Building an ultracold atom quantum simulator: defeating magic with magic!

Produced by: Science Popularization China

Author: Luan Chunyang (Department of Physics, Tsinghua University)

Producer: China Science Expo

"It is expected to become a milestone and major breakthrough in modern science and technology!" "It is an experimental masterpiece and a long-awaited achievement!"

What kind of scientific research results could make the reviewers of Nature journal exclaim in this way?

This achievement comes from a group of outstanding scientists from the University of Science and Technology of China, who have successfully developed a quantum simulator called "Tianyuan". The reason why this quantum simulator is so eye-catching is that it can handle a complex problem that has long puzzled scientists, that is, simulating the "Fermion-Hubbard model".

The Tianyuan quantum simulator has up to 800,000 optical lattice points, which means that its simulation capabilities far exceed the classical computers we use in daily life. Even more exciting is that it has achieved the first experimental verification of an important physical phenomenon called "antiferromagnetic phase transition", which marks that we have entered a new era of quantum science.

The significance of this work goes far beyond our current imagination. It not only provides scientists with a brand new tool to explore the special physical behavior at low temperatures, but also helps us to have a deeper understanding of complex phenomena such as high-temperature superconductivity. This not only opens up new horizons for scientific research, but also lays a solid foundation for future technological innovation.

The relevant research results were published in the journal Nature under the title "Antiferromagnetic Phase Transition in the Three-Dimensional Fermion-Hubbard Model" (Image source: Reference 1)

“Fermion-Hubbard Model”: A Simple but Not Simple Physical Model

Have you ever wondered why iron attracts a small magnet but wood doesn't? Although scientists have some understanding of this question, the mystery of magnetism is still not fully solved.

Scientists generally believe that the secret of magnetism lies in the interaction of electrons inside materials. Imagine that electrons are dancing a complex dance on the stage of atoms, and their every move affects the harmony of the entire team. But in the real world, this dance is too complex for us to fully capture with traditional methods.

Schematic diagram of the magnetic mystery

(Photo source: veer photo gallery)

Although we knew the answer lay in the interactions of electrons, to unlock the mystery we needed a key that seemed to be hidden in complex calculations.

Although it is complicated, we cannot give up. In the wonderful world of electrons, the interaction between electrons not only determines the basic properties of materials, but also gives rise to amazing physical phenomena such as high-temperature superconductivity and quantum phase transition. In order to explore these phenomena, we introduced the "Fermion-Hubbard model".

In 1963, physicist John Hubbard proposed a model to describe the behavior of electrons in a lattice, where "lattice" refers to the spatial structure in which atoms are arranged according to a certain rule. This model is not complicated. It simply simplifies the behavior of electrons into two types: one is the jumping of electrons between adjacent lattices, and the other is the repulsion between electrons in the same lattice point. Although the model is simple, it can explain many complex phenomena, including our understanding of high-temperature superconductivity.

Schematic diagram of the "Fermion-Hubbard model" in two dimensions

(Image source: Wikipedia)

However, solving this model accurately is like crossing a vast forest without a map. As the number of lattice points increases, the difficulty of calculation increases dramatically, and even the most powerful supercomputers find it difficult to cope.

But this does not mean that we are helpless. More than 40 years ago, physicist Richard Feynman proposed a bold idea: Why not use quantum systems to simulate these complex quantum phenomena directly? In this way, we can bypass those tricky numerical calculations and directly explore the mysteries of the quantum world.

Building an ultracold atom quantum simulator: defeating magic with magic!

As scientists explored the behavior of electrons, they encountered limitations of traditional computational methods. To overcome these challenges, they adopted Richard Feynman's forward-looking advice and began to build a new tool: a quantum simulator. This quantum simulator can accurately simulate the behavior of electrons in a lattice, helping us to gain a deeper understanding of the "Fermion-Hubbard model."

Among many artificial quantum systems, ultracold atoms in three-dimensional optical lattices are ideal for simulation due to their purity and controllability. Imagine that a "three-dimensional optical lattice" is like a perfect spatial grid woven with light, with each point being a precisely controlled node. "Ultracold atoms" are atoms that are almost still in these optical lattices through laser cooling technology.

Schematic diagram of the arrangement of atoms loaded into the optical lattice.

(Image source: Reference 1)

The process of building a quantum simulator can be simplified into three steps:

1) Use three orthogonal laser beams to create a uniformly distributed three-dimensional optical lattice, just like drawing perfect small boxes in space to provide a "stage" for electrons.

2) Cool the atoms to near absolute zero and cleverly arrange them in an optical lattice, ready to start their "performance".

