Transporting renewable energy to where it’s needed lies at the heart of the human endeavour to get rid of the need for fossil fuels. Superconductors can do so without loosing any of the precious electricity on the way, seemingly defying physical intuition. Find out in this article why many body physics is needed to understand their counter-intuitive behaviour, what role quantum entanglement plays and how quantum computation might lead to the discovery of materials which may give us the tools for a greener future.
Dealing with climate change and the shortening fossil resources of our planet is one of the most pressing problems of our generation. Physically, both issues arise from the fact that fossil fuels are incredibly convenient for solving the two most important human tasks: Producing energy and transporting it to where it’s needed. With oil, the former task has been done by nature in the last couple of million years. We just have to pump the ready-made product out of the earth. Transportation is also easy due to its incredible energy density. Just 50kg of oil can carry a car weighing 2 metric tonnes for a thousand kilometres!
The curse of Ohm
At first sight, both problems are not that hard to solve. We know how to harvest the sun’s energy with solar panels, so why don’t we just put a lot of them in the deserts of the earth and then transport the electricity to cities with long cables? The main reason is probably of political nature (deserts close to Europe for example have been war zones recently), but there is also a physical aspect: With current technology, transporting energy comes with a price in the form of Ohm’s law, which holds in all normal metals like copper and iron which we use to transport electricity.
Because of Ohm’s law, we inevitably lose energy when we transport it. And there is also another problem: Because the loss of energy happens in the form of heat, cables have to be thick enough (at a given energy throughput) so they don’t melt. Most current energy transport also happens at high voltage (U), because then the energy loss (P) due to heating is less. But a high voltage also means high electric fields and those fields can be damaging to electronics and humans and we need to make sure that there is no lightning jumping from the cable to the ground. All of these reasons justify why you see all these ugly masts everywhere in today’s civilized world.
While you may say that aesthetics is maybe not the most important thing when it comes to an issue endangering millions of people, reality is that most people would not like to have one of these masts in their front yard. In Germany, this fact has led to the stalling of the “Energiewende” because important electricity transport lines from the windy north (where most renewable energy is produced) to the population and industry centres in the south (where all those shiny cars are built) can’t be set up due to resistance of the population living along the planned route.
But there actually are materials with which you can transport the same amount of electricity as those huge masts in a single cable of just a few cms diameter under any old road! In superconductors, Ohm’s curse doesn’t hold and so they can conduct electricity without any energy loss (and with that I literally mean zero loss). How is that possible you may ask? Doesn’t this sound like a perpetuum mobile, something like a car that keeps on rolling when you just set it moving once?
Every time something counter-intuitive happens in physics, chances are that it’s quantum mechanics that lies at the base of it and it is no different in superconductors. In fact, you can describe a cable of superconducting material with a single wave function, as if it was just one huge quantum particle moving. Superconductors are one of the few examples where quantum effects become truly macroscopic; the kilometres of coherence length reached surmount the wavefunction extent of the electron in a hydrogen atom by a factor of 10’000’000’000’000! Put differently, if an electron wavefunction would be the size of a human then the wavefunction in a superconductor would be as large as the distance between Earth and Pluto!
Electron couple dance
How does this happen exactly and why does this lead to frictionless flow of electricity? While the exact explanation of this is quite involved and resulted in multiple (!) nobel prizes being awarded to the theory’s discoverers, I want to give a simple picture of analogy here. In a normal conductor, electrons are lone wolfs, they fight themselves through the mace of atoms and get pushed around by them, loosing energy to the crystal lattice every time they bump into something.
In a superconductor, something beautiful happens: As the temperature is lowered, electrons suddenly realize that they are note alone, and start to assemble in pairs (called “Cooper pairs” after their discoverer Leon Cooper, Nobel prize ’72). These pairs can then be regarded as one entity, just like a married couple often assumes one name. In the superconductor, this means that the “particles” with which we can base our theoretical description upon, are note the electrons any more, but the new “quasiparticle” (check out our article about those!) which we just called Cooper pair.
But there is something weird in this picture. Everyone knows that all electrons are negatively charged and equal charges repel each other. So how can they suddenly do the opposite? Overcoming this difficulty was the insight of Bardeen, Cooper and Schrieffer who jointly got awarded the nobel prize for this. They showed that what happens is that when an electron flies through the lattice it also distorts the regularly ordered arrays of atoms. If the relaxation of this distortion is much slower than the time between two electrons passing the same place, then a second electron will feel the effect of this distortion and gets attracted by it. Effectively, blending out the lattice, the first electron has therefore exerted an attractive force on the second. It’s also clear from this picture, that this attraction will be a quite long ranged force between the electrons. In fact, in typical superconductors, the distance between the two constituents of a cooper pair is hundreds of times larger than the distance between atoms in the crystal. This means that I should have drawn the arms on above picture much much longer!
The dancing couples merge
How does superconductivity arise from the cooper pairs? To understand that, we must first understand what’s so special about this pairs. They differ in one substantial property from the electrons: while two electrons can’t be in the same place at the same time (a purely quantum mechanical effect also termed “Pauli exclusion principle”), two,three, four, even hundreds of Cooper pairs can! And at very low temperatures, they also do. In fact, they get so close to each other that their quantum mechanical wave functions start to overlap, so strongly that all of them can in fact be descriped by one, macroscopically large wavefunction. A Bose Einstein Condensate (BEC) has been born, one of the only macroscopic quantum effects known so far.
One of the most counter-intuitive properties of this BEC is that it is also a superfluid, a fluid which can flow without any friction! This means that if you set this fluid in motion, it will never stop! And this is exactly how superconductivity emerges: a superfluid of cooper pairs has the property we were trying to explain all along: It flows without friction through its host material, i.e. without any resistance.
Can this even be used?
Yes and it already is! Ever seen those high-speed Maglev trains in Japan? They are based on yet another weird effect of superconductors: They push magnetic fields out of themselves! Maglevs are using this by levitating on superconducting magnets.
But also the application discussed in the beginning is not in the too far future. First kilometer-long cables of superconducting material have already been built and the proof of principle been shown. The problem however remains that one has to cool these materials with liquid nitrogen for them to be superconducting. There is however a whole different class of materials in which superconductivity ocurrs at much higher temperatures. Somewhat uncreatively, they are called “High temperature superconductors”. And even 30 years after their discovery it still remains a secret how superconductivity emerges in them as the picture which I presented above can’t be used for understanding them. One thing is clear however: Quantum mechanics deeply has its mysterious fingers in their inner workings.
Exciting times are ahead as today’s and tomorrow’s quantum computers study quantum materials like superconductors and they might lead to even more counter-intuitive, exciting and useful phenomena in the future! Stay tuned for more!