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Quantum information is called the second quantum revolution, why is it called the second?
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This article is the author's second speech at the 6th China Manufacturing Day event hosted by the Communist Youth League, the State-owned Assets Supervision and Administration Commission and other institutions on December 26, 2021.
Yuan Lanfeng: Thank you. Thank you to the three teachers for their wonderful speeches, and now I will give you my second lecture today, "Quantum Information: The Second Quantum Revolution". This morning I talked about "The Leading, Running and Running of Chinese Science and Technology", and this afternoon I will introduce to you in detail the quantum information I specifically mentioned.
In fact, every time quantum information has been mentioned before, there is a most basic question, that is, is this thing real technology or fake technology? There used to be a lot of folk science and they wrote a lot of articles saying that this thing is pseudo-technology, and then I also wrote a lot of articles to refute their fallacies. But recently, I suddenly found a new situation, that is, the US government came out to prove to us that this is a real technology.
On November 24, just a month ago, the Commerce Department issued an entity list that added a number of new agencies, including 12 in China. Why put them in a trade embargo? The answer is to prevent emerging U.S. technologies from being used in Chinese quantum computing technologies that support military applications, such as anti-stealth and anti-submarine applications, as well as the ability to crack codes or develop unbreakable ones.
In fact, all three of these are quantum information. Quantum information is divided into three major blocks: quantum communication, quantum computing, and quantum precision measurement. The anti-stealth and anti-submarine mentioned in it belong to quantum precision measurement, cracking the code belongs to quantum computing, and the unbreakable code belongs to quantum communication. Of course, the most interesting thing is to say, why does the undeterable code threaten the national security of the United States? This is worth thinking about.
How did the word quantum come about? In fact, it has a fundamental theory of physics called quantum mechanics. If you want to know the real principle of this quantum information, I can only make a very simple introduction here. But if you want to have a really deep understanding of it, welcome to read my book, "A Brief Introduction to Quantum Information," which I published not long ago. If you have any questions, you are welcome to read my book.
Most basically, how did the word quantum come about? It was first proposed in 1900 by the German physicist Planck. He discovered a phenomenon called blackbody radiation, the energy emitted by blackbody radiation, and he needed to assume that it was a portion. That is to say, you can send one, two, three copies, but you can't send half a copy, this phenomenon is called quantization. Since then, there has been an entire category called quantum mechanics. After Planck, many scientists, such as Einstein, Bohr, Dirac, etc., made great contributions to quantum mechanics.
With quantum mechanics, we call that traditional mechanics, Newtonian mechanics, classical mechanics. So now the words quantum and classical often appear in pairs, and they are used as adjectives. So when we say that something is quantum and something is classical, the implication is that the quantum is always smarter than the classical.
When you know the history of quantum mechanics, you will find a question: quantum is not a new word at all but an old word, which existed in 1900, why has it suddenly become hot now? The answer is that a new technology is now emerging, called quantum information, which is a new interdisciplinary discipline that emerged in the 1980s by the intersection of the two disciplines of quantum mechanics and information science. What is this quantum information for? As the name suggests, you know that it uses quantum mechanics to achieve effects that traditional information science cannot achieve. For example, one of the most sci-fi colors is teleportation. A lot of science fiction movies pass a person from here to there, and I can tell you that this is a real technology now, and its corresponding real technology is called quantum teleportation. But I must immediately remind that now we can't transmit a person, and quantum teleportation can transmit a particle. It's a real technology, so it's a huge step forward.
What other applications are there? For example, there is a quantum radar, which is part of the quantum precision measurement. Speaking of this quantum radar, what is it for? The first thing that comes to mind is the detection of stealth aircraft, but there are actually many units developing quantum radars with different principles. This quantum radar I'm familiar with is used to detect atmospheric wind fields. Atmospheric wind field means that there is no wind blowing in the detection atmosphere? If so, where does the wind blow? How fast is it? It's about probing this.
In fact, there is already a traditional technology for detecting this atmospheric wind field, which is lidar. Its only problem is that it can't be used during the day, why? Because the biggest disturbance during the day is the sun, the sun is too strong, it is a huge background noise. If you want to avoid this sunlight, it is best to use the infrared band, because the sunlight is mainly in the visible band. But why didn't this infrared band be used before? Because its detection efficiency is so low, we don't have a good detector for it.
They invented a new single-photon frequency conversion technique, which is to send you out that light is infrared light, but when you come back, you convert it into 863 nanometers of light. For this 863 nm we have very high detection efficiency. This increases the detection distance from 2.6 km to 8 km, which is a typical application.
Let's introduce the principle of quantum information very simply. Quantum mechanics itself has very, very rich content that we can't talk about for days, but there are three major parts used in information science: superposition, measurement, and entanglement. What exactly do these three words mean? You still have to read my book.
The basic operating object of quantum information is called qubits, not like traditional information science, which operates the object system called bits. Everyone who has studied computer science knows what is called bits, which means that you have and only have two states. But qubits are different, qubits are continuously tunable, it has an infinite number of states.
