Superfluid gyro detects Earth's spin.

Measuring fluctuations in Earth's rotation rate requires a sensitive laboratory instrument. Now, two groups of researchers have exploited the peculiar quantum properties of superfluid helium to build novel gyroscopes that can sense Earth's spin.

"We have demonstrated a new kind of instrument that can detect absolute rotation at a very sensitive level," says physicist Richard E. Packard of the University of California, Berkeley, who heads one of the groups. In principle, the new device has the potential to surpass the most sensitive gyroscopes available today for high-precision measurements of rotation rates.

Packard and his Berkeley colleagues Keith Schwab and Niels Bruckner describe their device in the April 10 NATURE. Eric Varoquaux of the University of Paris-South in Orsay, France, and his coworkers presented their findings last year at a conference in Prague.

A superfluid helium gyroscope takes advantage of the fact that the flow of a superfluid filling a doughnut-shaped container is quantized. In this case, the flow velocity multiplied by the length of the path along the center of the toroidal channel must be zero or a whole-number multiple of a fundamental quantity determined by Planck's constant and the mass of a helium atom.

Sitting on a lab bench, such a container actually rotates, owing to Earth's spin. For example, if the enclosed superfluid helium is in its zero-velocity state, displaying no net flow, the container must move relative to the helium. To an observer in the laboratory, however, the superfluid appears to be flowing around the torus.

The trick is to find a way of precisely measuring the relative motion of the helium inside the container.

A decade ago, Varoquaux and his colleagues demonstrated the possibility of making that measurement by placing a partition inside the torus and monitoring the flow through a tiny pinhole in the barrier. Because the superfluid has to maintain a zero net flow, liquid must squirt through the pinhole at a high speed in a direction opposite to the rotation.

Developing the requisite technology took much painstaking effort, however. Last year, Varoquaux and his coworkers succeeded in measuring Earth's rotation rate to within 2 percent. Then, Packard and his group made that measurement with a different device based on the same principle. They achieved a precision of 0.5 percent, independently corroborating and improving upon the French results.

Packard and his colleagues contend that it should be feasible to increase the sensitivity of their superfluid gyroscope by a factor of 10,000. Such an improvement would make it possible to monitor daily fluctuations in Earth's rotation rate using a laboratory instrument rather than by relying on astronomical or satellite-based methods.