The ‘topological’ quantum computer still exists only in theory but, if possible, would be the most stable and powerful computing machine in the world. However, it requires a special type of ‘qubit’ (quantum bit) that has yet to be realised and manipulated.

Conventional matter is made up of atoms which contain electrons, and scientists have long known that electrons are indivisible fundamental particles. Yet surprising new research shows that a weird feature of quantum mechanics can be used to produce objects that behave like half of an electron. These ‘split-electrons’ can act as topological qubits and might hold the key to unlocking the full power of quantum computation.

Recently published in Physical Review Letters, the discovery was made by Professor Andrew Mitchell at University College Dublin (UCD) School of Physics, Director of UCD Centre for Quantum Engineering, Science, and Technology (C-QuEST), and Dr Sudeshna Sen at the Indian Institute of Technology in Dhanbad, who are theoretical physicists studying the quantum properties of nano-scale electronic circuits.

“The miniaturization of electronics has reached the point now where circuit components are just nanometers across. At that scale, the rules of the game are set by quantum mechanics, and you have to give up your intuition about the way things work,” said Dr Sen. “A current flowing through a wire is actually made up of lots of electrons, and as you make the wire smaller and smaller, you can watch the electrons go through one-by-one. We can now even make transistors which work with just a single electron.”

In nano-scale electronic circuits, the phenomenon of ‘quantum interference’ between electrons can lead to states where electrons appear to split.

Professor Mitchell said: “In a nanoelectronic circuit, electrons going down different paths in the circuit can destructively interfere and block the current from flowing. This phenomenon has been observed before in quantum devices. The new thing that we found is that by forcing multiple electrons close enough together that they strongly repel each other, the quantum interference gets changed. Even though the only fundamental particles in the circuit are electrons, collectively they can behave as if the electron has been split in two.”

The result is a so-called ‘Majorana fermion’ -- a particle first theorized by mathematicians in 1937 but as yet not isolated experimentally. The finding is potentially important for the development of new quantum technologies, if the Majorana particle can be created in an electronic device and manipulated.

Professor Mitchell said: “There has been a big search for Majoranas over the last few years because they are a key ingredient for proposed topological quantum computers. We might have found a way to produce them in nanoelectronics devices by using the quantum interference effect.”

Quantum Interference explained by the ‘Double-Slit’ Experiment

When a nanoelectronic circuit is designed to give electrons the ‘choice’ of two different  pathways, quantum interference takes place. Professor Mitchell explained: “The quantum interference we see in such circuits is very similar to that observed in the famous double-slit experiment.” 

The double-slit experiment demonstrates the wave-like properties of quantum particles like the electron, which first led to the development of quantum mechanics in the 1920s. Individual electrons are fired at a screen with two tiny apertures, and the place they end up is recorded on a photographic plate on the other side. Because the electrons can pass through either slit, they interfere with each other. In fact, a single electron can interfere with itself, just like a wave does when it passes through both slits at the same time. Because the electrons can pass through either slit, the waves emerging on the other side can interact and recombine in complex ways, producing an interference pattern. When the peak of one wave collides with the trough of another, they cancel out -- the result is that the electron cannot pass through.

“It’s the same thing that is happening in a nanoelectronic circuit,” said Professor Mitchell. “Quantum interference can be used to produce the kinds of qubits we need for more powerful quantum computers.”

 

The research was published in August in Physical Review Letters, the flagship journal of the American Physical Society, and highlighted by the Editors as being among the most significant articles to appear that month.

Reference:  Physical Review Letters 133, 076501 (2024)