New discovery paves the way for the discovery of the holy grail of quantum computing | science and technology

QuTecn researchers are working on a quantum network node with mirrors and filters that guide laser beams to a diamond chip.Marieke de Lorijn

Accidental discoveries happen all the time in science and technology. Some of the best known examples include microwave oven, penicillin, Teflon, vulcanized rubber, and Viagra. This happened again at the Institute of Materials Science (ICMM) of the Spanish National Research Council (CSIC) in Madrid. Elsa Prada, Ramón Aguado and Pablo San José were part of the ICMM team that collaborated with the Institute of Science and Technology of Austria (ISTA), the Catalan Institute of Nanoscience and Nanotechnology (ICN2) to Barcelona and Princeton University in the United States. They were all working in search of the holy grail of quantum physics: the Majorana particle. Hypothesized by Ettore Majorana in 1937, this element of particle physics remained in the realm of theory for 86 years. Proving the existence of the Majorana particle, also called the fermion, requires exceptional stability which can only be provided by a special material called a topological superconductor. After two years, the international search team thought they had found the elusive particle, but further analysis revealed it was a mirage. What they actually discovered was something just as important: an impostor particle that mimics the behavior of a Majorana particle.

The discovery, recently published in Nature, is important for several reasons. Not only does it provide a deeper understanding of topological superconductors, but it demonstrates techniques for distinguishing impostor particles from true Majorana particles. It also identifies a source of error in the interpretation of experiments and points the way to an even more transcendental discovery which, according to physicist Ramón Aguado, “will be a Nobel Prize winner when it can be demonstrated irrefutably because its attributes will be so superior to the standard model of fermions and bosons.

Quantum computing exploits a unique property called superposition to exponentially increase processing capacity. While the classical bit can only have one of two values ​​(0 or 1), a qubit (the quantum analog) is capable of expressing multiple states simultaneously. But the superposition requires a still unfinished coherence of the quantum states that must be maintained for a minimum duration. Any environmental disturbance (temperature, vibration, remanent energy, electromagnetic radiation or other common phenomena) nullifies the property and causes decoherence and defects which reduce the computational capacity.

Jian-Wei Pan, China’s top IT expert, summed it up nicely. “Building a practical, fault-tolerant quantum computer is one of our biggest challenges. The most formidable obstacle to building a large-scale universal quantum computer is the presence of noise and imperfections.

The current approach is to recognize and address these limitations of quantum computing by correcting errors using traditional processing. Another approach is to isolate quantum computers as much as possible from the environment and keep them at temperatures close to absolute zero (-273.15°C or -459.67°F).

“A quantum computer, explains Pablo San José, must be completely isolated from the environment during its operation. There can be no light penetration, vibration or disturbance from the outside world. It must be in a bubble, like a mini-universe in itself. This makes it incredibly fragile.

A quantum computer must be in a bubble, like a mini-universe in itself. This makes it incredibly fragile

Pablo San José, physicist

To make the big leap forward in quantum computing, the Majorana particle must be found and mastered because this particle “is able to hide the quantum information it encodes, making it invisible to the outside world,” San said. José in an admittedly oversimplified explanation. “A qubit based on Majorana states would be much more robust against decoherence, because it is built from spatially separated quantum wavefunctions that make it insensitive to any local disturbance. This robustness would greatly ease the scalability problem [creating computers with more qubits to exceed traditional computing capacity]”, said Aguado. “We have been looking for this famous particle in topological superconductors for 10 or 12 years now.

Elsa Prada says the discovery of the particle requires the development of topological superconductors that can truly hide quantum information to protect it from external disturbances. “This type of material does not exist naturally, it is the product of materials engineering. Unfortunately, they harbor all sorts of impostor particles that can be misleading. To avoid deception by impostor particles, two things are needed: substantial improvements in material quality (a very delicate process that only a handful of materials producers know how to do), and subjecting the topological superconductor to very strict measurement protocols. sophisticated that reveal quantum entanglement.

A team led by Charles Marcus at the Niels Bohr Institute in Denmark has taken the first step by using proprietary topological material and an innovative technique to identify the Majorana particle. The measurements indicated an apparently correct trajectory. The Austrian team replicated the experiment independently using the same material, and the results initially matched. But two tests are insufficient in the world of science and technology, so an additional test was carried out. “They identified a contradiction in the findings, and it was an irreconcilable paradox that they couldn’t explain,” San José said. But the ICMM team found the answer: it was an impostor particle that behaved like a Majorana.

“Imposter particles often have some of the properties of true Majoranas, such as zero energy, zero spin, and zero charge. But they lack the fundamental property of shielding quantum information from the environment by means of “a quantum wave function that is analogous to an electron being split into two spatially separate halves. In that sense, impostor particles are useless in quantum computing,” San José said.

What initially seemed like bad news, Prada says, was nevertheless an important discovery. In the complicated quantum index game, the ICMM team located the impostor, which allowed them to identify the cause of errors in previous experiments – a producer of false positives. The discovery also paves the way for the development of more robust topological superconductors. “Making superconductors topological is really very complicated. You have to mix different materials very precisely, with very specific geometries, subject them to external fields, and more,” Prada said.

This scientific setback will in fact contribute to creating momentum in the field. “We’re too impatient,” Aguado said. “The materials that Elsa Prada was talking about are 13 years old, and everyone rushed to demonstrate them. For context, transistors were discovered in the 1940s. But we didn’t have consumer microelectronics until then. the 1980s. The first microprocessors were very large and had about 1000 transistors. Today, microprocessors contain more than 100 billion transistors which are barely larger than a few silicon atoms… The first quantum bit based on superconducting circuits was demonstrated in 1998, and it took Google and IBM more than 20 years to launch quantum computers with more than 10 qubits, and we’re just beginning to explore brand new concepts in physics that will eventually lead to the next stage – the Majorana-based topological qubit.

Thus, in quantum physics, detecting false shortcuts is equally important in finding the way forward. “We are entering a barely explored technological universe. Manipulating the quantum world is a much more complicated and tricky game,” San José said. “We lack the tools and materials needed to open the door all the way. But these first steps are crucial. In the long term, topological materials will spark a revolution that will go far beyond the quantum computer. We are facing a new frontier in the understanding of matter.

The ICMM team said: “Understanding the fundamental physics that governs these superconducting devices is very important. Our work allows the identification of false positives in the search for the elusive Majorana particle. When it is finally found, we will be able to harness the full power of quantum computing.

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