Peak to Peak Former Microsoft team achieves simplified version of Majorana fermions

Recently, researchers and engineers from QuTech and Eindhoven University of Technology have created Majoranas and measured their properties under controlled conditions. These Majoranas are "poor man's majoranas", based on two quantum dots in a nanowire that can be extended to a larger chain of quantum dots, the Kitaev chain that have a more robust Majorana behavior [1].

An artistic impression of the Kitaev chain in two coupled quantum dots (white with black arrows), where the larger green arrow represents the Majorana part of the system and the small white dots with arrows represent the electrons and their spins.

Discovery of a new Majorana particle

Quantum computers are a revolutionary technology that has the potential to solve certain problems faster than classical computers.

Because they use quantum bits, they can represent both 0 and 1: this allows quantum computers to perform multiple calculations at the same time. Quantum computers and the implementation of quantum bits have great potential in various fields, including drug discovery, financial modeling, and cryptography. Majorana particles are one of the candidates for the implementation of quantum bits.

However, the search for Majorana fermions has not been straightforward. in 2012, the lab first discovered the state; in 2018, researchers at Microsoft Labs in the Netherlands claimed they had clear experimental evidence for Majorana fermions, but later retracted their claim; now, a team of some researchers in this study has discovered a Majorana fermion's "cousin" - "Poor man's Majorana" [2]. The team expects the discovery to reinvigorate the field and the search for these elusive states.

"There was a crisis in the Majorana field when the experiment that discovered Majorana fermions in 2018 was withdrawn." Tom Dvir, of Delft University of Technology in the Netherlands and first author of the new study, said, "We hope that our findings will help refocus work in this field."

A Majorana fermion is a hypothetical collective electronic state that acts like a particle that is its own antiparticle. Theorists predict that Majorana fermions are topologically protected, which makes them highly stable to thermal or electrical perturbations from the environment. The discovery of Poor man's Majorana is also its own antiparticle, but it lacks the topological protection of Majorana. this difference means that the newly discovered Majorana particle is stable only within a very small range of parameters, Dvir said.

Tom Dvir explains, "Majorana particles can be made into a kind of quantum bit and gain attention for their unique properties." Unlike traditional quantum bits based on the properties of individual particles such as electrons, quantum bits based on Majorana particles are more resilient to certain types of quantum errors (resilient), which is a major challenge in developing scalable quantum computers.

Guanzhong Wang, PhD, Delft University of Technology

His colleague and co-first author Guanzhong Wang added: "The desirable properties of Majorana particles, and their exotic nature allowing the observation of new scientific phenomena, have inspired a large amount of research work, from academia to industry. The main direction of research to date has been in materials synthesis, with the aim of designing the right material properties so that devices made from them can operate immediately when cooled to low temperatures."

This time, the new approach shifts the focus to electrical control, which means that we observe and tune devices at low temperatures, more conducive to the Majorana phenomenon.

Successful observation using superconductors, electric gates

The topological conservation of Majorana fermions comes from the nature of the state: in a nanowire, Majorana states are predicted to be bound at both ends of the line. This localization means that if the positions of the two Majorana are switched, the states do not affect each other and thus retain their original encoding. This property makes these states very interesting for quantum bits, since it is possible to manipulate quantum bits without noise degrading and changing the information they contain.

However, Poor man's Majorana behaves a bit like a leaky Tupperware box: put two in the bag and their contents spill over and mix. says Dvir: "They can exchange information." Some researchers believe that achieving these "lesser" states is also an important step toward achieving the goal of Majorana-based quantum computing. "

Dvir and his colleagues implemented their Majorana particle in a quantum dot device in which two quantum dots are connected by a semiconductor wire coated with a superconducting film. This device is the shortest realization of a Kitaev chain - a one-dimensional fermion chain with zero spin; when there are five quantum dots or longer, the Kitaev chain should hold topological Majorana particles.

