# What is a quantum computer？ How powerful is the ＂quantum computing superiority＂ established in ＂Nine Chapters＂

Photo courtesy of China University of Science and Technology

"Nine Chapters" Quantum Computing Prototype Physical Map Data Picture

Schematic diagram of optical path system of "Nine Chapters" quantum computing prototype Photo courtesy of China University of Science and Technology.

On December 4th, many media, including this newspaper, reported a great achievement of quantum computing: Pan Jianwei and Lu Chaoyang of China University of Science and Technology built a quantum computer with 76 photons and 100 modes "Nine Chapters", which was 100 trillion times faster than Fu Yue, the fastest supercomputer at present. In other words, the supercomputer needs 100 million years to complete the task, and the "nine chapters" only takes one minute. At the same time, the "Nine Chapters" is equivalent to 10 billion times faster than the 53 superconducting bit quantum computer prototypes "Platanus acerifolia" released by Google last year.

However, many readers are amazed at this great scientific research achievement, but they don’t know the principle, the significance of the achievement and the application prospect of quantum computer. Some readers even report that "every Chinese character knows it, but they still don’t understand it". To this end, our reporter interviewed relevant experts and tried to uncover the mystery of the "Nine Chapters" and understand the principle of quantum computers.

** What is a quantum computer?**

"Quantum computer is a computer made by the principle of quantum mechanics, and it is still in a very preliminary stage. Correspondingly, the existing computer we are using is called a classic computer. " Yuan Lanfeng, an associate researcher at the National Laboratory of Microscale Material Science of China University of Science and Technology, has been committed to popular science writing. He told reporters that the calculation forms of the two are different. "The computer calculates by turning on and off the circuit, while the quantum computer uses the quantum state as the calculation form."

The computer we use every day, whether it is the image on the screen or the Chinese characters input, will be converted into 1 and 0 in the hardware circuit. Each bit represents either 0 or 1. These bits are information, and then they will be transmitted, calculated and stored. It is precisely because of this "calculation" process of 0 and 1 that computers are called "computers".

Quantum computing, on the other hand, uses the natural superposition of quantum to display the ability of parallel computing. "Quantum mechanics allows an object to be in multiple states at the same time. The coexistence of 0 and 1 means that many tasks can be completed at the same time, so it has the computing power beyond computers." Lu Chaoyang, a professor at the Chinese University of Science and Technology, said that each qubit can not only represent 0 or 1, but also represent that 0 and 1 are multiplied by a coefficient and then superimposed. With different coefficients, the possibility of this superposition will be many, many.

"At present, quantum computers use physical systems such as atoms, ions and photons, and different types of quantum computers use different particles. This time ‘ Chapter 9 ’ Using photons. " Yuan Yufeng said.

Yuan Yufeng told reporters that quantum computers do not surpass classical computers for all problems, but only for some specific problems, because they design efficient quantum algorithms for these specific problems. "For problems without quantum algorithms, such as the simplest addition, subtraction, multiplication and division, quantum computers have no advantage."

** What does "Nine Chapters" look like?**

In the Optical Quantum Laboratory of China University of Science and Technology, the reporter saw the "Nine Chapters" that established the superiority of China’s quantum computing.

From the appearance, it is not so much a computer as an open computing system: a grid of about 3 square meters on the experimental table is filled with thousands of components. "These are the optical paths of the quantum computer prototype," said Professor Yuan Zhensheng of the Pan Jianwei Research Group. "It is through the independent innovation of quantum light source, quantum interference and single photon detector in China that we have built a quantum computing prototype of 76 photons."

On another table, there is a receiver of "Nine Chapters". "If you stand between two tables, it means that you are in ‘ Chapter 9 ’ In the middle. "

It turns out that the mysterious "Nine Chapters" are just a bunch of optical paths and receiving devices.

Yuan Yufeng told reporters that optics is a means to realize quantum computing, which is parallel to many other means such as superconductivity, ion trap and nuclear magnetic resonance. "The University of Science and Technology of China has brought optics to the center of the world, greatly expanding the academic estimate of the upper limit of this means, which is the technical significance of this achievement."

** How powerful is the "quantum computing superiority" established in "Nine Chapters"**

The achievement of "Nine Chapters" is to realize the superiority of quantum computing. "Quantum computers surpass the strongest classical computers in existence on a certain issue, which is called ‘ Quantum superiority ’ Or ‘ Quantum hegemony ’ 。”

Yuan Lanfeng then explained, "Actually, ‘ Quantum hegemony ’ It is a scientific term and has nothing to do with international politics. It means that quantum computers far exceed existing computers on a certain issue. "

Based on the superposition of quantum, many quantum scientists believe that the computing power of quantum computer will far exceed that of any classical computer in specific tasks. In 2012, American physicist John Preskill described it as "the superiority of quantum computing" or "quantum hegemony". In 2019, Google was the first to announce the realization of quantum superiority. The quantum computer they use is called "Platanus acerifolia", and the problem they deal with can be roughly understood as: judging whether a quantum random number generator is really random.

"Google created ‘ Platanus acerifolia ’ A chip containing 53 qubits takes 200 seconds to sample a quantum circuit one million times, while the most powerful supercomputer in existence takes 10,000 years to complete the same task. 200 seconds to 10,000 years, if this is the best performance of both sides, then it is indeed an overwhelming advantage. " Yuan Lanfeng said that the difference between "Nine Chapters" and "Platanus acerifolia" is that the problems are different, and the physical system used to build quantum computers is different. "Nine Chapters" uses optics, while "Platanus acerifolia" uses superconductivity. "These two are not superior or inferior, just different technical routes."

