The slow and sometimes very intermittent progress of research during the last century, nevertheless, brought a number of advances in the fundamental knowledge of the superconductor phenomenon and its unique properties. It was in 1933 that the Germans Walter Meissner and Robert Ochsenfeld demonstrated the surprising diamagnetic effect (now known as the Meissner effect) by which a current passing through a superconductor exercises a strong repelling force on an external magnetic field. This property causes a magnet brought close to a material in a superconductive state to levitate spectacularly. The Meissner effect has consequently become the best way of testing whether a material superconducts or not. It is also the principle behind the current development of prototypes of the first Maglev (Magnetic Levitation) trains.

Scientists have been carrying out a wide range of full-scale experiments. These include attempts to discover all the elements that could become superconductors, identify their critical performances and investigate possible applications for them. Scientists have so far experimented on a series of metals and natural metalloids (numbering about 30 at ambient pressure), alloys and combinations of materials of all kinds.

Since the 1940s, a number of alloys have been developed – in particular the family of niobium nitrides and niobium titanium – showing excellent superconductive properties, at critical temperatures between 15 and 22 K, while attaining higher critical magnetic fields. But it was not until 1962 that the US firm Westinghouse first developed long lengths of superconducting electric wire produced from NbTi alloys on an industrial scale.

The super magnets

Meanwhile, liquid helium cryogenic technologies had made considerable progress and it became possible to use superconducting coils developed at temperatures close to absolute zero. This made it possible to pass currents of unprecedented intensities, in the region of a million amperes per square cm, which in turn generated uniquely intense magnetic fields.

It is in the field of these ‘super’ magnets that most of today's superconductive applications have been perfected. In particular, it has given rise to a boom in medical diagnosis technologies employing magnetic resonance imaging (MRI). These devices are now widely used in hospitals and represent a global market of almost €2.5 billion annually.

Other high-tech applications in the field of high magnetic fields are also of interest to the scientific world. These include the particle accelerators used at the European Organisation for Nuclear Research, CERN, the experimental nuclear fusion installations at the Wendelstein Stellarator in Germany, and the various spectrometric technologies used in the molecular study of materials or living organisms.

The Josephson junction

Another very interesting field of application for superconductivity was opened up in 1962 when a brilliant young doctorate student at Cambridge University, Brian Josephson, demonstrated the existence and very particular characteristics of a 'tunnel’ effect that can be produced between two superconducting materials separated by a thin insulating layer. In such an interference device (also known as the Josephson junction), the flow of electrons – even in the absence of any external voltage – can pass from one material to another, crossing the insulating barrier. This provides the capacity to detect even the weakest magnetic field.

This discovery has since come to acquire considerable importance. It is at the core of a revolutionary technique known as SQUID (Superconducting Quantum Interference Device). SQUID works by placing in parallel two Josephson junctions connected in a loop by a superconducting link. It was discovered that even a very weak magnetic field, known as a magnetic quantum, produced at the centre of the superconducting loop causes the latter to generate an electrical signal.

SQUID is the most 'sensitive' magnetic detection system available to scientists today. The technique is giving rise to highly advanced applications in medicine, particularly in the fields of neurological imaging by magneto-encephalography and cardiology. Important applications are also emerging in the field of metrology and the physico-chemical sciences.

The chips of the future

The unique properties of Josephson junctions and the phenomena of quantum interference devices have also opened the door to a potential revolution in superconductive computing. They were used in designing what is known as the Single Flux Quantum (SFQ) device, which acts like a new generation of switches for binary logic with a reaction time in the area of one picosecond.

The absence of resistance in a superconductor circuit minimises energy loss, which is currently one of the major obstacles to boosting computer performance. Superconductive electronics would, therefore, make it possible to produce processors 500 times faster than present semi-conductor components, while producing 200 times less heat.

A whole new ballgame

However, even if prototypes of these chips of the future have already been developed, the first generation of superconductive supercomputers has, to date, remained very much a promise rather than a reality. In the early 1980s, superconductivity seemed to have reached the limit of its potential as its applications were limited by the need to work at temperatures approaching absolute zero. The record, obtained from a niobium germanate alloy, was 23 K.

The cost and technological complexity of helium cryogenics closed the door to many 'dream' applications, whether in the field of electrotechnics or computing. It was against this backdrop that, in 1986, came a major breakthrough: the high temperature superconductors, which suddenly opened up a whole new world of possibilities.