Let's take a brief look at quantum computing. Many people have the impression that quantum computers can do everything faster than traditional computers. But this impression is wrong. I have to tell you, for example, that if you draw a diagram, the horizontal axis is a variety of computable problems, and the vertical axis is the performance of the computer to process it. If you draw a black line to indicate the performance of that classical computer, many people understand that he thinks that the quantum computer can do everything faster than the classical computer, that is, the red line as a whole is above. This is wrong! Quantum computers are much faster at certain things than classical computers, such as cracking passwords. What is correct is the middle graph, that is, for some problems, the performance of quantum computers is much higher, and those spikes are to stand out. But for other problems, such as the simplest problem, the problem of addition, subtraction, multiplication and division, classical computers have handled it very well, and quantum computers do not have any advantage in doing it. Because its approach is the same as that of classical computers, and its cost is certainly much higher than that of classical computers, if you use a quantum computer to add, subtract, multiply and divide without any meaning, its performance is not as good as that of classical computers. So you can understand that the prospect of quantum computers is not to replace classical computers, but to say that the two are used together. For quantum computers, if you have an advantage, you should use a quantum computer, and if you have no advantage in a quantum computer, you will continue to use a classical computer.
To what extent are quantum computers doing now? I can tell you that the biggest progress in this experiment is that recently, we have achieved quantum superiority. The word quantum superiority was originally called quantum supremacy when it was first proposed, and later everyone felt that the word sounded like hegemonism, which sounded not very good, so now it is more renamed quantum superiority. But the actual meaning is the same, it is to say that for a problem, the quantum computer far exceeds the current classical computer.
Why emphasize an issue? You have to have an advantage over certain problem quantum computers, so you have to find a specific problem where quantum computers actually outperform the strongest classical computers today. Everyone knows that the classical computer is now very strong, it can do tens of billions of operations a second, beyond this is to achieve the superiority of quantum computers.
The first milestone was in 2019, when the Google quantum computing team achieved it with a superconducting physics system, and the problem they were dealing with was called random line sampling. But immediately after the results were published, many teams working on classical computers came out to improve the classical algorithm. Because doing classical computing is also very smart, the software and hardware of classical computing can also be improved. Soon a lot of people improved classical computing, and one of the big changes recently was that some people overtook it. Also a month ago, in November 2021, a very clever classical algorithm was proposed by Pan Zhang's team at the Institute of Theoretical Physics of the Chinese Academy of Sciences. They say that if the algorithm is executed on the strongest supercomputer, it will take less time than the plane tree, and it will beat the plane tree head-on. So the plane tree first achieved quantum superiority, and then was surpassed by classical computers, so its quantum superiority was canceled.
The other is that in December 2020, Pan Jianwei, Lu Chaoyang and others at HKUST used optical systems to achieve quantum superiority, this quantum computer is called Jiuzhang, and the problem it deals with is Gaussian boson sampling. If you want to understand what this means, you still have to read my book.
What does Chapter Nine look like? Ordinary people generally see news reports like the picture in the middle, and a set of light paths looks very unconscious. But what I see is this on the right. On December 5, 2020, after the report of the nine chapters was issued, the Central Committee of the Youth League asked me to make a propaganda film and let me publicize the research of quantum information at HKUST. Professor Yuan Zhensheng of Pan Jianwei's research group was very enthusiastic to take us to the laboratory. In a small space, I don't know what it was, and when I went in, he suddenly told us that the things in the grid in front of us were the optical devices of the nine chapters, and the core devices of the nine chapters in the grid behind us. So we're in chapter nine! I discovered that nine chapters is a geographical term, and we are introducing nine chapters to you in nine chapters.
So there are a lot of people who say that Nine Chapters is an optical experiment, yes it's an optical experiment, but this optical experiment can be equivalent to solving a mathematical problem, in this sense it is a computer. And when you solve this mathematical problem, it's much faster than today's traditional computers, how much faster? Chapter Nine took 50 million samples in 200 seconds, when the strongest classical computer took 600 million years to do the same task, 200 seconds to 600 million years, which is 100 trillion times or 10 to the 14th power. Of course, classical computers can also progress, you chase me, everyone tries to catch up with it. But what's actually happening in this area is that it's getting harder and harder to catch up. After a year, the nine chapters have been upgraded again, and it is now the second number of the nine chapters, and many of its indicators have been improved.
For example, the diagram above is the dimension of the state space, which is raised from the 30th power of the 10th of the ninth chapter to the 42nd power of the 10th of the ninth chapter of the second. The figure below is the number of photons used, which can be sampled up to 76 in chapter 9 and 113 in chapter 2. Someone asked me why the photon count didn't just double? I told it this thing wasn't linear growth, this thing was exponential growth. Every time you add a photon, the difficulty increases exponentially, and the amount of computation you bring is also exponentially increased, so every photon you add is very difficult.
Where does this difficulty manifest itself? It was when the question of Gaussian boson sampling was first raised, and the question was raised as a theoretical question in 2013. At that time, there were a lot of people who thought that this thing was not experimentally achievable, and it was already very difficult for you to do boson sampling of up to 5 photons, and 10 was impossible. They did 76 all at once, so you can see what a big breakthrough in technology.
