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As a former member of the Junior Class of USTC, Professor Lu Dawei of Southern University of Science and Technology is probably a "hero from a young age" in the eyes of his peers.

Although when looking back on his undergraduate experience, he admitted: "It's really easy to lose control of yourself in college in the morning."

However, he still obtained a bachelor's degree and a doctorate degree from USTC, and after completing his postdoctoral research at the University of Waterloo in Canada, he returned to China and joined the Southern University of Science and Technology as a scientist.

At the same time, as a teacher, he also cultivated a young talent like himself - Huang Keyi.

Not long ago, a paper with Lu Dawei as the corresponding author and Huang Keyi as the first author was published in the top physics journal Physical Review Letters. When Huang Keyi completed this paper, he was just an undergraduate student.

So, what does this paper talk about? According to the introduction,By using the nuclear magnetic resonance quantum system, they verified a "self-sufficient" quantum refrigeration (refrigerator) principle.

Figure | From left to right: Huang Keyi, the first author of the paper, and Lu Dawei, the corresponding author of the paper (Source: Lu Dawei)

By designing and regulating a specific form of "three-body" interaction between three atoms, the research team built this "self-sufficient" quantum refrigerator.

No additional energy is required during the entire refrigeration cycle to cool down one of the atoms (similar to how a classic refrigerator does not require electricity).

This is not only a phenomenon unique to the microscopic quantum world, but also does not violate the classical laws of thermodynamics.

Regarding the application prospects, Lu Dawei said: "A practical quantum refrigerator cannot be built for the time being, and I think the difficulty of realizing such a device is no less than building a general-purpose quantum computer.But as the reviewer said, it is still very promising to use this method to cool down quantum bits in quantum computers."

(Source: Physical Review Letters)

Let’s start with the “monsters” in physics

It is reported that classical thermodynamics studies a macroscopic system, describes the average behavior of a large number of particles, and follows the laws of Newtonian mechanics and statistical mechanics.

People have thus defined a series of macroscopic quantities such as temperature, internal energy, and entropy, and invented equipment such as heat engines and refrigerators through heat transfer and work analysis.

Quantum thermodynamics studies microscopic quantum systems, especially systems with only a few particles.

It relies on the basic principles of quantum mechanics, involving superposition, entanglement and measurement of quantum states, etc. This makes quantum thermodynamics fundamentally different from classical thermodynamics, and also means that almost all classical thermodynamic quantities need to be redefined.

For example, in the classic cylinder-piston model, work can be defined by the movement of the piston. The piston compressing the gas is "positive work", while the gas pushing the piston apart is "negative work".

In the quantum world, there are only a few quantum states of particles and the Schrödinger equation that governs their evolution. At this time, work is defined as the change in the energy of the system when the quantum state remains unchanged. An increase in energy is "positive work", and a decrease in energy is "negative work".

The difference between quantum and classical work lies not only in the definition. There is also a close connection between work and information in the quantum world, which is reflected in the famous "Maxwell's demon" paradox.

In 1867, British scientist James Clerk Maxwell proposed an experiment to explore and challenge the second law of thermodynamics, which states that heat always flows spontaneously from a hotter object to a cooler one.

Maxwell imagined a tiny demon guarding a doorway separating two gas chambers that were initially at the same temperature, or in thermal equilibrium.

The Maxwell demon can observe the speed of each gas molecule and control the opening and closing of the gate. When the demon sees a fast molecule moving from the left chamber to the right chamber, it opens the gate and lets the molecule pass.

When the demon sees a slow molecule moving from the right chamber to the left chamber, it also opens the gate and lets the molecule pass.

By doing this, the "demon" can concentrate fast molecules in the right ventricle and slow molecules in the left ventricle, thereby raising the temperature of the right ventricle and lowering the temperature of the left ventricle.

This seems to violate the second law of thermodynamics, because the two gas chambers that were originally in thermal equilibrium actually achieved separation between the high-temperature area and the low-temperature area under the guidance of the "monster".

For this reason, the "Maxwell's demon" experiment has triggered extensive discussion and research in the academic community, and expanded the understanding of the relationship between thermodynamics and information theory.

Modern physicists believe that Maxwell's demon does not violate the second law of thermodynamics because the demon needs to obtain information when observing molecules and deciding to open and close the door.

Processing this information itself requires work, especially when the "demon" erases information, the entropy will increase, which can make up for the decrease in system entropy.

Based on this, the "Maxwell's demon" experiment promoted the application of information theory in thermodynamics, revealed the profound connection between information processing and physical processes, and laid the foundation for the development of quantum thermodynamics.

In quantum thermodynamics, the thermodynamic processes of quantum systems mainly involve the regulation and measurement of quantum states, that is, the processing of "quantum information".

By defining the entropy of quantum states, the academic community has gradually established a connection between quantum information and thermodynamics, which is similar to the changes in information and entropy recorded by the "Maxwell demon".

