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The robust qubit: the Zen particle

A couple of years ago, theoretical physicist Carlo Beenakker tracked down the Majorana particles and inspired Leo Kouwenhoven from TU Delft to try to create them in a superconducting nano-structure. He succeeded in doing so in 2012, and the news made headlines around the world. But a lot of work still needs to be done to get multiple Majorana qubits to work together.

Majorana particles: robust and entangled

A full-fledged quantum computer needs to have at least a hundred or so qubits working together in order to perform calculations beyond the range of a conventional computer. Individual qubits already exist that can assume a mixture of values of 0 and 1 simultaneously. In fact, there are even various types of them. The main problem is that these qubits are extremely sensitive to disturbances. If the qubit is hit by even one little vibration or ray of light, it reverts to an ordinary bit with a value of either 1 or 0. So, you have to isolate them very well from the environment for the duration of the calculation process.

However, the qubits need to be closely linked together (physicists refer to this as ‘entanglement’), or else the quantum computer will not work and you’ve got nothing more than a conventional computer. Majorana particles meet these contradictory requirements. They are relatively robust, but they can still easily be made to entangle, allowing them to be manipulated from outside.

The art of Nothingness

Beenakker says: ‘I always call it a Zen particle. Zen is the art of nothingness. The Majorana particle is also ‘nothing’: it has no charge and no mass. But you can still store quantum information in it.’ Marjoranas occur at both ends of a minute, superconducting thread. This is why Majoranas always come in pairs. When in 2012 Kouwenhoven’s research group first showed that Majorana particles existed in their superconducting nano-structure, the news made headlines across the world: the existence of Majoranas in theory had now been shown in practice. But that was only the first step. The researchers still needed to work out how you could entangle multiple Majorana qubits together, how you write the data to them and then how to read the result when the quantum calculation was completed.

Leo Kouwenhoven and his team in the lab. Photo © Sam Rentmeester

Architecture of the quantum chip

This is what Beenakker’s PhD student Bernard van Heck has been working on for the past few years. In his dissertation, he works out in detail the architecture of a chip that can perform calculations with Majorana qubits. Manipulating the qubits is done using electric and magnetic techniques that are all used in other superconducting electronic devices. The fact that the techniques already exist makes it easier to quickly construct a working prototype.

Software giant Microsoft is investing millions of euros in the Q-tech ‘factory’, a conglomerate of research groups that include the Kouwenhoven group. It’swhere potential components for the quantum computer are produced and tested. Leiden serves as a think tank for Q-tech, and there is intensive contact between quantum researchers in Leiden and Delft. Components of van Heck’s chip have already been tested in Delft, but a couple of years are still needed to construct and test all the different designs in his dissertation.

Beenakker says: ‘You can use various strategies to construct a quantum computer. Either you take our current capabilities and try to scale them up by a factor of a hundred, or you can take on something that is conceptually brand new. Both strategies have their pros and cons, but here in Theoretical Physics we’re focusing on the latter, and thus on Majorana particles.’

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