Now it?s our turn

Leiden theorists to continue work on the discovery of Majorana particles

Sam RentmeesterProfessor Leo Kouwenhoven from Delft with PhD students Kun Zuo (left) and Vincent Mourik at the lab.

By Bart Braun

“The experimental physicists have made their move, and now it’s the theorists’ turn.” Two Leiden scientists are to analyse the results of measurements produced by Professor Leo Kouwenhoven from Delft. “Together, we’re going to find out what’s going on.”

Even those who have only the slightest interest in science news cannot have missed it: the Majorana fermions. A group of physicists from Delft and Eindhoven headed by Professor Leo Kouwenhoven announced on the Science website last Friday that their measurements indicated that they had discovered particles in their nanowires.
Kouwenhoven was invited to join Prime Minister Rutte and go on the TV show Pauw & Witteman and there are even rumours of a Nobel Prize.
But that’s getting ahead of ourselves. First of all, Nobel Prizes are only awarded after years of waiting and Kouwenhoven’s discovery still holds too much uncertainty. Not without reason, his article in Science is called “Traces of Majorana fermions” rather than “Evidence for Majorana fermions”. Kouwenhoven announced that he would be joined by scientists from Leiden in his work on the measurements.
Michael Wimmer and Anton Akhmerov are two of those Leiden scientists; they are two theoretical physicists from Professor of Theoretic Physicist Carlo Beenakker’s group and they are looking forward to this partnership.
But first, let’s go back to the basics: the stuff around us – air, cats, the earth – consists of stuff that is much, much smaller and physicists have divided that stuff into two categories. On the one hand there are bosons, named after Satyendra Nath Bose, and on the other you have fermions, named after Enrico Fermi. The difference between bosons and fermions is in the property physicists call “spin”: tiny particles such as electrons act as if they are rotating on their axis and bosons and fermions have different spin values. Electrons and quarks are all fermions: if you put them together in the right proportions, you get atoms. And if you put atoms together in the right proportions, you get a cat, or the earth.
Physicists describe the properties of all these tiny, tiny particles in a system of mathematical equations we know today as the standard model. Seventy years ago, Mr Majorana – one of Fermi’s PhD students – established that a certain kind of particle must exist, according to the maths: the Majorana fermion. And that is what Kouwenhoven has discovered, isn’t it?
Not quite: “What Leo has found is not precisely what Majorana was talking about and actually they aren’t really fermions”, says Akhmerov. People studying Solid State Physics like to make ordinary matter act in unusual ways, he explains. The electrons in Kouwenhoven’s nanowire act as if a Majorana is present; however, that does not mean that Majoranas are things in their own right as Ettore Majorana imagined them to be. Wimmer adds: “You could compare it to the Cooper electron pairs in a superconductor. Superconductors work because two electrons together will work as one thing: a Cooper pair. But the Cooper pair isn’t a real thing like an electron, it’s a quasiparticle.” Just like these Majoranas.
Akhmerov conjures up a picture on his laptop. “This image accompanied the article in Science, but the media hardly used it because it looks boring. The peak in the middle is caused by the Majoranas, according to the people in Delft; you see, they have excluded all the other obvious possibilities. But that doesn’t prove that these things are Majoranas, it’s a statement about the likelihood of it.” Wimmer continues: “It might seem as if we’re playing down Leo’s discoveries, but we’re not. That lot in Delft have done some really impressive work.”
Akhmerov explains: “The experimental physicists have made their move, and now it’s the theorists’ turn. We are going to get involved in this game, that’s for sure, and we’ll see if we can explain these measurements properly in terms of Majoranas: should we expect these particles to act like that in those circumstances? Together, we’re going to find out what’s going on.”
One of the reasons why the Majoranas are so interesting, and the reason why Microsoft has poured so much money into Kouwenhoven’s research, is that, in theory, you could use them to build a quantum computer. Ordinary computers use noughts and ones, or bits, as they are called. But in a quantum computer, a bit can be both nought and one at the same time, and that means that you can solve certain problems much faster than with the most powerful of regular computers, for things like simulations of folding proteins or quantum mechanical phenomena. Majorana fermions would be useful because quantum computers are very susceptible to disturbances. The Majoranas form pairs, catch an electron and then split up towards opposite ends of the Delft nanowire. According to the amazing laws of quantum mechanics, a very special situation arises, as the electron would be in both Majoranas simultaneously. If you allow the two Majoranas to cancel each other out, an electron would, or would not, appear. “Which makes them very suitable as quantum bits”, adds Akhmerov.
The fact that the Delft Majoranas are quasiparticles which, for now, can only exist in an indium antimonide wire just above absolute zero is not a problem for the Leiden physicists. Akhmerov exclaims: “So what? Just keep your computer cool!”and Wimmer adds: “At the moment, the electrons in your computer can’t work without wire.”

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