At the same time, people have discovered that quantum information theory provides a series of tools and techniques. For example, for new quantum resources such as quantum coherence, quantum entanglement, and quantum measurement, they can all provide support for the study of quantum thermodynamics.

For example, the efficiency of heat engines or refrigerators can be improved through resources such as quantum entanglement.

Experimental research on quantum thermodynamics has been put on the agenda

Although quantum information can bring new possibilities to the development of thermodynamics, there are still many challenges in exploring and verifying the theory of quantum thermodynamics in actual quantum systems.

Fortunately, with the continuous investment of academia in quantum information technology over the past twenty years, people have become more and more proficient in controlling quantum systems, and have achieved great experimental breakthroughs in quantum computing, quantum communication, quantum precision measurement and other fields.

Then, it is natural that the experimental research on quantum thermodynamics, which also takes the processing of quantum information as its core technology, has reached the time to be put on the agenda.

A hot research topic in quantum thermodynamics is quantum heat engine/refrigerator.

Taking a quantum refrigerator as an example, its basic function is the same as that of a classical refrigerator, which is to absorb heat from a colder object and then release the heat into a warmer environment to achieve cooling.

However, unlike a classical refrigerator, a quantum refrigerator uses quantum information processing to achieve this refrigeration process, and various rich quantum resources can provide support for this process.

Therefore, if we can precisely control the quantum system, prepare these quantum resources and utilize them effectively, we can experimentally realize a quantum refrigerator.

Nuclear magnetic resonance quantum system is one of the main research directions of Lu Dawei's team, which is to study the resonance phenomenon between atomic nuclei and magnetic fields.

It is reported that the spin information carried by the atomic nucleus can be both regulated and read out. In fact, this is an ancient yet novel technology.

It is said to be old because its development has gone through nearly a century. Isidor Isaac Rabi, the supporting actor in the movie "Oppenheimer", won the 1944 Nobel Prize in Physics for discovering the nuclear magnetic resonance phenomenon.

Today, modern nuclear magnetic resonance technology has become an indispensable "tool" for medical examinations, which determines the health condition of patients by detecting hydrogen atoms in human water molecules.

It is novel because nuclear magnetic resonance, as a function of quantum systems, has only been developed for more than 20 years.

In fact, in the field of quantum information, nuclear magnetic resonance is a pioneering experimental platform.

By encoding quantum information into the spin of the atomic nucleus and then controlling and reading it out through a magnetic field, it is possible to complete various quantum information tasks and reveal many wonderful quantum phenomena.

Since he started his doctoral studies in 2007, Lu Dawei has been working in this field for nearly twenty years.

In 2012, Lu Dawei came to Canada as a postdoctoral fellow, where he read a theoretical article on a “self-sufficient” quantum refrigerator[1].

At that time, he thought the idea of ​​three-body interaction was very clever. However, since he mainly studied nuclear magnetic resonance quantum computing during his postdoctoral period, he focused his main energy on promoting the number of quantum bits and control accuracy.

In 2017, Lu Dawei joined the Southern University of Science and Technology to set up an independent group. With the development of quantum systems such as superconducting circuits and ion traps, the disadvantages of using nuclear magnetic resonance systems to realize quantum computers have been continuously magnified.

In 2020, Lu Dawei read a review paper on quantum thermodynamics by chance. After reading it, he suddenly realized that nuclear magnetic resonance is very suitable for studying quantum thermodynamics.

After all, this system studies thermodynamics at the real atomic and molecular scale, and has many unique advantages such as room temperature and ensemble.

“It gives me the feeling of a novel called The Three-Body Problem”

At that time, the graduate students in the group all had other tasks, so Lu Dawei brought in two undergraduates in the group: Zhu Xuanran, a junior, and Huang Keyi, a sophomore.

Later, they used nuclear magnetic resonance to realize a quantum refrigerator with "indefinite causal order".

"Indefinite causal order" is a wonderful quantum resource that allows the order in which two events A and B occur to coexist.

That is, the classical world only allows A then B, or B first then A, but the quantum world allows these two orders to exist "simultaneously".

On a MRI machine,The team used the four carbon atoms in the crotonic acid molecule to realize this "indefinite causal order" and used it as a quantum resource to drive the quantum refrigeration process, realizing a quantum refrigerator.

In 2022, the related paper was published in Physical Review Letters[2].

Lu Dawei said: "Zhu Xuanran, who was the co-first author of this paper, had already graduated, so it didn't help him apply for a scholarship. But gold will always shine. He later received a scholarship from the Hong Kong government and is currently studying for a doctorate at the Hong Kong University of Science and Technology."

At the same time, this paper also caused quite a stir in 2022. Dr. Philip Ball, a famous science book writer and winner of the Royal Society Science Book Award, specifically introduced the experiments of Lu Dawei's research group in his Nature Materials column [3].

