An array of detectors close to the head of a patient suffering from focal epilepsy picks up tiny magnetic-field fluctuations, pinpointing the location of the lesion in the brain responsible for the disorder. A five-ton aluminum bar suspended in a vacuum chamber at a temperature near absolute zero awaits the minuscule disturbance that would signify the arrival of a gravity wave from a supernova. A lonely instrument in Baja California records subtle variations in the magnetic field, helping geophysicists to locate a source of geothermal energy below the surface.
Each of these disparate measurements is made possible by an instrument called the SQUID, short for super-conducting quantum interference device. The SQUID, which picks up changes in magnetic field, is the most sensitive detector of any kind available to scientists. Only inherent quantum effects set its limits. It has become by far the most widely used small-scale superconducting device. Although it is not a particularly new invention—it celebrated its 30th birthday last year—it has recently undergone a revolution in its accessibility. The advent of the high-temperature superconductors in the late 1980s has enabled SQUIDs to operate in liquid nitrogen, at a “warm” 77 kelvins (-196 degrees Celsius). As such, newer SQUIDs will be simpler to use and more widely applicable than those built from conventional superconductors, which function only at temperatures near absolute zero.
The SQUID derives its phenomenal properties from a combination of several quantum mechanical effects. The resistanceless flow of electric current is the most apparent. The Dutch physicist Heike Kamerlingh Onnes earned a Nobel Prize for his 1911 discovery that mercury became a superconductor when cooled by liquid helium to 4.2 kelvins. Subsequently, many metals—for example, tin, lead and niobium—and a vast number of alloys were shown to lose all resistance to current when cooled to low temperatures.
On supporting science journalism
If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
An explanation of why materials became superconducting was a long time coming. It waited until 1957, when John Bardeen, Leon N. Cooper and J. Robert Schrieffer published their seminal paper, reporting theoretical work that also earned a Nobel Prize. The central feature of their idea, called the BCS theory, is the Cooper pair: two electrons of opposite spin and momentum are bound together so that they have zero net spin and momentum. The attractive force behind this pairing is a subtle interaction between the negative charge of electrons and the positive charge of ion cores in the superconducting material. These ion cores are simply atoms that have lost one or more of their outer-most electrons, which become free to conduct electricity. The ion cores are pulled in toward an electron as it moves through the lattice of a solid, creating a region of enhanced positive charge. This region attracts another, nearby electron. The effect is analogous to two baseballs on a water bed: if the indentations caused by the baseballs overlap, the baseballs become attracted to each other. The two electrons are weakly bound together, with an energy typically of one millielectron volt.
How do paired electrons move without resistance, whereas single electrons do not? In ordinary conductors, impurities, defects and, especially, lattice vibrations called phonons deflect the movement of single electrons. Such scattering of electrons endows the substance with resistance. The energy binding the electrons in a Cooper pair, though low, is high enough to prevent the pair from being separated by scattering. Hence, Cooper pairs propagate through the material without resistance. Deep cooling is essential because it quiets the lattice vibrations. At higher temperatures, the thermal energies become large enough to disrupt the Cooper pair.
A remarkable fact about a superconductor concerns its wave function. A wave function is a mathematical tool physicists use to represent particles in quantum systems. Like any wave, this function has both amplitude and phase—a simple example is a sine wave. It gives the probability for a given particle to be in a particular place at a particular time. What is curious about a superconductor is that a single wave function can describe the entire collection of Cooper pairs. When no current flows, all the pairs have the same phase—that is, they are said to be phase coherent.
A third piece of Nobel-winning work at the heart of the SQUID comes from Brian D. Josephson, who predicted the effect that now bears his name. As a research student in 1962 at the University of Cambridge, Josephson considered two superconductors separated by a layer of an insulating material, which acts as a barrier to the flow of current. The quantum-mechanical wave functions associated with the Cooper pairs leak into this “forbidden” region from each side. Provided the barrier is not too thick, the two wave functions will overlap. If this overlap is sufficiently large, the phases of the two wave functions “lock together.” Under these conditions, Cooper pairs can “tunnel” through the barrier without breaking up. The junction hence acts as a weak superconductor. The critical current—the maximum super-current that can flow through the junction—depends on the size of the junction, the superconducting material and the temperature.
This phenomenon is described as the direct-current (dc) Josephson effect. Experiments conducted a few months later verified it. Philip W. Anderson and John M. Rowell, then at Bell Telephone Laboratories, made the observations. An alternating-current (ac) effect exists as well. Here a voltage maintained across the junction causes the amplitude of the supercurrent to oscillate in time.
