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ERC Advanced Grant for Carlo Beenakker to ‘braid’ Majorana fermions

Theoretical physicist Carlo Beenakker has been awarded a €2m Advanced Grant by the European Research Council (ERC). He will use this to try to create the ideal building blocks for a quantum computer: ‘braided’ Majorana fermions. An ambitious project that may just lead to a major breakthrough.

Researchers from all around the world are racing to build the first quantum computer. Software giant Microsoft, for instance, is investing hundreds of millions of euros in Delft research institute QuTech. Carlo Beenakker and his group of theoretical physicists act as a kind of think tank for QuTech, developing new concepts for quantum computing.

Bits are the smallest building blocks of information processed by the ‘classic’ computers that we all have at home or in our smartphones. The value of a bit is 0 or 1, and it can be visualised as a switch that is either ‘on’ or ‘off.’ Each computer chip contains millions of these switches.

A hundred qubits

In contrast, the building blocks of information that a quantum computer uses are qubits, and their value is a mix of 0 and 1 simultaneously. This odd, but well-known, quantum-mechanical property means that a quantum computer of only about a hundred qubits is, in theory, more powerful than today’s biggest supercomputer. 

A qubit can consist of one electron trapped in a microscopically small cage. The problem with this type of qubit, however, is that it is very sensitive: a minute vibration or flicker of light can reduce it to a regular bit with a value of 0 or 1, ruling out quantum computing. Despite this, experiments are being carried out with sets of 20 such qubits.

Topological quantum computing will have been achieved if the world lines of Majorana fermions are braided in space (x) and time (t).

Radically different approach

The aim is to gradually increase this number until you end up with a working quantum computer. Beenakker describes this approach as: ‘A lot of hard work and very gradual progress.’ The proposal that has won him the ERC Grant takes a daring, radically different approach by focusing on qubits that consist of ‘braided’ Majorana particles. High-risk, high-reward is how he terms it in his research proposal: if he is successful, this will constitute a giant leap forwards that will immediately outpace the slowly but surely approach.

Half electrons

Majorana fermions are ghost-like particles that can come into being in ultra-cold superconducting materials. Leo Kouwenhoven’s group in Delft was first to show this, hitting world headlines at the time. Majorana fermions always appear in pairs; in a certain sense, they are two half electrons. In Kouwenhoven’s study, these two halves were stuck to the ends of a microscopic piece of superconducting metal. Majorana fermions can stand up better to interference than other forms of qubit, and although theoretically they make good qubits, in recent years it has proved more difficult than expected to make working qubits from these immobile particles.

Impression of the microscopic structure of a chip that creates a topological qubit.

Topological qubits

Beenakker now wants to go one step further and make ‘topological’ qubits from braided Majoranas. Theoretical research has already shown that a voltage pulse can cause the two halves of a Majorana fermion to switch places. In a graph of their position plotted against time, two braided ‘world lines’ can be seen (see image). As the Majorana fermions switch places, the topology of the situation changes. In theory, this is a very robust way to store the information of a qubit, while making it possible for multiple qubits to communicate with each other – something that is essential to quantum computing.

ERC Advanced Grant

Every year, the European Union awards about 200 ERC grants to projects that will run for up to five years in all scientific domains. Beenakker will use most of his €2m to appoint PhD candidates and postdocs. Beenakker: ‘We will definitely have the theoretical calculations within five years. And within that time it should be possible to build a prototype in the lab too.’ That would be a breakthrough in the development of the quantum computer.

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