“‘ Chapter 9 ’ On the same track, than ‘ Platanus acerifolia ’ Ten billion times faster, this is the equivalent speed, which also means that China has achieved ‘ Quantum hegemony ’ " . Yuan Lanfeng further explained that the achievements of the "Nine Chapters" firmly established China’s position as the "first phalanx" in international quantum computing research. This is because "Platanus acerifolia" comes first, and "Nine Chapters" is the second one after all, so it just means that China is not far from the United States. "As for quantum communication, we won’t say anything ‘ The first phalanx ’ Yes. Because there is no phalanx there, China is clearly the most advanced in the world. "

** What is Gaussian Bose sampling?**

All the reports mentioned, "‘ Chapter 9 ’ Handle ‘ Gaussian bose sampling ’ Its speed is faster than the fastest supercomputer ‘ Fuyue ’ A hundred trillion times faster. " So, what is Gaussian Bose sampling?

"Bose sampling is one of the specific tasks used to demonstrate the superiority of quantum computing and has always been highly anticipated by scientists." Yuan Lanfeng said, "It can be roughly understood that there are many exits in a light path. Ask how much light goes out of each exit."

Because quantum mechanics endows photons with many incredible properties, the different paths of photons can not only be superimposed on each other, but also offset each other. The specific results depend on the situation and are very complicated. "In the face of such a difficult problem, the Bose sampling device has its place. Like a computer, it can solve specific mathematical problems with high accuracy, and at the same time, it applies the quantum mechanical characteristics of photons, so it can be called a ‘ Optical quantum computer ’ 。” Yuan Yufeng said.

This time, the research team composed of Pan Jianwei and Lu Chaoyang of China University of Science and Technology, in cooperation with Shanghai Institute of Microsystems of Chinese Academy of Sciences and National Research Center for Parallel Computer Engineering, successfully developed the "Nine Chapters". The main difference from the previous Bose sampler is the input photon state, that is, upgrading the previous Bose sampling device. "Bose sampler input is an independent photon, and ‘ Chapter 9 ’ The input is a group of interrelated quantum light waves. " Yuan Lanfeng said, therefore, the "Nine Chapters" is many times faster than the classic computer, which truly reflects the "advantages of quantum computing".

** Should quantum computers be equipped with systems?**

"The quantum computer itself is a set of ‘ System ’ 。” Professor Lin Mei of China University of Science and Technology said that independent optical components provide hardware, and the complex optical structure determines its "algorithm". "For example, a quantum computer with photons as qubits needs a single photon source that can generate photons, change the state of photons, and complete ‘ Algorithm ’ The specific optical path structure also requires a single photon detector to observe the final state of photons. "

It is understood that for the control of quantum computers, it is still necessary to input and output information through ordinary computers. Workers need to input the initial data on the ordinary computer, and the data will be converted and calculated in the quantum computer control system, and the final result will be transmitted back to the ordinary computer of the workers.

** How far is quantum computer from practical use?**

Can quantum computers handle practical problems? The answer is: yes.

"For example, factorization, quantum computers have fast algorithms. The difficulty of factorization is the basis of RSA, the most commonly used cryptosystem at present, so the quantum computer can factorize quickly, which means that the password can be cracked quickly. " Yuan Lanfeng said, "The only problem is that the existing quantum computer can only decompose very small numbers, which is not enough to crack practical passwords. Therefore, after realizing quantum superiority, the next important goal is to create a quantum computer that transcends classical computers for a practical problem. "

"When the train was first invented, even the speed of the carriage couldn’t catch up; When the plane was first invented, it could only fly in the sky for 1 minute; When the quantum computer was first invented, the calculation process could not last for a few minutes. " Yuan Lanfeng said that with the development of quantum computing today, the "Nine Chapters" we have developed are not only fast and stable, but also have potential application value. "No matter how elementary the quantum computer is now, one day, it will come to us step by step like the previous trains and planes. Maybe in the future, we can use optics to realize a truly powerful quantum computer, that is, a programmable quantum computer that can handle many practical problems. "

** How does the scientific community evaluate the "Nine Chapters"**

After the publication of "Nine Chapters", it won unanimous recognition from the scientific community. The reviewers of Science magazine thought that this achievement was "a most advanced experiment" and "a major achievement".

Ignacio Cirac, director of Max Planck Institute in Germany and winner of Wolff Prize and Franklin Medal, said, "Overall, this is a major breakthrough in the field of quantum technology and a big step towards developing quantum devices with quantum advantages over classical computers." Anton Zeilinger, president of the Austrian Academy of Sciences, Wolff Prize winner and academician of the American Academy of Sciences, commented, "This work is very important, because Pan Jianwei and his colleagues have proved that quantum computers based on photons can also be realized ‘ Advantages of quantum computing ’ . I predict that it is very likely that quantum computers will be widely used one day. Even everyone can use it. "

Dirk Englund, a professor at Massachusetts Institute of Technology, winner of the American Young Scientists Presidential Award and Sloan Prize, called it "an epoch-making achievement". He said, "This is a milestone in the development of medium-sized quantum computers. It shows that we are at a very special moment in the frontier field of complex systems, and complex systems have complexity that we can’t predict on computers today. Therefore, this is a remarkable achievement. "

Barry Sanders, a professor at the University of Calgary in Canada and director of the Institute of Quantum Science and Technology, said, "I think this is an outstanding work and has changed the current pattern. We have been trying to prove that quantum information processing can overcome classical information processing. This experiment is beyond the reach of classical computers. I think this is one of the most important achievements in the field of quantum computing. There is no dispute about this experiment, and there is no doubt that the results obtained by this experiment far exceed the simulation ability of traditional machines. What I want to say is that this experimental technique is very challenging. In order to get this result, they must solve many very difficult technical problems. On the technical level alone, their achievements are also impressive. This is an experiment that people dream of, and they have done it to make their dreams come true. "