Beyond their role in SQUIDs, Josephson junctions have many other applications. Because they can switch rapidly from the superconducting state to the resistive state—in just one or two picoseconds—hey appear in experimental ultrafast digital components, including logic circuits, shift registers and analog-to-digital converters. Standards laboratories also use the Josephson junction to maintain the reference for the volt. Irradiating a junction with microwaves of a given frequency induces voltage steps. These steps occur at voltages that are precisely some integer multiple of that frequency.
Besides zero resistance and the Josephson effect, the SQUID exploits a third quantum-mechanical phenome-non: flux quantization. We are accustomed to thinking of quantization as something that happens on an atomic scale--for example, the occupation by electrons of discrete energy levels as they move around the nucleus. An analogous effect occurs in superconducting rings on a macroscopic scale. Suppose a current flows around the ring. The current produces a magnetic field threading through the ring. The product of the magnetic field and the area enclosed by the ring—the magnetic flux—cannot take on an arbitrary value. It must equal an integral number of a quantity called the flux quantum. A flux quantum is extremely small: a red blood corpuscle, about seven microns in diameter, in the earth’s magnetic field (about 0.00005 tesla) embraces roughly one flux quantum.
A dc SQUID is rather simple. It consists of two Josephson junctions formed into a superconducting ring. Applying current to the SQUID (biasing it) sends Cooper pairs of electrons tunneling through the junctions. A magnetic field applied to the ring, however, alters the flow. Specifically, it changes the quantum-mechanical phase difference across each of the two junctions. These phase changes, in turn, affect the critical current of the SQUID. A progressive increase or decrease in the magnetic field causes the critical current to oscillate between a maximum value and a minimum one. The maximum occurs when the flux administered to the SQUID equals an integral number of flux quanta through the ring; the minimum value corresponds to a half-integral number of quanta.(The flux applied to the SQUID can assume any value, unlike the flux contained within a closed superconducting ring, which must be an integral number.) In practice, we do not measure the current but rather the voltage across the SQUID, which also swings back and forth under a steadily changing magnetic field.
This quantum interference effect provides us with a digital magnetometer. Each “digit” represents one flux quantum. In fact, conventional electronics can detect voltages corresponding to changes in magnetic flux of much less than one flux quantum. The SQUID in essence is a flux-to-voltage transducer, converting a tiny change in magnetic flux into a voltage.
In my early days as a research student at Cambridge, my supervisor, Brian Pippard, proposed that I use a SQUID to make a highly sensitive voltmeter. In those days, procedures for making Josephson junctions were in their infancy and not practicable for manufacturing instruments. One day early in 1965, over the traditional afternoon tea at the Cavendish Laboratory, I was discussing this problem with Paul C. Wraight, a fellow student. He suggested that a molten blob of solder (an alloy of lead and tin that becomes superconducting in liquid helium) deposited onto a niobium wire might just conceivably make a Josephson junction. His rationale was that niobium has a native oxide layer that might behave as a suitable tunnel barrier.
We rushed back to the laboratory, begged a few inches of niobium wire from a colleague, melted a blob of solder onto it, attached some leads and lowered it into liquid helium. As we hoped, Josephson tunneling! The fact that Wraight’s idea worked the first time was important. If it had not, we would never have bothered to try again. Because of its appearance, we christened the device the SLUG. Later I was able to make a voltmeter that could measure 10 femtovolts (10-14 volt), an improvement over conventional semiconductor voltmeters by a factor of 100,000.
Needless to say, the technology of SQUID sensors has evolved beyond recognition in the intervening years. Most modern dc SQUIDs follow a design pro-posed by Mark B. Ketchen and Jeffrey M. Jaycox of the IBM Thomas J. Watson Research Center. They consist of multiple layers of thin films deposited on silicon wafers. The photolithographic and etching techniques of the semiconductor industry pattern these films. These methods can produce as many as 400 SQUIDs on a four-inch wafer. The wafer is then diced into individual chips, each bearing one SQUID. The SQUID it-self consists of a square washer of niobium that has two Josephson tunnel junctions. The barriers consist of aluminum oxide, an electrical insulator, grown on top of one of the niobium layers.
Just how sensitive is such a SQUID?A convenient criterion is the energy associated with the smallest change in magnetic flux the device can detect in one second: typically about 1032 joule. This incredibly tiny amount is roughly equal to the mechanical energy required to raise a single electron one millimeter in the earth’s gravitational field. In fact, the best SQUIDs ever manufactured are 100 times more sensitive than that. They approach the limit set by Heisenberg’s uncertainty principle, which sets fundamental boundaries on the accuracy of measurements.
