NEWS RELEASE, 4/10/97 UC Berkeley physicists develop ultrasensitive gyroscope based on superfluid helium

by Robert Sanders

Berkeley -- An ultrasensitive, superfluid gyroscope developed by physicists at UC Berkeley has the potential to surpass today's most sensitive devices for measuring absolute rotation or spin. In a paper in this week's issue of Nature, physics professor Richard Packard and his colleagues, graduate students Keith Schwab and Niels Bruckner, report a proof-of-principle demonstration of the new device. Their prototype superfluid gyroscope already is quite sensitive and they believe its sensitivity will eventually surpass that of the ring laser gyroscope, a highly sensitive device used in advanced commercial aircraft inertial guidance systems. Packard's immediate goal is to create a version with a sensitivity 10,000 times greater than the team has achieved to date. "We have demonstrated a new kind of instrument that can detect absolute rotation at a very sensitive level," Packard said. "If these devices obey the theoretical design equations, they will surpass the ring laser gyroscope." He predicts that in the future, if he and his colleagues can boost sensitivity by very large factors, the superfluid gyroscope may be able to detect some of the strange effects of general relativity predicted by Albert Einstein more than half a century ago, such as gravitomagnetism. "This may be a better mousetrap, but we don't know if there are any mice to be caught," Packard said. "It is really too early to predict what other uses there may be in future." A rival team in France achieved a similar feat last year, the results of which they published in the Czechoslovakian Journal of Physics. The gyroscope is based on the fact that superfluids such as helium-4 have an uncanny ability to sense absolute rotation. The team developed a way to detect the quantum changes in superfluid helium-4 as the device's rotation changes. Their prototype has a sensitivity equal to one-half percent of the Earth's spin rate. Although Packard thinks it will be possible to achieve 10,000 times greater sensitivity than at present, they will need even much greater sensitivity to detect general relativistic effects due to spinning objects such as the Earth. Such sensitive rotation or spin detectors are needed in fields such as geodesy, where geologists look for slight changes in the Earth's rotation to provide clues to what is happening in the planet's interior. At present such changes are determined using techniques of radioastronomy. It would be very appealing to have a single instrument in a laboratory that could output a number showing changes in the Earth's daily rate, Packard says. An extremely sensitive rotation sensor could also, in principle, detect gravitomagnetism, an analog of electromagnetism seen when massive objects move. A different but also very sensitive gyroscope -- a spinning ball -- is due for launch by NASA around the turn of the century in an attempt to detect gravitomagneitc effects from the rotating Earth. The superfluid gyroscope works because of a well-known property of superfluids: if you travel around any closed loop in the fluid you find that the net flow is zero. Put in mathematical terms, the integrated velocity around any closed path in the fluid must be zero. That means if you start spinning a superfluid-filled doughnut, analogous to spinning a bicycle wheel on its axle, the fluid inside initially remains still while the doughnut spins. If you place a wall inside the doughnut to force the fluid to move, it will not flow like water -- as a solid block of fluid -- but rather develop complex flow patterns so that the fluid still retains the property of zero net flow around a closed loop. The researchers went one step further and put a sub-micrometer sized pinhole in the wall, which causes a high velocity backflow through the pinhole, in a direction opposite to the rotation -- a result also of the requirement for zero net flow. Packard saw that the high velocity fluid squirting through the hole essentially amplifies any change in rotation of the doughnut. Therefore, if he could detect these changes, he could measure very small alterations in spin. The device is essentially a hollow doughnut filled with helium-4, chilled to near absolute zero (about 3/10 degree above zero Kelvin). The instrument they built was a one-centimeter-square silicon chip, etched with a channel for the superfluid and then capped by a niobium-coated diaphragm. The diaphragm operates like a drumhead, which they vibrate up to 60 times a second to drive the superfluid back and forth in the channel, forcing it in each cycle to squirt through a microscopic hole. At some point during each cycle the fluid achieves a critical velocity through the hole -- the velocity at which a small vortex (a superfluid tornado) spins off from the submicron-sized hole. This event causes a detectable glitch in the vibration of the diaphragm that is detected by a sensitive SQUID (superconducting quantum interference device). Because any rotation of the device changes the velocity of the superfluid in the aperture, any rotation will also affect the apparent critical velocity. This permits the researchers to detect the flow in the aperture created by very small rotation rates of the silicon chip. The researchers demonstrated the principle of the device by using it to detect the Earth's rotation. As they slowly moved the device from an east-west orientation (that is, oriented so that the plane of the silicon chip faces east-west) to a north-south orientation, the spin of the Earth was gradually added to motion of the superfluid, causing an increase in the velocity of the fluid squirting through the hole. This changed the point in the cycle of the vibrating membrane at which the quantum vortex spun off, providing a sensitive measure of the Earth's motion. By increasing the number of "turns" of the doughnut from one to several -- in essence creating a spiral tube -- and by increasing the size of the doughnut, they can dramatically increase the gyroscope's sensitivity. Packard and his team plan to scale up to about 30 turns and a device about 6 centimeters across in an attempt to increase the sensitivity by a factor of about 10,000. "We can scale this up to have multiple turns, but unfortunately we can't do that indefinitely without making the device unwieldy. Also, Mother Nature usually puts up roadblocks that make achieving theoretical limits much more difficult than at first sight." Packard says. Packard also plans to try to create a gyroscope using superfluid helium-3, which is even colder than helium-4 (1/1000 degree above zero Kelvin) and potentially more sensitive. The work has been supported by grants from the Air Force Office of Scientific Research, the Office of Naval Research, and the National Science Foundation.