The world’s first “quantum” computer — a machine that harnesses the magic of quantum phenomena to perform memory and processing tasks incredibly faster than today’s silicon-based computer chips — was recently sold by D-Wave Systems of Canada to Lockheed-Martin. Although some question whether the machine is truly a quantum computer, its designers have published articles in peer-reviewed journals demonstrating that the basic elements of this novel computer are indeed superconducting quantum bits.
This spring marked the 100th anniversary of the discovery of superconductivity — the ability of materials to carry electrical current with no loss. Currents set up in superconducting wires can exist for years without any measurable decay.
Because of this property, superconductors have unique features that can be exploited in many ways. They can carry enormous amounts of current, making them ideal for urban power grids. And, when wound into coils, they can produce extremely strong magnetic fields.
Such superconducting magnets have been applied in a variety of technologies. The best-known examples are the magnets that drive the magnetic resonance imaging (MRI) machines found in most hospitals. Perhaps the most exotic are the huge magnets used to accelerate particles in the Large Hadron Collider, which seeks to discover the fundamental principles of matter.
Despite their great promise, however, superconductors have limits, the primary one being that most superconduct at very low temperatures — indeed, near absolute zero (minus-273?C). Such temperatures can be achieved only through liquid-helium cooling. Thus, Swiss researchers caused excitement in 1986 by announcing the discovery of superconductivity in an oxide of copper at twice the temperature of the previous record holder.
Shortly thereafter, researchers in the US found a related material that superconducts above the temperature at which air liquefies. As Time magazine proclaimed in May 1987, with the discovery of these so-called “cuprates,” the superconducting revolution had begun.
Alas, the revolution soon bogged down. Cuprates are notoriously difficult materials to work with, because they are very brittle. This is exacerbated by their strong anisotropy — the materials have a quasi-two-dimensional structure consisting of a weakly coupled stack of conducting sheets. As such, they are a challenge for industry, though applications are beginning to appear.
Since the cuprates first appeared, a variety of other “high temperature” superconductors have been discovered — one is a simple compound of magnesium and boron, and another involves a mixture of iron and arsenic. Although none of them superconduct at temperatures as high as liquid air, they may ultimately be better materials with which to work. Given the vast number of combinations of elements that can form compounds, there is a good chance that better superconductors await our discovery.
In the coming years, superconductors are expected to play a growing role in technology. Already, “second-generation” cuprate wires are being used to make high-capacity cables for electric power transmission and lighter-weight generators for wind turbines. Stronger superconducting magnets are leading to the development of MRIs with more sophisticated diagnostic capabilities. Superconductors are being used for levitated trains in high-speed rail transport and as microwave filters for improved signal bandwidth in cellular base stations. The discovery of a new superconductor with enhanced properties could lead to even greater technological innovation.
This brings us to the intellectual challenge of superconductors. It took 46 years from the discovery of superconductivity to the 1957 Bardeen, Cooper and Schrieffer (BCS) theory of how the phenomenon occurs. Along the way, a number of famous physicists tried and failed to get the answer — Albert Einstein, Werner Heisenberg and Richard Feynman being notable examples.
Discovering the solution required the development of advanced theoretical techniques. What had been difficult to figure out was how to get electrons to superconduct. The basic discovery of BCS was that if the electrons pair up, those couples could indeed superconduct.
Fortunately, the mechanism for such coupling was known. Although electrons are negatively charged, and therefore repel one another, the positive ions that they leave behind when they flow through a metal can mediate an effective attraction between two electrons under restrictive conditions (for example, the metal must be very cold).
The suspicion, though, is that this is not the case in the new superconductors.
Cuprates superconduct at much higher temperatures, but more importantly, they possess some exotic properties: They are formed by doping electrical carriers into a host material that is a magnetic insulator — the last place one would look for a conventional superconductor. And unlike the BCS theory, in which the pairs are isotropic — with identical properties in all directions in space — the pairs in cuprates are strongly anisotropic, resembling a cloverleaf.
How can one pair electrons without ions holding them together, thereby enabling higher-temperature superconductors? While ideas about this abound, new theoretical breakthroughs most likely will be needed to develop the machinery required to solve such electron-electron theories, perhaps even involving black holes. Whatever the theory turns out to be, it is certain to revolutionize physics.
Michael Norman is the head of the Materials Science Division at Argonne National Laboratory, a principal investigator in the Center for Emergent Superconductivity and Fellow of the American Physical Society.
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