I should also mention that SQUIDs based on alternating current exist. This kind of instrument is known as the radio-frequency (rf ) SQUID, because it is biased with a flux oscillating in the megahertz range. The device consists of a single Josephson junction in a superconducting loop, which is coupled to an inductor connected across a capacitor. This design forms a so-called resonant circuit, which is driven by an rf current. The amplitude of the rf voltage across this circuit oscillates in response to a magnetic flux.
Manufacturers sold rf SQUIDs long before they did dc SQUIDs, even though the direct-current variety is generally more sensitive. Presumably, rf SQUIDs were easier to make, since each one requires only a single junction. Now advanced thin-film technology allows reliable production of large numbers of junctions. As a result, most SQUIDs sold today are of the direct-current type. Nevertheless, rf SQUIDs have not disappeared from the shelves, as many investigators find them quite adequate.
To take advantage of the extraordinary sensitivity of the SQUID, the devices are almost always coupled to an input circuit. For magnetometers, this circuit enhances the SQUIDÕs sensitivity to magnetic fields, often by 100-fold. This so-called flux transformer simply consists of a loop of superconducting material coupled to a SQUID. The flux transformer boosts the field sensitivity because the loop encloses a much larger area than can a SQUID. An external magnetic field causes a persistent supercurrent to circulate in the loop. This current induces a flux in the SQUID. With a flux transformer, a SQUID can reach femtotesla (1015 tesla) resolution. One femtotesla corresponds to one part in 1011 of the earth’s magnetic field.
Another variation of this SQUID magnetometer is the SQUID gradiometer. It measures the difference between magnetic field values at two different places, that is, the gradient. The gradiometer relies on two flux-transforming loops wound in opposite directions. An alternative approach employs two SQUID magnetometers separated by a short distance; electronic circuitry then subtracts one output from the other to determine how the field changes over this distance.. This method is particularly appealing when applied to large arrays of magnetometers. In one variant, Roger H. Koch and his colleagues at the IBM Watson center developed the “three-SQUID gradiometer.” In this device the output of one magnetometer channel cancels the ambient magnetic noise at two others, thereby creating a quiet environment for the gradiometer.
Measuring magnetic-field gradients is especially useful in medical diagnosis: the electric currents in the human body provide a rich source of time-varying magnetic signals. The tiny magnetic signals vary from a few femtoteslas from the brain to 50,000 femtoteslas from the heart. Until the development of SQUIDs, these signals were too weak to be studied. In addition, fluctuations in the earth’s magnetic field and magnetic noise produced by the motion of elevators and automobiles and, particularly, by the 60-hertz hum from the electric grid system overwhelm the body’s magnetic signals. The SQUID gradiometer attenuates background noise because the sources are usually far from the patient and tend to be nearly uniform. Hence, they evoke only a weak response in the SQUID gradiometer, which is sensitive to nonuniform fields.
In practice, an array of SQUID sensors maps the spatial variation in the magnetic fields produced by the body. From this contour map, a computer can reconstruct the region inside the body that produced the signals. This procedure is entirely noninvasive. During the past 20 years, the number of SQUIDs in the array has grown from about seven to more than 100; recently the Super-conducting Sensor Laboratory in Inzai Chiba, Japan, announced a prototype system consisting of 250 channels.
Such instruments provide the physician with crucial information about various illnesses. For example, in patients who have focal epilepsy, a relatively localized electrical discharge in the brain triggers a seizure. Mapping magnetic-field spikes by a SQUID array can pinpoint the source of the discharge. When superposed on an image produced by magnetic resonance imaging, this source may correlate with some abnormality, such as scar tissue. Under favorable circumstances, the surgeon can excise it or destroy it with a “gamma knife”—collimated gamma rays. In a different approach, one evokes a magnetic response by means of a specific stimulus. For example, in San Diego, Eugene C. Hirschkoff of Biomagnetic Technologies and Christopher C. Gallen of the Scripps Research Institute use a 74-channel SQUID system to map the response of the cortex surrounding a brain tumor to tactile stimulation.
Another medical application of widespread interest concerns the heart. Cardiac arrhythmia—erratic heartbeat—comes from spurious electrical pathways connecting the atria and the ventricles, short-circuiting the normal cardiac signals. In severe cases, the arrhythmia can prove fatal. To treat this ailment (with an electrical discharge from a catheter), one must localize this pathway, which can sometimes entail a prolonged search with one or more catheters. Several groups, including Gerhard Stroink and his colleagues at Dalhousie University in Halifax, and researchers at Siemens in Erlangen and at the University of Erlangen-Nurnberg have shown that SQUID imaging can localize the site of the electrical discharge. In this way, SQUIDs can substantially reduce the time required to find the anomaly.
