The miraculous property of the Majorana fermion.
Text: Tim Schröder
Quantum computers of the future promise to outshine their conventional forebears with their powerful ability to perform arithmetic operations in parallel. The problem is that qubits, the building blocks of quantum mechanical memory, can still only store information for a short space of time.
Qubits store information using their “spin”, or rather using the alignment of the magnetic field of a fundamental particle such as the electron, for example. This spin can adopt certain states – that is, it can exhibit different spatial orientations. Applied to the computer, different orientations would correspond to the states “on” and “off”, or “1” and “0” in binary code.
Today, qubits are still a decidedly unstable form of memory and are sensitive to interference from their surroundings – especially heat or electromagnetic fields, such as those found in electronic systems. The spin can flip over within a few seconds, or even fractions of a second, at which point the information is lost.
Accordingly, quantum computers of the future will, to some extent, need memory modules with greater stability. The development of such modules is the focus of researchers at the University of Basel, including Jelena Klinovaja, Professor of Physics and an expert in the physics of condensed matter. Klinovaja is a specialist dedicated to searching for an extremely unusual fundamental particle. This particle should, in theory, be far less sensitive to interference by electromagnetic fields and would therefore make a promising candidate for a future quantum computer.
Insight from an Italian physicist
The specific fundamental particle concerned is the Majorana fermion, named after the Italian physicist Ettore Majorana who first postulated the idea over 70 years ago based on theoretical calculations. More precisely, as is so often the case in physics, the Majorana fermion actually refers to two counterparts: a fundamental particle and its antiparticle. There are plenty of examples of pairs of this kind, such as the proton and the antiproton or the electron and the positron. A common feature of all such pairs is that the two partners have very similar properties, but also that there is always a key difference. This is not true of the Majorana fermion, however. Based on theoretical considerations, Ettore Majorana concluded that there must be particles of matter that are simultaneously their own antiparticle.
Moreover, these exotic particles have an almost miraculous property: if you exchange the positions of two identical particles of this kind by revolving them around one another, it is possible to use this change as a way of storing quantum information. As this exchange is not dependent on a specific path, qubits of this type are often also described as “topological”. In this context, “topological” means that a property remains the same regardless of the type of change – in this case, regardless of the exchange and of the path that it describes.
Evidence of Majorana states
A few years ago, a team of Dutch researchers actually succeeded in using a complex experimental setup to detect the first reliable signs that particles with Majorana properties exist. They attached a nanowire made of semiconducting material to a superconductor, and then altered the magnetic fields and voltages within the system in such a way that signals could be detected at the ends of the nanowire. In theory, these signals corresponded to those of Majorana particles. This principle has been perfected by multiple research groups around the world – including by Jelena Klinovaja and other researchers at the University of Basel.
To even detect these exotic Majorana particles in the first place, it is key above all to synthesize very pure materials for the experiments. “I’m a theorist,” says Jelena Klinovaja. “In my research group, we focus on developing material systems that can most likely be used to generate quantum mechanical states corresponding to those of a Majorana fermion or of even more exotic particles – so-called parafermions.”
The theoretical ideas are then put into practice by other research groups at the University of Basel, who use them to produce tailor-made semiconducting and superconducting surfaces. “That’s the really great thing about working here in Basel – theory and experimentation take place next door to one another,” says Jelena Klinovaja. “Our theoretical ideas can be put into practice immediately.”
Understanding interference better
Jelena Klinovaja’s work is a bit like researching an object when all you can see is its shadow: she has not yet been able to grasp the particle, but rather only its electromagnetic shadow – that is, its properties. What makes her work so difficult is the fact that she is essentially looking for a needle in a haystack. Majorana states only occur in extremely small numbers. “It’s almost as if we’re working with a system the size of Earth but need to find a single person,” says the researcher. This is compounded by further challenges: Majorana states are indeed more robust than other fundamental particles, which makes them a promising form of qubit memory for future quantum computers. “However, our theoretical reflections also point to other confounding variables in the environment that can influence the Majorana particles.”
Before an application in quantum computers can become tangible, it is essential that we gain a more precise understanding of these confounding variables. Jelena Klinovaja therefore wants to study effects of this kind on Majorana particles, as well as parafermions, in greater depth in the future. It is an almost mystical approach to research: so far, the existence of Majorana states has not been definitively proven, but Klinovaja and her colleagues nevertheless want to research the properties of Majorana fermions. It is almost as if they were trying to use a blurry shadow to ascertain an object’s properties and weak points – in other words, it presents an enormous challenge. Klinovaja has just been awarded a prestigious ERC Starting Grant in order to push ahead with this research, underlining the significance of her work in this highly competitive field.
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