Majorana particles back on the radar in Delft thanks to new approach – New Scientist

After TU Delft researchers had to withdraw their Nature publication about a promising building block for future quantum computers two years ago, they are now coming up with a new approach. By taking a step back, the coveted stable Majorana particles seem to be within sight.

Delft researchers, led by Leo Kouwenhoven, have been looking for majorana particles for more than ten years. In 2012, they found the first hopeful signs. Majorana particles are not elementary particles, such as electrons, but so-called quasiparticleswhere particles in a material collectively behave as if they were a particle.

Majorana particles

The material in which you excite these majorana quasiparticles could protect them from outside disturbance. This stability makes majoranas on paper very suitable as building blocks for a future quantum computer, which can perform certain tasks much faster than is possible with current computers.

A Nature publication was published in 2018 in which the Delft researchers showed that they had succeeded in finding majorana particles at the ends of ultra-thin superconducting nanowires. But after criticism of the measurements, they had to withdraw this article in 2020.

Since then, the research group has not been idle. They are working on a new approach with which succeeded now seems to make simpler, ‘austere majoranas’. That is a first step towards more stable Majorana particles.

‘In recent years it has become clear that with accurate measurements you can see phenomena emerging in your material that initially look very similar to majoranas, but which do not have all the necessary properties,’ says Guanzhong WangPhD candidate at QuTech, the research institute for quantum technology of TU Delft and TNO.

Short chain

That raised the question of why the nanowires did not behave as hoped. The Delft researchers believe that the materials are not pure enough, causing disruptions. In theory, majoranas should be able to withstand such disturbances, but there is a limit to that. It seems there were too many disruptions.

That is why the researchers have now taken a step back. They have made a shorter, simpler nanowire, in which they can better control the conditions. Majorana particles always appear in pairs, one at each end of such a nanowire. You can think of a majorana pair as two half electrons.

The shorter nanowire is somewhat reminiscent of a string of beads, because it consists of two so-called quantum dots, small islands of semiconductor in a non-conducting environment, with a piece of nanowire covered with a superconducting material in between. Electrons can move back and forth between the quantum dots and the superconducting piece of nanowire in specific ways. ‘By very precisely controlling the behavior of electrons in the superconducting part, we can ensure that majoranas are formed at the ends,’ says Tom Dvirpostdoctoral researcher at QuTech.

The researchers have now demonstrated that they have sufficient control in the simplified system with two quantum dots to allow simple, ‘sober majoranas’ to emerge.

Chewing gum

‘The next step is not only to measure the Majorana particles, but also to manipulate them so that we can use them as qubits,’ says Dvir. ‘At the same time, we will see if we can make the chain longer, so that the Majoranas are further apart, which should make them more stable and robust.’

‘I think it’s a wonderful experiment,’ says theoretical physicist Carlo Beenakker of Leiden University, who was not involved in the study. ‘They have taken a step back to first build something they understand well, and then carefully, step by step, work towards the final goal: stable Majoranas.’

Beenakker compares the two majoranas at the ends to a piece of chewing gum that you pull apart. If you pull it apart until it breaks into two pieces, you have two Majorana particles. If you put them back together you will have your old electron back.

First step

‘What they have now done in the experiment is to pull the chewing gum apart a bit,’ says Beenakker. ‘But the parts are not separated from each other. So they can shoot right back into an electron.’

Beenakker therefore sees it as a first step towards a ‘real Majorana particle’. “Maybe if you make a longer chain and pull harder, you’ll get two independent particles, but we’re not there yet.”