Despite the impressive results of bio-magnetic measurements, the high cost of multichannel machines has kept them from achieving general acceptance. Yet the technology has the potential to reduce health care costs dramatically. Locating the epileptic focus with SQUIDs may take three hours, whereas the alternative method of implanting electrodes on the surface of the brain may last as long as one week. The savings could easily reach $50,000. Similarly, the removal of a brain tumor without significant loss of neural function prevents trauma for the patient as well as saving the enormous expense of rehabilitation. The insurance industry has begun to accept the cost-savings possibilities of biomagnetic measurements; eight companies have reimbursed patients for the presurgical screening of brain tumors at Scripps. Blanket approval for the SQUID-based procedure may emerge by the end of this year.
SQUIDs also play an essential role in countless nonmedical applications, both in fundamental science and in routine measurements. A SQUID recently set an upper limit on the mass of the photon (if it has one at all: conventional theory dictates that it does not). The data indicate that the photon’s mass must be less than about 1046 gram. This limit is the strictest yet set in a cryogenic laboratory experiment.
Another fundamental application of a SQUID lies in the attempt to detect gravity waves. Relativity predicts that such waves—displacements in space and time—ought to be abundant in the universe. Collapsing stars, black holes and other movements of dense celestial bodies should ripple space in all directions. Researchers try to pick up these shifts with a giant metal bar. Weighing typically five tons and cooled by liquid helium, the bar would be sent into minute longitudinal oscillations by an incident gravity wave. A displacement in the end of the bar is detected as a flux change in a SQUID at a resolution of 1018 meter—roughly 0.001 the diameter of an atomic nucleus. Indeed, several gravity-wave antennae are deployed worldwide. To date, the antennae have not recorded gravity waves. But the next generation coming on-line, with per-haps two orders of magnitude greater sensitivity, will most likely do so.
Probably the most widespread SQUID-based system is a sophisticated “susceptometer.” Manufactured by Quantum Design in San Diego, the instrument enables scientists to measure the magnetic properties of samples from a few kelvins to well above room temperature. Hundreds of such machines figure in the workplaces of physicists, chemists, materials scientists and biologists.
Although SQUIDs have proved their utility for many years, interest in them has recently exploded, thanks to J. Georg Bednorz and K. Alexander Muller of the IBM Research Laboratory in Zurich. These researchers dis-covered the so-called high-temperature superconductors (which promptly earned them the fourth Nobel Prize in the field). Other workers quickly pushed the transition temperature to over 100 kelvins (-173 degrees C).
The popular media heralded this breakthrough as the greatest scientific revolution since the lightbulb (or, perhaps, merely the transistor). The hype promised an almost instant multibillion-dollar business. Of course, as with all scientific innovations, progress takes time. Today the only devices commercially available that rely on this break-through are SQUIDs.
Unlike most conventional superconductors, the high-temperature materials are ceramics organized in complex layers. For devices such as the SQUID at least, the most common material is an oxide of yttrium, barium and copper. It bears the formula YBa2Cu3O7-x, where x equals approximately 0.15. The substance is known as YBCO (often pronounced “ibco”). Its transition temperature is about 90 kelvins, so that it be-comes a superconductor at the temperature of liquid nitrogen.
Because they are ceramics, the new superconductors are brittle and difficult to work. Workers cannot readily bend the wires into shape as they can with the relatively ductile low-temperature superconductors. Researchers have de-vised various schemes to make high-quality thin films out of them. Of those approaches, deposition by a high-power pulsed excimer laser has proved very useful, not least because the process is relatively rapid. The laser pulses reside in the ultraviolet part of the light spectrum, with a typical wavelength of 248 nanometers. They strike a rotating YBCO target mounted in a chamber containing oxygen. Each pulse vaporizes a small amount of YBCO, forming a beautiful rose-colored plume. The material collects on a nearby substrate kept at about 800 degrees C. The film grows epitaxially—in other words, its crystal structure mimics that of the substrate—with the correct chemical makeup.
Besides forming them into thin films, SQUID makers also need to form Josephson junctions on the wafer. Investigators have invented many clever processes to form them in high-temperature compounds. Duane Dimos and his co-workers at the IBM Watson center have developed an especially successful method. One starts with a crystal, usually of strontium titanate, that has been cut and fused together to produce a deliberate misorientation of the crystal axes along a line. This dislocation is called a grain boundary. When the YBCO film is grown epitaxially on the substrate, it replicates the abrupt change in crystal orientation. The grain boundary greatly diminishes YBCOÕs super-current-carrying capability, hence be-having as a Josephson junction. Another successful way of making junctions calls for an epitaxial sandwich; workers interpose a thin slice of normal material between two superconductors.
