Text | China Science News, trainee reporter Jiang Qingling, reporter Li Chenyang
In November 1957, Chairman Mao Zedong visited the Soviet Union and met with Chinese students at Moscow University. In the midst of the surging crowds, there was a 25-year-old young man from Hebei who couldn't squeeze into the front row, so he had an idea and climbed the high windowsill of the auditorium to get a better view for himself.
This young man's name is Wang Yuzhu. Although he could see the beloved Chairman Mao at the moment, he couldn't understand the chairman's Hunan dialect. This made him anxious, and he hurriedly asked his classmates to help "translate".
When he finally understood these words, he felt a deep shock: "The world is yours and ours, but in the end it is yours. You young people are full of vigor and are in their prime, like the sun at eight or nine o'clock in the morning. Hope is in you. ”
The ardent expectations contained in the words illuminated the deepest part of Wang Yuzhu's heart. He eagerly awaited that he could contribute to the construction of the motherland.
Time flies, and today's Wang Yuzhu is already a 92-year-old academician of the Chinese Academy of Sciences. Of course, he is no longer the sun at eight or nine o'clock in the morning, but he has dedicated the light of his life to the motherland and the party.
As one of the pioneers of China's atomic clock, he was responsible for the successful development of the mainland's first rubidium atomic clock, which has been used in a number of important national defense tasks, among which the rubidium atomic clock installed on the "Yuanwang" survey ship has ensured the success of all satellites. The space cold atomic clock developed by the team led by him was launched with Tiangong-2, becoming the most accurate "timing needle" in the nine skies.
Wang Yuzhu Courtesy of Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences
"Xiaosakubo" Zode Atomic 钟
Wang Yuzhu's academic achievements in his life can be called heavy, and if it can be summed up in just one sentence, it is to make a better atomic clock.
Atomic clocks, as the name suggests, are timekeepers that use electromagnetic waves that are absorbed or emitted by atoms as they transition between different energy states. Because the electromagnetic wave cycle is very stable, the atomic clock has become the most accurate timekeeping tool in the world, and plays an extremely key role in national defense and military, positioning and navigation, aerospace and other fields.
In 1955, Louis Essen and Jack Parry built the first reliable cesium atomic clock at the UK's National Physical Laboratory to calibrate the frequency of a quartz oscillator. Six years later, Wang Yuzhu returned to China from the Soviet Union and joined the Beijing Institute of Electronics, Chinese Academy of Sciences. Building on his doctoral work, he built a laboratory from scratch and successfully developed a sodium atomic clock laboratory device in 1964.
In 1963, the Chinese Academy of Sciences decided to form a new research institute with some research laboratories of the Beijing Institute of Electronics and the Changchun Institute of Optics and Mechanics - the Shanghai Branch of the Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences (now the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences). Wang Yuzhu and his lab members moved to Shanghai en masse, where they have since taken root.
Soon after, Wang Yuzhu learned that a foreign country had successfully developed a rubidium atomic clock with superior performance, but the technology was blocked from China. He was very aware of the important position of rubidium atomic clocks in navigation and positioning technology, so he began to independently develop rubidium atomic clocks in 1965.
It was a long and difficult journey. At that time, there were no rubidium atom isotopes that could be used to make atomic clocks in the mainland, and Wang Yuzhu's team finally used the initial device to isolate the isotopes after several setbacks. Due to the influence of the historical background at that time, Wang Yuzhu's research work was interrupted several times, and it took four years before and after to develop the key components of the rubidium atomic clock and the required testing equipment.
In September 1969, Premier Zhou Enlai put forward the decision to abandon the "foreign crutch" and establish China's "atomic time", and the development of rubidium atomic clocks gradually got on the right track.
In 1970, the Chinese Academy of Sciences put forward the policy of "opening the door to the office". 38-year-old Wang Yuzhu led the frequency standard group, went to the factory to receive "re-education" of the working class, and settled in Shanghai Guorong Lighting Factory. This is a small factory for manufacturing enameled wire, and the working environment and conditions are extremely poor, they set up an atomic clock experimental platform in the small attic of the factory kitchen, and began the development work with the workers.
Using the three key technologies proposed by Wang Yuzhu, the team has developed three generations of rubidium atomic clock prototypes after 7 years of hard work, "the performance is stronger from generation to generation, reaching the international level at that time".
The rubidium atomic clock developed by them has participated in and completed a number of national defense tasks, including ultra-long-wave navigation, submarine navigation, time synchronization between base stations, long-range missile and communication satellite launches, and the national atomic time system.
