On April 8, 1911, at the Leiden Cryogenic Laboratory in the Netherlands, Heike Kamerlingh Onnes and his collaborators immersed a mercury capillary in liquid helium and saw the mercury's electrical resistance drop to nothing once the temperature reached about 3 kelvins, or 3 degrees above absolute zero (around –270 Celsius).
This phenomenon of "superconductivity" was one of the first quantum phenomena to be discovered, although back then quantum theory did not exist. In subsequent decades theoreticians were able to put quantum physics on a solid foundation and explain superconductivity. Since then, researchers have discovered new families of materials that superconduct at higher and higher temperatures: the current record-holder works at a balmy 138 K.
So where's my maglev train?
Indeed, the promise of superconductors—power grids that waste no energy, computers that run at untold gigahertz of speed without overheating and, yes, trains that levitate over magnetic fields—has not fully materialized.
Still, superconductors have made it possible to build the strong magnets that power magnetic resonance imaging machines, which are the most important commercial application of the phenomenon to this day. And scientists use superconductors in advanced experiments every day. For instance, particle accelerators at the Large Hadron Collider in Geneva rely on superconducting coils to generate magnetic fields that steer and focus beams of protons. Some of the most accurate measurements in all of science are done thanks to superconducting quantum interference devices, or SQUIDs.
And finally, superconducting electrical transmission lines are here. Wires based on high-temperature superconductors (with liquid nitrogen–based cryogenics, which are technically simpler and much cheaper than liquid helium–based ones) have recently become commercially available. A South Korean utility plans to install them on a large scale. Some U.S. scientists now say that it may be easier to get permits for and build a national superconducting supergrid than construct a conventional high-voltage system.