Also on October 26, the same day that they released The Ninth Chapter 2, they released another thing called Zu Chong No. 2. In May, they released The First Zuchong, and five months later they released the Second Zuchong. This Zu Chong No. 2 is the same system as the plane tree, both are superconducting systems.
It deals with the same problem, which is random line sampling, but it is much larger than the plane tree, and its computational complexity is 6 orders of magnitude higher than the plane tree. So it is very interesting that Zhang Pan's classic algorithm surpasses the plane tree, but can it surpass the zu chong no. 2? The answer was that I didn't know, I went to ask Zhang Pan, and Zhang Pan said that I had not yet taken care of studying Zu Chong No. 2. Very interesting, this is the Chinese scientists are doing a lot of left and right.
Let's sum up from the perspective of quantum computing, what is the picture now? At least one system in China has achieved quantum superiority, that is, optics, and superconductivity may also be achieved. The United States originally achieved quantum superiority in superconductivity, but now it is surpassed by classical. So China's system of achieving quantum superiority is 1 or 2, and the United States has changed from 1 to 0, becoming the same as other countries. So in this sense, China is leading in the field of quantum computing in this indicator.
Finally, let's introduce quantum communication a little bit. Quantum communication is also a large area of research, such as the previous teleportation technique. The technology that is really used in practice is quantum cryptography, and when you see them in the media reporting how quantum communication is, nine times out of ten they are referring specifically to quantum cryptography. What is the effect of quantum cryptography? The principle of quantum cryptography is very complex, and I can't say it clearly here, but I can tell you that its effect is unconditionally secure and confidential transmission. What is unconditional security? Unconditional security means that the enemy cannot crack your password even if they have unlimited computing power. And most of our current passwords are conditionally secure, that is, you must assume that the enemy's computing power is limited, it cannot be cracked in a relatively short period of time, in this sense is safe. Therefore, unconditional security is essentially much more than conditional security.
How can there be unconditional security in the world? You need to read a book on cryptography to understand why this thing exists. If you look at my book, you will understand. One of the keywords here is called a disposable note key, and the other is the BB84 protocol, which is the basic technology of quantum cryptography.
What can be told is that China is undoubtedly the world leader in this quantum cryptography. In 2016, we launched the world's first quantum science experimental satellite "Mozi", and in 2017, we opened the world's first quantum secure communication backbone network "Beijing-Shanghai Trunk Line", which can achieve quantum secure communication at a distance of 2,000 kilometers from Beijing to Shanghai. And the two are connected, and they have realized a quantum communication network that integrates heaven and earth. In fact, we have connected it to Europe, realizing the intercontinental quantum secret call between Academician Bai Chunli, president of the Chinese Academy of Sciences, and Anton Salinger, president of the Austrian Academy of Sciences.
But this space-earth integrated quantum communication network, what it can do is not limited to quantum confidential communication, it can also be used to do other things. For example, the year before, they used the space-earth integrated quantum communication network to test the unity of quantum mechanics and general relativity. Some Australian scientists have come up with a theory called the "event form," saying that quantum mechanics and general relativity may be unified in such a form, and then it has explicit predictions. They did experimental tests, and the results of the test were no, at least indicating that their current version is not correct, and if you want to make this theory alive you have to modify the theoretical parameters. In the future, we will launch higher orbit quantum science experimental satellites to continue testing the revised theory.
The quantum communication network is not only of practical value, but also an important infrastructure for basic research of all mankind, which is a major contribution to the scientific community of all mankind.
Finally, to sum up. Quantum information is divided into three blocks. We are undoubtedly the world leader in quantum communications, we are three to five years ahead of Europe and five to eight years ahead of the United States. In terms of quantum computing, we and the United States jointly form the first square, but we need to tell you that it is important to note that quantum computers have no practical value yet. For example, those quantum superiority problems are specifically designed to achieve quantum superiority, and its problem itself has no practical value at present, and cracking the code is its vision. I hope that everyone will work hard to join this industry and help quantum computing have practical value as soon as possible.
Quantum precision measurement is now the most explicit use value, such as the satellite navigation system flying in the sky, GPS, Beidou and other core technologies are atomic clocks, and atomic clocks belong to quantum precision measurements. So some of the quantum precision measurements are already practical, and some are being put into practice quickly. Either way, the practical outlook is very great.
Quantum information is called the second quantum revolution, why is it called the second? Because at the beginning of the 20th century, Planck, Einstein, bohr proposed the theory of quantum mechanics, which was the first quantum revolution. But at that time, our ability to manipulate the microscopic world was not strong enough, so you could only face a large number of particles, and you could do very little. Now we can manipulate a single particle, and then we can fully tap into its quantum properties and achieve some effects that were previously impossible to achieve, so we call it the second quantum revolution.
Finally, welcome everyone to read my book, pay attention to my Weibo, WeChat public number and video program technology Yuan Ren, etc., thank you!