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Quantum ‘faucet’ puts researchers on the threshold of seeing a new state

May 9 / 15
Image above: The tiny ‘faucet’ used by researchers to search for a new state of matter required cooling to near absolute zero temperatures, and used a pipe only a few tens of atoms across in width. (Images courtesy of Pierre-François Duc)

After eight years of persistence, researchers are standing on the threshold of observing a new state of matter predicted by quantum theory.

The researchers succeeded at drawing superfluid helium through the smallest channel yet — only a few tens of atoms across in width, and drilled by a beam of electrons. Guillaume Gervais (McGill University), a fellow in the Quantum Materials program, and collaborators from McGill, Universität Leipzig, and the University of Vermont recently published the details of their experiment in Science Advances.

Helium is a superfluid at very low temperatures, and the only element that does not freeze when cooled to near absolute zero (-273 °C). Instead, it begins to flow freely, without viscosity. The researchers confined helium into an extremely narrow space to test a theoretical state of matter known as the Luttinger liquid. Quantum theory predicts that when confined to one dimension, superfluid helium should change its state and slow down. By contrast, most fluids flow faster when moving through a narrower space.

As Gervais’ team confined low-temperature helium in channels with widths of 45 nanometres, 16 nanometres, and finally 6 nanometres, they observed that the superfluid began to slow down. This was an indication they were finally approaching the grey zone near the Luttinger liquid state that Gervais has chased for years. Gervais first learned of the Luttinger theory in 2002, while driving through a tunnel with Nobel Laureate Horst Störmer. While discussing quantum conductance, they were held up by the stop-and-start flow of confined traffic. Gervais recalls: “He looked at me and said: ‘Luttinger liquid!’ And do you know, I had no clue what he was talking about!”

That simmered until a CIFAR Nanoelectronics program meeting in 2005, where Gervais discussed it with then-Associate Fellow Ian Affleck (currently a Quantum Materials Senior Fellow). “Because no one had ever considered doing it in a real liquid,” Gervais says, “if it weren’t for Ian Affleck being part of CIFAR, I would not have done this experiment. That’s the bottom line…I needed to know if I was just plain crazy!”

With this advance, the researchers now intend to confine the helium in even smaller channels, less than 1 nanometre in diameter, with different lengths. If the Tomonaga-Luttinger theory holds, the liquid’s flow should also decrease when moving through the longer channels. The technical challenges will likely be greater than the first time, but Gervais is excited about what they will learn. An understanding of one-dimensional physics could one day lead to novel technologies, such as very precise rotating sensors that GPS systems could use, as well as insights into the small-scale physics of electronic circuits.