Wang Yuzhu still remembers that in 1982, the mainland issued a notice to the whole world to launch rockets into designated sea areas. When he learned that the "Yuanwang" survey ship was about to go to sea, he was very nervous: "When it will be launched, where it will be launched, and the time-related data to be measured all require good time synchronization. It was not until the good news of the "successful launch" came that Wang Yuzhu was relieved and "burst into tears with excitement," feeling that he had made a truly valuable contribution to the motherland's national defense.
"Mr. Wang's heart has always followed this important national mission, and every time the 'Yuanwang' goes to sea, he always remembers whether the atomic clock is working properly. Xu Zhen, an associate researcher at the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, said, "Until the retirement of the 'Yuanwang', the atomic clock has been serving the country well, and Mr. Wang is very pleased with this." ”
Young Wang Yuzhu.
"Passing by the Nobel Prize," he said
In 1997, Steven Chu, William Phillips, and Cohen Tanhoudji were awarded the Nobel Prize in Physics for their work on laser-cooled gas atoms.
At this time, people suddenly realized that "the physical idea of laser cooling gas atoms proposed by Wang Yuzhu is consistent with the mechanism used now", but due to various reasons, "Wang Yuzhu is only one step away from the Nobel Prize".
To make it clear how to "pass by the Nobel Prize", we have to turn back the clock to 20 years ago.
In 1977, after completing the rubidium atomic clock mission, 45-year-old Wang Yuzhu returned to the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences to find the next research direction. It was then that he became aware of a paper published in 1975 by renowned physicists Arthur Schawlow and Theodor Hansch. In this paper, we propose a Doppler cooling mechanism for laser-cooled atoms, and calculate that sodium atoms can be cooled to a minimum of 240 μK, which is also known as the "Doppler limit".
The accuracy of an atomic clock is limited by the thermal velocity of the atoms, and if the temperature of the atoms is lowered, the accuracy of the atomic clock will be greatly improved. Wang Yuzhu immediately realized that cold atoms would have a revolutionary impact on the study of atomic clocks, and decided to devote himself to the study of laser cooling gas atoms.
In 1979, after understanding the correlation between laser-cooled gas atoms and Doppler shifts, Wang Yuzhu put forward several suggestions on the atomic mechanism of laser-cooled gases related to the Doppler effect, among which "integrating sphere red-shifted diffuse reflection laser cooling gas atoms" is similar to Phillips's work in 1983. However, due to limited conditions, Wang Yuzhu did not complete the experiment of laser cooling atomic beams with integrating spheres until 1993. After another 15 years, the team finally achieved direct cooling of atoms from the 87Rb background gas, obtaining 109 orders of cold atomic gas with a minimum gas temperature of 25μK.
It was also in 1979 that Wang Yuzhu began to think about the physical mechanism of laser cooling of gas atoms from a new perspective. "Since the Doppler frequency shift can be used to cool the gas atoms by the laser, why can't the optical frequency shift be used to cool the gas atoms by the laser?" After in-depth thinking, Wang Yuzhu proposed a new cooling mechanism different from the Doppler cooling mechanism-using the AC Starck effect (optical frequency shift effect) laser to cool the gas atoms. This is in turn consistent with the Sisyphean cooling mechanism below the Doppler cooling limit proposed by Steven Chu and Tanugi in 1989.
This is the first time in the world that the mechanism of applying the optical frequency shift effect to laser cooling gas atoms has been proposed. When Xiao Luo visited Wang's laboratory in 1979, he agreed with his new cooling mechanism: "The idea is new and reasonable, the expression is direct and clear, and I suggest you publish it." With his encouragement, Wang Yuzhu has expounded relevant physical ideas in two domestic journals, "Science Bulletin" and "China Laser".
However, in 1980, at the international laser conference organized by the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, a foreign scientist did not agree with this cooling mechanism. Wang Yuzhu recalled the scene at that time: "He suppressed and criticized you on the spot, saying that this is not good, it is really domineering." ”
Wang Yuzhu proposed a new idea of laser cooling gas atoms 5 to 10 years earlier than abroad, but limited to the experimental conditions at that time, it was not experimentally realized. In 1998, when Steven Chu visited the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences and visited Wang Yuzhu's laboratory, he asked him why he had not carried out the experiment earlier, and Wang Yuzhu reluctantly explained: "I have no conditions. ”
Looking back on these past events, Wang Yuzhu has never been saddened by missing the Nobel Prize. What he was more concerned about was the difficulty of carrying out basic research work in the context of the times, and the contempt that Chinese were treated in the international academic community at that time.