In 2024, the team studied another quantum resource, the three-body interaction, which sounds a bit sci-fi and gives people the feeling of the novel "Three Body".

In fact, whether it is the electrostatic (Coulomb) force or the gravitational force that everyone is familiar with, they are all interactions between two objects. Even in the quantum world, natural three-body interactions still do not exist.

(Source: Physical Review Letters)

However, once this three-body interaction is constructed, many interesting phenomena will arise.

For example, more than a decade ago, researchers at the University of Bristol in the UK envisioned achieving "self-sufficient" quantum refrigeration through three-body action.

The experiment conducted by Lu Dawei's team in 2024 just verified the above idea.

That is, by using the three carbon atoms in crotonic acid and combining them with various control methods developed for the nuclear magnetic resonance quantum system, they successfully modulated the three-body interaction required for the experimental plan.

In the study, they measured the changes in work and heat throughout the process and found that there was indeed no net energy input into the quantum system.

In addition, they tracked the temperature changes of the target atoms and found that the temperature of the target atoms had been decreasing spontaneously as the cooling process progressed.

"Although the academic community has predicted this in theory a long time ago, it was still quite a sense of accomplishment when I actually observed this phenomenon in the experiment," said Lu Dawei.

Recently, a related paper titled “Experimental Realization of Self-Contained Quantum Refrigeration” was published in Physical Review Letters[4].

Huang Keyi, an undergraduate student at Southern University of Science and Technology, is the first author, and Professor Lu Dawei and Assistant Professor Nie Xinfang of Southern University of Science and Technology serve as co-corresponding authors.

Figure | Related papers (Source: Physical Review Letters)

Deliberately "exposing flaws" when submitting the paper, but the reviewer kindly pointed them out

Lu Dawei said: "We are indeed very lucky that both reviewers from PRL gave very positive comments. In fact, due to the niche nature of quantum thermodynamics research, especially this experiment, I was prepared to engage in a long tug-of-war with the reviewers, and even deliberately "sold a flaw" in the text."

As a result, one of the reviewers not only discovered this flaw, but also took the initiative to help them come up with a solution to this "flaw".

"When I saw the review comments, I was not only very happy, but also a little bit amused and helpless. It felt like when I was a child and did something wrong and tried to hide it, but my mother still pointed it out to me in good faith," said Lu Dawei.

In addition, the reviewers believed that this experimental study on quantum thermodynamics was both important and novel. Moreover, Lu Dawei was even more impressed by the fact that the reviewers had even thought about the application prospects of the results.

The reviewers believe that the quantum refrigerator is currently only a principle verification and will not produce any substantial applications in the short term.

However, quantum refrigeration technology can indeed further cool quantum bits and suppress the error rate during quantum computing.

Following the reviewer's ideas, Lu Dawei et al. proposed a theoretical framework for using three-body interaction refrigeration for quantum computing initialization.

"The feeling is like that of a graduate student who receives earnest care and guidance from his supervisor. I am really grateful to the reviewers of PRL. They are of high academic level and strong insight, and have made us more deeply aware of the importance of scientific discussion and cooperation," said Lu Dawei.

(Source: Physical Review Letters)

As mentioned earlier, Huang Keyi is the first author of this paper in 2024. As a mentor, Lu Dawei praised this undergraduate student highly.

Lu Dawei said that at the beginning of this research, he spent 50,000 yuan to buy a piece of old equipment that was more than 20 years old.

At that time, he told his students that he could leave this device for everyone to practice with. Then, he also shared with Huang Keyi the theoretical paper on the "self-sufficient" quantum refrigerator that he had kept in his closet.

Lu Dawei told Huang Keyi: "Since you are learning to do experiments, don't just study. No matter what new things you come up with, at least it is an achievement. Even a small amount of money counts."

Unexpectedly, Huang Keyi's execution ability was off the charts. He quickly sorted out all the details and learned about instruments while doing experiments. "Then he got his first PRL paper as the first author in his second year of graduate school," said Lu Dawei.

However, there are still relatively few experimental studies on quantum thermodynamics in the field. After the paper was published, they were mostly contacted by colleagues in theoretical physics, who also expressed their willingness to cooperate in nuclear magnetic resonance quantum thermodynamics.

"Right now, we are conducting experimental collaborations with three theoretical groups, covering many broad areas of thermodynamics, such as quantum thermodynamics, information theory, and resource theory. Recently, a paper co-authored with the team from City University of Hong Kong was just accepted by PRL," Lu Dawei concluded.

References:

1.PRL 105, 130401 (2010)

2.PRL 129, 100603 (2022)

3.Nat. Mat. 21,1099 (2022)

4.Huang, K., Xi, C., Long, X., Liu, H., Fan, Y. A., Wang, X., ... & Lu, D. (2024). Experimental Realization of Self-Contained Quantum Refrigeration.Physical Review Letters, 132(21), 210403.

Operation/Layout: He Chenlong

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