As with their low-temperature counterparts, high-temperature SQUIDs rely on a flux transformer to improve sensitivity to magnetic fields. A simple means of accomplishing this task is to form the flux transformer in the same YBCO layer as the SQUID. A second flux transformer can further enhance the sensitivity. With the latter of these designs, Dieter Koelle and his co-workers at the University of California at Berkeley, Lawrence Berkeley Laboratory and Conductus, Inc., in Sunnyvale, Calif., achieved a noise level of about 30 femtoteslas. Michael Muck and his colleagues at the Julich Research Center in Germany achieved 24 femtoteslas using a similar transformer coupled to an rf SQUID.
It should be noted that SQUIDs operating in liquid nitrogen will never achieve as good a resolution as can their counterparts working in liquid helium. So why are the high-temperature devices creating such a stir? The popular view was expressed many years ago by Nobelist Ivar Giaever, now at the Rensselaer Polytechnic Institute: liquid helium is as expensive as Scotch, whereas liquid nitrogen is as cheap as milk.
Although this statement is more or less true, the heart of the matter is that liquid nitrogen vaporizes much more slowly than does liquid helium. Instead of having to refill a dewar of liquid helium every few days, one replenishes the supply of liquid nitrogen every few weeks. Furthermore, liquid helium is available only in major metropolitan areas; in more remote regions of the world it is an exotic substance that can-not readily be procured every few days. Liquid-nitrogen cooling thus offers a superconducting technology that would otherwise be denied.
The growing promise of high-temperature SQUIDs is especially apparent in geophysics research, which is often conducted in inaccessible locations. One such application is in the study of magnetotellurics (“magnetism of the earth”). Its practitioners measure the electrical resistivity of the ground below the surface to infer the underlying structure. In essence, very low frequency (0.001 to 100 hertz) electromagnetic waves from the upper atmosphere propagate down to the earth’s surface. The solar wind blowing on the charged layers in the magnetosphere or ionosphere generates these waves, which are reflected by the ground. But a component of these waves decays into it. By measuring the tiny fluctuating magnetic fields (using magnetometers) and electric fields (using buried electrodes), the geophysicist can map out the resistivity of the earth’s crust at depths of as much as several tens of kilometers. From this information the researcher infers the subsurface hydrology and porosity, seeking valuable clues on the whereabouts of oil or geothermal energy sources. In complementary techniques the geophysicist supplies magnetic pulses and measures the response. Pulsed methods are widely ap-plied down exploratory boreholes to search for oil and may be able to locate buried hazardous wastes.
The ease granted by liquid nitrogen will likely encourage the proliferation of SQUID systems in medicine. Already, several research groups have used high-temperature SQUIDs to obtain magnetocardiograms and even magnetoencephalograms. An intriguing extension of the technology may aid fetal cardiology. Heart rate variability is one way physicians assess the health of the fetus, and electrocardiograms can be taken until the seventh or eighth month of pregnancy. Beyond that period, the signal declines because the fetus becomes electrically insulated from the mother. A magnetocardiogram, however, remains unaffected. Moreover, the superior spatial resolution of magnetic measurements makes distinguishing the fetal signal from the maternal one easier.
Workers have begun to obtain magnetic images by sweeping SQUIDs over an object. Such a “scanning SQUID microscope” can resolve objects down to a few microns wide. This technique has applications in studying not only magnetic materials but also metals and electronic circuits, which produce magnetic fields when a current flows. Scientists are also exploring the use of SQUIDs in nondestructive testing—say, in looking for corrosion of aluminum sheets rivet-ed together in aircraft. The SQUID measures the influence of the aircraft skin on an applied oscillating magnetic field; a change in electrical conductivity reveals the defects.
At this juncture, the most sophisticated high-temperature SQUID commercially available is “iMAG,” a version of the single-layer YBCO magnetometer. Made by Conductus, a complete system with electronics, dewar and probe sells for about $10,000. Highly versatile, this instrument is suitable for laboratory experiments as well as for geophysics and magnetocardiology studies.
To be sure, such SQUIDs will by no means replace their low-temperature cousins in all situations. Gravity-wave detectors and other applications with the most demanding requirements will continue to call for liquid helium. The liquid nitrogen-based devices will, however, open up a range of possibilities that were not previously realistic, bringing this wonderfully sensitive technology out of the laboratory and research hospital and into the marketplace.
