Discover a ‘trickster’ pretending to be a Majorana particle, quantum holy grail | Technique

QuTecn researchers are working on a node in a quantum network, where mirrors and filters direct laser beams toward a diamond wafer.
QuTecn researchers are working on a node in a quantum network, where mirrors and filters direct laser beams toward a diamond wafer.Marek DeLoregen

Finding one thing, finding another, and that the result is closely related to the desired goal is a common occurrence in science and technology. Suffice it to recall a few examples such as microwave, penicillin, Teflon, vulcanized rubber or Viagra. Something similar happened to a team from the Institute of Materials Sciences of Madrid (ICMM), of the Supreme Council for Scientific Research (CSIC), consisting of Elsa Prada, Ramón Aguado and Pablo San José, in collaboration with researchers from the Institute. Science and Technology in Austria (ISTA), the Catalan Institute of Nanoscience and Nanotechnology (ICN2) and Princeton University in the United States. They were looking for the holy grail of quantum physics: the Majorana particle, a theoretical proposal made 86 years ago by Ettore Majorana in the context of elementary particle physics that had not yet been experimentally proven. Finding and perfecting them in a special material that guarantees their stability, known as a topological superconductor, will be a critical step in condensed matter physics and quantum computing. After two years of investigation, they thought they had found him. But further analysis revealed that the discovery was a mirage. In its place they discover something different, but also fundamental: a trickster particle, which mimics the behaviors of the Majorana, but is not.

The significance of the discovery published in natureAnd It is multifaceted: it deepens our understanding of topological superconductors, demonstrates techniques capable of distinguishing between fraudulent particles and a real Majorana particle, identifies the source of error in the interpretation of experiments, and points the way to discovering it, according to physicist Ramón Aguado. , “The Nobel Prize will be when its existence and, above all, its quantum statistics, which are far from the norm in the Standard Model of fermions or bosons, are conclusively proven.”

Quantum computing takes advantage of a unique property, superposition, to double processing power. This property allows, while a classical bit has two values ​​(0 or 1), a qubit (the quantum analogue) multiplies its capacity exponentially by being able to express several states simultaneously. But such superposition requires a coherence of quantum states, which is currently out of reach and lasts for a minimal amount of time. Any change caused by the environment (temperature, vibration, residual energy, electromagnetic radiation or other usual phenomenon) cancels the property, generates decoherence, throws errors and limits computational power.

Jian-Wei Pan, a leading computer expert in China, summed it up powerfully: “Building an error-resistant and practically useful quantum computer is one of the biggest challenges for humans. Having noise and imperfections is the most fascinating thing to build a universal, large-scale quantum computer.”

So far, these limitations are assumed and attempts are being made to mitigate them by correcting errors. suffixWith the help of classical processing, or by building computers that are as isolated as possible from the environment and capable of maintaining temperatures close to absolute zero, equivalent to minus 273.15 degrees Celsius.

Pablo San José explains that a “quantum computer” requires that, while performing the calculation, it remain completely detached from the environment: there can be no interaction with light, nor with vibrations, nor with anything from the outside world. It must be in a bubble, as if it is its own little world. This makes them incredibly fragile.”

A quantum computer should be in a bubble, as if it were a small universe in itself. This makes it incredibly fragile.

Pablo San Jose, physicist

In this way, a quantum leap to building quantum computers that is both fault-tolerant and practically useful would be in finding and perfecting Majorana particles, “capable of masking the quantum information that they encode in such a way as to make it invisible to the outside,” warns San Jose, but it’s a simple way to explain it. . “A qubit built on Majorana states will be much stronger against decoherence, because it is built of spatially separated quantum wave functions, so it is immune to any local perturbation. This strength will make the scalability problem much easier.” [crear ordenadores con más cúbits para superar la capacidad de la computación clásica]Aguado adds. “We’ve been looking for this famous particle in topological superconductors for 10 or 12 years now,” he says.

In order to find it, it is also necessary, as Elsa Prada points out, to develop topological superconductors that can hide quantum information in order to protect it from the outside and “not be fooled”. “This material does not spontaneously exist in nature; they are products of engineering. Unfortunately, these contain all kinds of rogue particles and can be misleading. In order not to be deceived, two things are needed: Significantly improving the quality of the materials used (which is a delicate process). Very few of them farmers of materials in the world) and subject the topological superconductor to highly sophisticated measurement protocols, which reveal quantum entanglement. “

Charles Marcus’ group, at the Niels Bohr Institute in Denmark, has taken a first step using their topological materials and a new technique to identify a Majorana particle. The measurements seemed to point to the right track. The Austrian team repeated the experiment independently of the same material, and the results were initially consistent. But in science and technology, twice is not enough and they have done a supplementary test. “They saw that there was a contradiction in the conclusions and this was an irreconcilable contradiction that they didn’t know how to explain,” says San Jose. The CSIC team’s theoretical calculations yielded the answer: a trickster particle that behaved like a Majorana, but wasn’t.

“These fraudulent particles often have some of the original Majorana properties, zero energy, zero spin, zero charge…but not the essential element, which is the protection of quantum information from the environment via a quantum wave function that we can understand as an electron splitting into two spatially separated halves. In this sense, they are not useful for quantum computing,” he explains.

What might sound like “bad news,” Prada admits, is nonetheless a major discovery. In the complex Quantum Cluedo game, the CSIC team has located the impostor, which makes it possible to identify the cause of errors in previous experiments, which is the source of the false positives.

In addition, it also lights the way for the development of topologically durable superconductors. Prada adds, “Making it topological is really very complicated: you have to mix different materials in a very precise way, with very specific geometries, and subject them to external fields…”.

Discovery is actually a step back to gain momentum. “We’re impatient. The first proposals for the materials Elsa Prada is referring to date back 13 years ago and there was a rush to show them. To put it in context, let’s think that the transistor was discovered in the 1940’s and we didn’t have microelectronics of mass use until the 1980’s. With The first massive microprocessors had about 1,000 transistors, while they currently have over 100,000 million transistors inside them, with sizes a little larger than a few atoms of silicon,” Aguado explains. “The first quantum bits based on superconducting circuits were demonstrated in 1998 and it took more than 20 years for Google or IBM to release their own quantum computers with several dozen qubits. We were simply beginning to explore very new physics concepts that would eventually lead to the step Following; a topological qubit based on Majoranas,” he adds.

Thus, in quantum physics, finding the way forward is just as important as spotting false shortcuts. “We are entering a technological world that we have barely explored. Manipulating the quantum world is a different, more complex and delicate game. We lack tools and materials that we do not know yet to fully open the door, but these initial steps are crucial. In the end, topological materials will enable a revolution beyond the quantum computer. We We are facing a new frontier in understanding the matter,” adds San Jose.

“It is very important to understand the underlying physics that govern these superconducting devices. Our work dramatically narrows the chances of false positives in the search for the elusive Majorana. We have taken another step toward its discovery and the future exploitation of all its quantum computing power,” the researchers conclude.

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