Quantum swirls in superfluid helium.

Quantum swirls in superfluid helium

At sufficiently low temperatures, liquid helium becomes a superfluid, flowing without friction. This curious property is one of the more startling consequences of the role quantum mechanics plays in determining the bulk properties of liquid helium.

Physicists have known since 1938 that helium-4, the most abundant helium isotope, can turn into a superfluid. Obtaining clear evidence that the much less common helium-3 becomes a superfluid has proved more difficult.

Physicist Richard E. Packard and his collaborators at the University of California, Berkeley, now report the best experimental evidence yet that the behavior of liquid helium-3, cooled to temperatures below 0.0003 kelvin, matches the current theoretical prediction of how a helium-3 superfluid should behave.

"It is truly exciting to see such a fundamental and exotic prediction come true in the laboratory," Packard says. He described his group's findings this week at a conference in Brighton, England, on low-temperature physics.

Packard and his co-workers studied the flow properties of liquid helium-3 in a specially constructed, rotating refrigerator containing a long, thin cylinder. A fine wire made from a superconducting material and stretched along the cylinder's central axis served as a sensor, allowing the researchers to measure the flow of liquid helium-3 within the rotating cylinder. Sending a pulse of electrical current through the sensor would cause the wire to vibrate like a guitar string, and any fluid motion around the wire would alter its vibrations in a characteristic, detectable way.

The team found that chilled liquid helium-3 remains at rest when the cylinder rotates at low speeds. "When we reach a certain critical speed, we can see the [helium] circulation begin to change in a fairly erratic way," Packard says. "We stop rotation, and either the circulation disappears or it goes to a new stable state." A similar pattern holds for higher rotational speeds, with the liquid helium either coming to rest or jumping to a well-defined quantum flow state. In contrast, an ordinary liquid would always flow so that it kept up with the container's walls.

The Berkeley experiments clearly show that the flow of liquid helium-3 is quantized: Its rate increases in steps, each a multiple of a fundamental unit involving double the mass of a helium-3 atom. That finding confirms a theoretical prediction that helium-3 becomes a superfluid only if its atoms are coupled in what are known as Cooper pairs.