"Although we concentrated on making a specific device, the technology that we developed is much more broadly applicable," says Clarke. "It will be the basis for virtually any kind of future electronic circuitry involving high-temperature superconductors."

The new high-Tc magnetometer consisted of two components, each fabricated on its own chip -- a high-Tc squid and a high-Tc flux transformer. The squid was developed and fabricated at Conductus, using a novel technique that produced grain-boundary Josephson junctions in YBCO films deposited on a precisely controlled substrate. The flux transformer was developed and fabricated by the Clarke group and it consisted of an insulating layer of strontium titanate sandwiched between two layers of YBCO film. All three layers were laser-deposited onto a strontium titanate chip that had been heated to 800 degrees Celsius so that they grew epitaxially, that is, with aligned crystal structures.

When the high-Tc squid and flux transformer were combined to make the magnetometer, the two components were inductively coupled without actually touching. Through extensive testing, this magnetometer showed a sensitivity to magnetic field strength that was about ten times better than any previously reported from a magnetometer made with high-Tc superconducting films.

By the summer of that year, Clarke and his group had used this magnetometer to record the magnetic fields produced by the human heart. The magnetic signals produced by the beating of a heart were detected and recorded as a wave-form that could be examined much like an electrocardiogram. Magnetocardiograms had been recorded before but always with low-Tc squids. This was the first such measurement with a thin-film high-Tc squid magnetometer.

One of the major problems that has to be overcome in the use of any squid is magnetic noise; e.g., from the earth's magnetic field, the electrical grid system, or the motion of elevators. This noise must be muffled for the squid to perform to its maximum capabilities. "External" magnetic noise can be blocked out with the use of Mumetal (an alloy of high magnetic permeability) shielding; however, for high-Tc squids there is also a problem with "intrinsic" magnetic noise.

Gene Dantsker, a former member of Clarke's group (now with TRW in Los Angeles) who worked with Saburo Tanaka and others on eliminating excess magnetic noise, says the intrinsic noise of a high-Tc squid can be a serious liability.

"We say that a high-Tc squid is only as good as its intrinsic noise," he says. "We now understand that the problem has largely been an artifact of design."

When a high-Tc squid is cooling down to its operational temperature, it can capture "flux vortices" -- the flowing lines of force generated in the material by ambient magnetic fields. Thermal activation can cause these vortices to move, generating magnetic noise that obliterates the signal being read by the squid.

Says Dantsker, "This excess noise from within has been particularly limiting for the use of high-Tc squid magnetometers required to operate in the earth's magnetic field with high sensitivity at low frequencies."

The researchers showed that intrinsic noise could be largely eliminated for single-layer YBCO film squids and magnetometers by reducing the width of the squid's superconducting material.

"If the width of the squid body is reduced so that it is less than the distance separating the flux vortices in a given magnetic field, the vortices will not be captured," Dantsker explains. "The result is greatly reduced low-frequency noise over a wide range of ambient magnetic fields."

Dantsker, Clarke, and Tanaka were able to fabricate a new family of washer-shaped high-Tc squid bodies that feature an array of slots or holes. (The former resembles a concentric series of moats, the latter a gridiron for waffles.) The YBCO film in these squids is patterned along narrow lines no more than four micrometers wide. In tests to date, the devices exhibit no excess intrinsic noise when cooled and operated in an ambient magnetic field equivalent in strength to that of the earth.