Physicist Steven Thompson explains how the introduction of impurities and the study of condensed matter physics are bringing us closer to achieving the ultimate in superconductors and quantum computing.
The paradox of perfection is a philosophical idea that for something to be perfect, it must be imperfect. By making something less perfect in form, it can often be made more perfect in function. Whatever the deeper philosophical implications of this statement, it’s almost a literal truth in the field of materials science.
As far back as the Crusades, blacksmiths knew that adding controlled amounts of impurities into sword blades could vastly improve the quality of the steel, making it harder and tougher. This led to the development of the famed Damascus steel, allegedly the strongest of its time, with an unmistakeable rippled appearance.
In the modern world we have incredible control over the properties of our materials. From simple alloys where elements are mixed together to the more complex doping procedures used in semiconductors, the process of deliberately changing the chemical composition of materials in order to improve their properties is commonplace and well understood. In the realm of quantum mechanics, however, there’s still a lot left to learn and it turns out that even trace impurities and randomness can have surprisingly profound effects.
When some materials are cooled to a few degrees above absolute zero (-273C), something remarkable occurs. They become perfect conductors of electricity, known as superconductors. This is a uniquely quantum mechanical effect which typically only happens at extremely low temperatures. Superconductors could have amazing technological applications, but they’re just not practical unless they can be made to operate at higher temperatures.
It turns out that the careful addition of chemical impurities into certain materials can stabilise superconductivity up to a relatively balmy -140C or thereabouts. These materials are known as ‘high temperature superconductors’. Though they haven’t yet reached a high enough temperature to be practical, they’re a step in the right direction. Figuring out how to improve upon this is one of the biggest open questions in modern condensed matter physics.
A different sort of impurity could prove extremely useful in the race to build a quantum computer. Take diamond, for example. By creating impurities known as nitrogen-vacancy centres, diamond can be turned into a promising candidate for practical quantum computing.
Nitrogen-vacancy centres are created when pairs of carbon atoms are removed from diamond and one of the resulting holes is filled by a nitrogen atom. These nitrogen-vacancy centres can act as quantum bits (qubits) and can be used to store quantum information similarly to the way in which a regular computer stores information in the form of 0s and 1s. Other forms of impurities in different materials can achieve similar quantum computing promise in different ways, some taking advantage of a phenomenon known as quantum entanglement which will be dealt with in a future article.
These are just a few examples of how impurities can be useful but this is really just the tip of the iceberg. Fundamentally, there are still a lot of unknowns as to how impurities and randomness can affect the deep quantum mechanical properties of materials. This is an active and fast-growing area of research, both in the UK and elsewhere. If we can figure out the underlying physics of disorder, we just might find the key to designing the materials of the future.
The world isn’t perfect, at least not in form. But if we can harness these imperfections, understand them and use them, we can perhaps make our world function a whole lot better.