In the face of all kinds of lamentations in the media, Wang Yuzhu just said lightly: "There is no point in talking about the Nobel Prize, but recalling the journey we have traveled and recording the real situation will make us pay more attention to systematic basic scientific research and cherish our current research work more." ”
Wang Yuzhu instructs students in the laboratory.
Holding up the "sun at eight or nine o'clock"
Wang Yuzhu is not only a pioneering scholar, but also a teacher of peach and plum garden. The field of cold atom research, which was cold and lonely at the beginning, is now full of talents. At academic conferences, which often involve hundreds of people, the vast majority of participants are either from Wang Yuzhu's disciples or have studied in his laboratory.
"Mr. Wang is a very peaceful teacher, and he values the interest of young people in research. Wei Rong, a researcher at the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, said. He was not initially assigned to Wang Yuzhu's research group, but he was very interested in the research direction of the group. A good friend of his happened to be studying under Wang Yuzhu and expressed his willingness to exchange research groups with him, and also told Wang Yuzhu about it. It didn't take long for Wei Rong to be invited by Wang Yuzhu to visit the laboratory. "I was excited, but I was also nervous. Later, Mr. Wang called my mentor at the time and arranged for the two of us to change the group. Wei Rong said, "Mr. Wang is like this, he thinks you are really interested in this direction, so he is very willing to recruit you in." ”
Xu Zhen had a similar experience. He was unusually preparing for his Ph.D. exam, and he thought he was going to say goodbye to his academic career. Unexpectedly, Wang Yuzhu took the initiative to ask him where he failed in the exam and what he wanted to do in the future. Seeing that Xu Zhen really liked scientific research, Wang Yuzhu invited him to work in the research group, and later helped him to study for a doctorate on the job.
According to Lu Desheng, a researcher at the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, the essence of Wang Yuzhu's education is that in addition to his love for scientific research, everything must be done from the national interest.
In March 2002, with a small amount of funds, Wang Yuzhu's laboratory realized China's first Bose-Einstein condensate (BEC), a rarefied gas with rubidium atoms, and gave birth to a new state of matter, making China the 11th country in the world and the second in Asia to have a new state of matter. When the five-star red flag fluttered on the international BEC website, Wang Yuzhu, who never cared about personal fame and fortune, was very excited at that moment.
When it comes to property, Wang Yuzhu is both generous and "picky". In the face of various disasters and epidemics, he is often the person who makes the most donations in the firm. But even if it is a small object in the laboratory for a few cents, he cherishes it very much, and does not allow students to throw it away at will. In his words, no one has the right to waste a penny of the state's money!
Under Wang Yuzhu's subtle influence, his students, like him, are concerned about "national affairs" and shoulder "national responsibilities", and constantly promote the development of atomic clock research.
The space cold atomic clock is another breakthrough in the development of atomic clocks. In the 90s of the last century, Wang Yuzhu began to carry out the relevant demonstration of space cold atomic clocks, and his student Wang Xinqi had already mentioned the "new space atomic clock proposal" in his doctoral dissertation. After entering the 21st century, Wang Yuzhu led the team to gradually promote the feasibility study of miniaturized cold atomic rubidium clocks and space cold atomic clocks.
In 2006, under the call of his mentor Wang Yuzhu, Liu Liang gave up his high-paying job in the United States and returned to the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences, where he soon led the establishment of the Space Cold Atomic Clock team. In 2010, the development of the prototype of the space cold atomic clock and the demonstration of ground scientific experiments were completed; in 2011, the experimental plan of the space cold atomic clock officially entered the design and development stage of the engineering prototype; in 2016, the world's first space cold atomic clock for scientific experiments in orbit was launched into space on Tiangong-2, with an accuracy of up to 30 million years and an error of less than 1 second.
It is worth mentioning that the scheme of a space cold atomic clock was first proposed by the Paris Observatory in France, but China was the first country to send it into space. From missing the opportunity to research laser cooled gas atoms to catching up with the research of space cold atomic clocks, in just a few decades, it seems to have changed the world. Wang Yuzhu is pleased with the increasing improvement of the domestic scientific research environment, and is also pleased that the younger generation has shouldered the important task of strengthening the country with science and technology.
Time flies, and the young man who climbed on the windowsill and had light in his eyes has entered his old age, but the research team of cold atoms is growing day by day and in its prime. This is the most gratifying thing for Wang Yuzhu - everyone will grow old, but in his limited years, he has lifted more "eight or nine o'clock suns" for the motherland's scientific and technological undertakings.