David Hall
David S. Hall ’91, the Paula R. and David J. Avenius 1941 Professor of Physics. Photo by Ryan Donnell.

Physicists have long predicted the possibility of tying knots in quantum fields. But no one has been able to make or observe a three-dimensional quantum knot—until now.

Inside the lab of David S. Hall ’91, Amherst’s Paula R. and David J. Avenius 1941 Professor of Physics, scientists have found a way to create knotted solitary waves in a quantum-mechanical field. This represents a major step forward in understanding the nature of quantum fluids.

“First we cooled a gas of rubidium atoms down to billionths of a degree above zero,” says Hall, who, with Mikko Möttönen of Aalto University in Finland, led the team that made the discovery.

The cooling process created “a superfluid—a tiny, well-ordered environment,” which the scientists then exposed to a rapid change of a magnetic field, “which tied the knot in less than a thousandth of a second.”

Previous experiments have identified knotlike structures only in one and two dimensions, where the curves do not close. The new three-dimensional knots exist within a tiny droplet of superfluid just barely visible to the human eye. Each knot is approximately 10 times smaller than the thickness of a human hair.

Hall and his colleagues have now tied hundreds of knots in his basement laboratory in Merrill Science Center. They published their results in Nature Physics. Among the co-authors is Andrei Horia Gheorghe ’15, who assisted Hall and Möttönen as part of his senior thesis.

The next step is to experiment with the knots and study their properties, to see what the quantum knots can do. Fundamental research often has the potential to revolutionize people’s lives in ways that are impossible to predict, Hall says: “We don’t know what this particular discovery might lead to, but the possibilities are exciting. When scientists invented lasers, they certainly weren’t thinking about grocery store scanners."