The mysterious mysterious particle of Majorana is now closer to the control of quantum computing

As mysterious as the Italian scientist who named it, the Majorana particle is one of the most fascinating quests in physics.

Its fame derives from its strange properties – it's the only particle that is its own antiparticle – and its potential to be exploited for future quantum computing.

In recent years, a handful of groups, including a Princeton team, have reported finding Majorana in various materials, but the challenge is how to manipulate it for quantum computing.

In a new study published this week, the Princeton team describes a way to control Majorana's quasiparticles in a context that also makes them more robust. The framework (which combines a superconductor and an exotic material called topological insulator) makes the majoranas particularly resistant to destruction by heat or vibration of the external environment. In addition, the team has demonstrated a way to turn on or turn off the Majorana with the help of small magnets built into the device. The report appeared in the newspaper Science.

"With this new study, we now have a new way of engineering Majorana's quasi-particles in materials," said Ali Yazdani, professor of physics in the 1909 class and lead author of the study. "We can verify their existence by imaging them and we can characterize their predicted properties."

The Majorana owes its name to the physicist Ettore Majorana, who predicted the existence of the particle in 1937, only a year before disappearing mysteriously during a ferry trip off the Italian coast. Building on the same logic with which the physicist Paul Dirac had predicted in 1928 that the electron had to have an antiparticle, later identified as the positron, Majorana theorized the existence of a particle that would be its own antiparticle.

Generally, when the material and antimatter meet, they mutually cancel each other by a violent release of energy, but the majoranes, when they appear in pairs at each end of specially designed cables , can be relatively stable and interact weakly with their environment. The pairs make it possible to store quantum information at two distinct locations, which makes them relatively robust against disturbances because the change of the quantum state requires simultaneous operations at both ends of the wire.

This capability has captivated technologists who imagine a way to create quantum bits – quantum computing units – that are more robust than current approaches. Quantum systems are prized for their ability to solve problems that can not be solved with today's computers, but they require the maintenance of a fragile state called overlay, which, if disturbed, can cause failures of the computer. system.

A Majorana-based quantum computer would store the information in pairs of particles and perform the calculation by plaiting them around each other. The results of the calculation would be determined by the annihilation of Majoranas between them, which can result in the appearance of an electron (detected by its charge) or nothing, depending on how the pair of Majoranas were braided. The probabilistic result of the annihilation of the Majorana pair underlies its use for quantum computing.

The challenge is to easily create and control the Majoranas. One of the places where they can exist is at the ends of a magnetic atom chain of an atom of thickness on a superconducting bed. In 2014, reporting ScienceYazdani and his collaborators used a tunneling microscope (STM), in which one end is dragged on atoms to reveal the presence of quasiparticles, in order to look for the majoranes at both ends of a chain. 39 iron atoms resting on the surface of a superconductor.

The team then detected the Majorana quantum spin, a property common to electrons and other subatomic particles. In a report published in Science in 2017, the team said that the Majorana spin property is a unique signal to determine that a detected particle is indeed a Majorana.

In this latest study, the team explored another predicted location for Majoranas' research: in the channel that forms at the edge of a topological insulator when it is brought into contact with a superconductor. Superconductors are materials in which electrons can travel without resistance, and topological insulators are materials in which electrons flow only at the edges.

The theory predicts that Majorana quasiparticles can form at the edge of a thin sheet of topological insulator that comes into contact with a block of superconducting material. The proximity of the superconductor forces the electrons to flow without resistance along the topological insulating edge, which is so thin that it can be considered a wire. Since the majoranes are formed at the ends of the threads, it should be possible to make them appear by cutting the thread.

"It was a prediction, and it was just sitting there all these years," Yazdani said. "We decided to explore ways to achieve this structure because of its greater Majorana manufacturing potential in the face of imperfections in materials and temperature."

The team constructed the structure by evaporating a thin layer of topological bismuth insulator over a block of niobium superconductor. They placed nanoscale magnetic memory bits on the structure to create a magnetic field that derails the electron flux, producing the same effect as cutting the wire. They used STM to visualize the structure.

However, when they used their microscope to hunt Majorana, the researchers were first confused before what they saw. Sometimes they saw majorana appear and at other times they did not find it. After further exploration, they realized that the Majorana only appeared when the small magnets were magnetized in a direction parallel to the direction of the flow of electrons along the channel.

"When we started to characterize small magnets, we understood that they were the controlling parameter," Yazdani said. "The way the magnetization of the bit is oriented determines whether Majorana appears or not.This is an on-off switch."

The team reported that the quasi-Majorana particle that forms in this system is quite robust because it occurs at energies distinct from other quasi-particles that may exist in the system. Robustness also comes from its formation in a topological edge mode, which is inherently resistant to disturbance. Topological materials derive their name from the mathematics branch, which describes how to deform objects by stretching or folding them. Electrons circulating in a topological material will therefore continue to move around bumps and imperfections.