According to Du Lingjie, a professor at the School of Physics at Nanjing University, quantum physics research is interesting and pure, and he devotes himself to it and constantly explores the mysteries of science. In March this year, a new study by Du Lingjie's team was published in the journal Nature, which attracted attention. Bravely exploring the frontiers of scientific research, building experimental equipment from scratch, and constantly breaking through the mindset in research, Du Lingjie said that it was his strong interest that led him to persevere and keep walking.
▲The picture shows Du Lingjie (middle) discussing with students. Li Jiahao/photo
Recently, Du Lingjie, a professor at the School of Physics at Nanjing University, who was born in 1986, became "popular".
On March 28, the journal Nature published a new study by Du Lingjie's team. They measured the collective excitation of the fractional quantum Hall effect in gallium arsenide quantum wells using polarized light scattering techniques under extreme conditions. This is the first time in the world that quasiparticles with graviton characteristics have been found in real systems since the concept of gravitons was proposed, and they are called fractional quantum Hall effect gravitons by theorists in the field.
This achievement has attracted great attention from the academic community and the media. Du Lingjie said that it was his interest that led him forward, immersed in the world of pure quantum physics, and wanted to see what changes it would bring to the world.
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Part 1
Interest leads
Challenge the frontiers of scientific research
Du Lingzhuo was born in Zhenjiang, Jiangsu, since he was a child, he likes to dig into natural phenomena, went to high school, he found that the questions he was curious about belonged to the category of physics, so he chose science, and entered the intensive department of science of Nanjing University as an undergraduate.
When he was in graduate school, Du Lingjie chose the direction of quantum physics. This choice seemed somewhat surprising at the time, as quantum physics was still a "somewhat seemingly nonsensical" field – and it was unclear what the "usefulness" of studying it would be. And Du Lingjie took a fancy to its "interesting and pure".
After graduating with a master's degree, Du Lingjie chose semiconductors as a carrier for further research in quantum physics during his doctoral studies. In order to expand his research horizons, in the postdoctoral stage, he began to try new optical experimental research, and began to get in touch with the fractional quantum Hall effect.
In the fractional quantum Hall effect, there is a completely new state of matter in the microscopic world that has never been discovered before, and can only be observed under extreme conditions. Its emergence has opened a window for human beings to understand the world, but because of the strict observation conditions, the research is extremely difficult, and there are relatively few researchers.
In 2019, Du Lingjie and his collaborators discovered for the first time in optical observations that the fractional quantum Hall effect occurs in collective excitation, that is, the energy transition of a large number of electrons collectively – like the sudden innumerable ripples of different shapes on the surface of a calm lake. After the paper was published, it was considered by theoretical physicists to be evidence of the existence of fractional quantum Hall effect gravitons.
In fact, many years ago, it was theoretically predicted that there may be a fractional quantum Hall effect graviton in condensed matter. Because its behavior is similar to that of gravitons, it is figuratively called the "projection" of gravitons in condensed matter. But the experiment of finding this "projection" and figuring out the mechanism behind it is very difficult.
In 2019, Du Lingjie finished his postdoctoral work and returned to the School of Physics of Nanjing University to engage in scientific research. He decided to challenge this frontier of scientific research.
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Part 2
It lasted 3 years
Building an observation telescope from scratch
The premise of the experiment is to have appropriate instruments and equipment. However, at that time, there was no laboratory equipment in the world that met the requirements. Because the conditions of the experiment were extremely harsh and seemed to be contradictory. The necessary conditions for the experiment are a strong magnetic field, 100,000 times stronger than the Earth's conventional magnetic field, and an extremely low temperature, 0.05 degrees Celsius above the physical sense of absolute zero (minus 273.15 degrees Celsius).
Although strong magnetic fields and extremely low temperatures can be achieved by the device, optical measurements are required during the experiment, and "windows" are installed on the equipment. With light, the temperature of the sample rises, disrupting the sensitive fractional quantum Hall effect and causing the observation to fail.
The foreign team used wet dilution refrigeration and poured liquid helium into the experimental device every day to achieve cooling. However, we use dry dilution refrigeration, i.e. compressor refrigeration. Du Lingjie explained that the process of compressing the gas causes vibrations, which makes the experiment even more difficult. In addition, the humid environment in the south is also a major challenge for optical experiments.
How to eliminate the effects of temperature rise, vibration, humidity changes, etc.? After trying again and again, Du Lingjie found that the road of "subtraction" did not work, so he and his students could only "do addition": he and his students designed from scratch, using new special materials to absorb shocks, and reducing thermal radiation through optical interference, so as to overcome the problem little by little.
In order to make the indicators meet the experimental requirements, from August 2019 to August 2022, which lasted 3 years, Du Lingjie finally led the team to design and assemble the experimental device from scratch and placed it in an "ultra-clean room" with constant temperature and humidity. This large device is about 8 meters long, 5 meters wide, and 9 meters high, and has a light source and detector, like a super-large "microscope" and a super "telescope".
In the second half of 2022, the "telescope" began to be put into operation. For the next four months, Du's team worked day and night in quantum wells to find evidence of graviton excitation, but found nothing.
Du Lingjie was not discouraged. He locked himself in his room, compared a large amount of data back and forth, and finally found a faint signal of graviton excitation in the massive data. They then measured the signal with a spin of 2 in the fractional quantum Hall effect, further confirming that it was graviton-excitation.
Du Lingjie was very happy, he thought that after a long journey, he finally ushered in success. However, reality gave him a slap in the face. The paper, which had been written for many years, was rejected by the journal Nature on the grounds that there was "insufficient evidence".
The road ahead is confused again.
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Part 3
Think outside the box
Usher in scientific breakthroughs
has not yet come out of the shadow of the rejection of the submission, Du Lingjie has been hit again.
At an international conference held in July last year, an expert asked Du Lingjie the question: "The graviton excitation spin is 2, but is the spin 2 necessarily the graviton excitation?"
This stopped Du Lingjie. "Previously, I had been stuck in a stereotype, and like my peers, I was shy away from the 'hard bones' in the field. Now I have to tackle it head-on, and I realize that there is still a long way to go. Du Lingjie said.
Back in the lab, he pulled himself back together. "It is important to publish articles, but for scientists, it is more important to understand the scientific problems. We challenge the frontiers of scientific research, and even if we fail, at least one solution can be ruled out, which can be regarded as a success. He encouraged himself.
This time, Du Lingjie paid attention to the data that he had not paid attention to before, and realized that the excitation measurement of extremely small momentum was the key to solving the problem, so he designed a new experiment, "After half a year of measurement, we found that in addition to the most significant feature of spin 2, the excitation signal also has a key characteristic - characteristic energy. ”
In January this year, Du Lingjie was invited to participate in an international conference on the field of fractional quantum Hall effect. This time, he came up with strong experimental evidence to answer the questions that were questioned last year. Scholars, including the previous expert, applauded the young scientist. The experimental discovery of fractional quantum Hall effect gravitons has been recognized by the international academic community. In March, the paper of Du Lingjie's team was published in the journal Nature, which is also the world's first observation of the novel quasiparticle of graviton in condensed matter.
"There is no standard answer in life, and doing scientific research is a process of constantly breaking the mindset. This is Du Lingjie's personal experience and his attitude towards life. "My interest in the purely physical world has led me to explore new fields, research new topics, and explore the mysteries of natural science." ”
Published in People's Daily on April 22, 2024, page 11
People's Daily reporter Yao Xueqing