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Real quantum wire confirms theoretical predictions

by Kurt Kleiner Feb 3 / 14

One day in 2002, after a post-doctoral job interview, Guillaume Gervais was riding in a car driven by Horst Störmer, the Nobel Prize-winning physicist. They entered a busy tunnel, and Störmer had to slam on his breaks when traffic came to a stop. “Guillaume, this is a Luttinger liquid!” Störmer joked.

The researchers built a device on a chip that confined electrons into two quantum wires.
Photo c/o the researchers

Gervais, a CIFAR Fellow in the Quantum Materials program, had never heard the term before. But 12 years later and in collaboration with Sandia National Laboratories, he would manage the difficult trick of fabricating an electronic device that supports predictions made 50 years ago by Japanese Nobel laureate Sin-Itiro Tomonaga and American physicist Joaquin Mazdak Luttinger.

“It’s an old academic problem,” says Gervais. “The bottom line is that when electrons are confined on a line the underlying physics is completely different.”

A Luttinger liquid isn’t a liquid in the everyday sense. Instead, it’s the quantum state you obtain when you confine a collection of electrons in a line with no freedom to move anywhere than back and forth. It turns out that the physics of electrons confined to one dimension this way is different than that of electrons that are free to move around in two or three dimensions.

When Luttinger used the laws of quantum mechanics in 1963 to work out how these lines of electrons should behave it was impossible to test the predictions.

Since then technology has advanced to the point that real Luttinger liquids can be created, for instance by confining lines of electrons inside carbon nanotubes, or confining atoms in special “traps” designed specially for it.

Gervais and his team took a different approach. Along with McGill University student Dominique Laroche and Michael Lilly at Sandia, they created a device on a chip that confined electrons into two quantum wires, separated by only 15 nanometers, or roughly 150 atoms.

One prediction about how Luttinger liquids should behave has to do with something called “Coulomb drag” – this is the friction that occurs between two close-packed electronic wires. According to prediction, Coulomb drag in a Luttinger liquid should increase sharply below a certain temperature.

Gervais and his colleagues were able to show that such an increase in resistance does in fact occur at a temperature of about 1.6 Kelvin, which is close to absolute zero.

The result, which was reported in the journal Science in January, is of interest because it supports a fundamental prediction of quantum physics. But it also could assist in developing practical applications. As computers and other electronic devices continue to shrink, quantum interactions become more important to how they operate, and results like these could ensure further progress in high-end technologies.