When electrons in the smallest Kitaev chains move between quantum dots, they do so by one of two processes: normal tunneling, where electrons simply jump from one dot to another, or Andreev tunneling, where electrons from both dots enter the superconductor at the same time. There, they combine to form a Cooper pair, which is then injected into both points, joining them together. The Majorana state occurs when the normal tunneling rate and the Andreev tunneling rate match.

Dvir and his colleagues demonstrated that by adjusting the voltage applied to each element in their device, they could electrically switch the device from a state in which normal tunneling dominates to a state in which Andreev tunneling dominates to a state in which the two processes have equal rates. In their experiments, they detected signals for all of these states and those predicted for the two Poor man's Majorana states.

Dvir's team demonstrated three regimes for their quantum dot system: (left) normal tunneling dominates; (center) Andreev tunneling dominates; (right) normal tunneling and Andreev tunneling have equal rates, which is the "sweet spot" for an experiment that would Poor man's Majorana state appears.

Karsten Flensberg of the University of Copenhagen, Denmark, said previous proposals for connecting quantum dots and tuning the system involved multiple magnetic fields that were difficult to control with the nanoscale precision required for the Majorana experiment. "(The researchers) came up with a clever way to tune the ratio of normal and Andreev tunneling between the two quantum dots." By using superconductors and electrical gates, the team solved the problem.

The researchers first made two quantum dots close to each other, separated by a short semiconductor/superconductor nanowire. These quantum dots were electrically connected to each other in two ways: the first by electrons hopping between the two dots; the second involved pairs of electrons entering and leaving the semiconductor/superconductor nanowire at the same time [3].

"This is a breakthrough in Majorana's research." This sentiment is echoed by Jagadeesh Moodera, an experimentalist at MIT who is actively seeking topological Majorana. The new platform, he says, "opens up a new area of research in Majorana physics. Majorana research "has been mired in some controversy, and it's refreshing to see good basic science being done on (the subject)."

Breaking new ground in Majorana physics

The demonstration does have one limitation, however. The two wave functions that implement poor man's Majorana appear to overlap slightly, meaning they are not completely confined to their respective quantum dots. "These states are imperfect." However, Dvir notes that it is unclear whether this imperfection matters for quantum bits based on poor man's Majorana; they are working with theorists to figure this out. Indeed, it is not clear whether topologically protected Majorana (for quantum computing) is better.

For Dvir, this poor man's Majorana implementation is special for another reason: for most physics experiments, significant analysis and interpretation is needed to get plots from the experimental data that can be compared to the output of the theoretical model. "It's rare for abstract physics to appear in the lab exactly as predicted," he says.

"Currently, we are using a simplified version of Majorana, using only two quantum dots." Dvir explains, "Our ultimate goal is to have more quantum dots, perhaps even up to five, so that the two halves of the electron can be separated more widely. The farther the Majorana particles are separated, the better the resulting quantum bits are protected against noise."

"However, the difficulty of adding more quantum dots to the device is expected to increase linearly, rather than exponentially. This is because we can tune each dot individually, allowing us to achieve the ideal configuration more easily."

"Looking ahead, there are two main goals. The first goal is to create a complete topological Majorana based on the Majorana from this work; the second goal is to use these Majoranas to create quantum bits. This will require multiple copies of the system and further tuning."

In summary, while this simple system implemented this time can be extended to simulate a complete Kitaev chain with a new topological order, it can also be immediately used to explore physics related to non-abelian arbitrators in condensed states. In the long run, this approach can produce topologically protected Majorana states on longer chains [4].

Reference link：

[1]https://qutech.nl/2023/02/15/new-approach-for-majorana-research-in-short-nanowires/?cn-reloaded=1[2]https://physics.aps.org/articles/v16/24[3]https://phys.org/news/2023-02-approach-majorana-short-nanowires.html[4]https://arxiv.org/pdf/2206.08045.pdf