3) Observe the "wonderful physical phenomenon" on stage - the antiferromagnetic phase transition, to verify the accuracy of the model.

The "antiferromagnetic phase transition" may sound complicated, but its essence is: at low temperatures, the electron spins inside the material tend to point in opposite directions, forming a stable state; but when the temperature rises, this ordered arrangement will be broken and the magnetism will change accordingly.

Although the "Fermion-Hubbard model" has been proposed for many years, it is still a big challenge to directly observe the antiferromagnetic phase transition in experiments. This requires lowering the temperature of the quantum simulation system to an extremely low level to ensure the accuracy of the simulation.

If the temperature of the quantum simulation system can be lowered below a certain temperature, scientists will be able to simulate the process of antiferromagnetic spin fluctuations, which not only verifies the theory of high-temperature superconductivity mechanism, but is also a key step in understanding this phenomenon.

A major breakthrough in quantum simulation

Exploring the mysteries of magnetism is like climbing Mount Everest, with every step full of unknowns and challenges. Even a concept that sounds very professional, such as "antiferromagnetic phase transition," has never been verified in experiments. Not to mention, simulating this phase transition in a doped state is an almost impossible task for traditional supercomputers.

However, scientists including Pan Jianwei, Chen Yuao, Yao Xingcan, and Deng Youjin from the University of Science and Technology of China bravely met this challenge. They cleverly combined advanced technologies to build a quantum simulator of ultracold atoms.

The quantum simulator they built has jumped from dozens of lattice points to an astonishing 800,000 lattice points, which is a qualitative leap. Usually, the optical lattice system constructed in the experiment always has the problem of uneven potential wells, just like building a house on uneven ground. But the "box-type optical potential well" technology is like a "plastic surgery" for the optical lattice system, making each potential well regular and providing a perfect stage for electrons.

Ultracold atoms are loaded into a three-dimensional optical lattice like eggs

(Photo source: veer photo gallery)

The "flat-top optical lattice" technology further optimizes the experimental process. By fine-tuning the laser, it makes the central area of ​​the optical lattice more uniform, just like laying a flat carpet on an uneven ground, providing a space for atoms to be evenly distributed.

On this basis, the research team further reduced the intensity noise in the potential well and optimized the loading process of ultracold atoms. This is like providing a warm and quiet home for the atoms on a cold winter day, ensuring that they can conduct experiments in the best condition.

Finally, scientists successfully built an ideal ultracold atom quantum simulator. In this simulator, scientists can precisely control every parameter and finally observe the long-awaited phenomenon - antiferromagnetic phase transition. This is not only a verification of the theory, but also an important breakthrough in the exploration of the physical mechanism of high-temperature superconductivity.

This research not only successfully verified the antiferromagnetic phase transition of the "Fermion-Hubbard model" (including the situation in the doped state) for the first time in the world, but also provided a solid experimental foundation for our understanding of the physical mechanism of high-temperature superconductivity.

This research is like lighting a beacon on the long journey of science, illuminating our way forward and bringing new hope and direction to our exploration of a deeper understanding of nature.

Taking a big step into the era of quantum 2.0

This groundbreaking research result is not only a brilliant victory for the scientific community, but also marks an important step forward in the field of quantum computing. It proves to the world that quantum systems are not just theoretical wonders, they have demonstrated the ability to surpass traditional computers and solve complex problems in the real world.

It provides scientists with an ideal research platform, allowing them to explore the difficult problems in condensed matter physics more deeply. It is like opening a door to an unknown world, allowing us to glimpse the mystery of the strong correlation interaction between electrons.

Schematic diagram of the Tianyuan ultracold atom quantum simulator

(Photo source: China University of Science and Technology News)

As the performance of ultracold atom quantum simulators continues to improve, we have reason to believe that in the future it will not only become a tool for verifying antiferromagnetic phase transitions, but also a powerful tool for exploring various exotic physical phenomena. It will help us uncover the mystery of physical phenomena such as high-temperature superconductivity and quantum phase transitions, and give us a deeper understanding of the interactions between electrons.

The success of this research is not only a verification of existing scientific theories, but also a bold prediction of future scientific exploration. It indicates that quantum computing will play a more important role in the future, becoming a key force in solving scientific problems and promoting technological progress, and allowing us to feel the infinite possibilities of the quantum world.

References

[1] Shao HJ, Wang YX, Zhu DZ, et al. Antiferromagnetic phase transition in a 3D fermionic Hubbard model[J]. Nature, 2024: 1-6.

[2] Gaunt AL, Schmidutz TF, Gotlibovych I, et al. Bose-Einstein condensation of atoms in a uniform potential[J]. Physical review letters, 2013, 110(20): 200406.