Ghose, “was is there a quantum butterfly effect? Is there a quantum version? The quantum world operates under a different set of laws. Do they give rise to quantum behaviour corresponding to chaos?”

Jessen’s lab was looking at cesium, a metal with a particular internal structure that has “just the right values to control them and see something interesting at the quantum level.”

What the team set out to do, over a period of about two years, was “kick the top.” Ghose explains:

“An atom has a spin, like a toy top. Using lasers and magnetic fields to introduce small perturbations (changes) to the system, we were able to ‘kick’ the top, kicking the atomic spins. Suddenly we saw that something happened. As the experimental parameters were varied, the behaviour changed from periodic motion to unpredictable motion… like a chaotic kicked top. We had observed the signatures of chaos!”

Ghose says the team had three “firsts” that made their work special.

  1. Implementation of the kicked top, an important paradigm for studying chaos. “It was a mathematical model and now it was a physical experiment, for the first time.”
  2. It was the first direct observation of the signatures of chaos at the quantum level by completely reconstructing the quantum state of the atoms.
  3. It was the first observation of the fingerprints of chaos in the quantum correlations (or “entanglement”) between the electrons and nuclear spin. “In our experiment, the electrons and nuclear spin are talking together at the quantum level.”

The decision to submit the results to Nature was made jointly, Ghose says. “We knew that we were aiming very high. We sent in a pre-submission in December.” When that was accepted, they went to work on the formal paper.

“We spent a lot of time making sure the introduction was strong, and it took several months to write the paper. Every detail had to be just right.”

Evidently it was.

Among the articles Nature has published in its 140-year history were the first account in English of Wilhelm Röntgen’s research on x-rays (1896), John Cockcroft’s and Ernest Walton’s research on splitting the atom (1932), and J. Tuzo Wilson’s paper on plate tectonics (1966).

Barry Ries
Office of Research Services

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