On December 4, many media, including this newspaper, reported a great achievement in quantum computing: Pan Jianwei, Lu Chaoyang and others of the University of Science and Technology of China built a "Nine Chapters" quantum computer with 76 photons and 100 modes. , It processes "Gaussian Bose sampling" faster than the current fastest supercomputer "Fuyue" one trillion times faster.
In other words, the task of a supercomputer that takes 100 million years to complete, the "nine chapters" only need one minute.
At the same time, "Nine Chapters" is also equivalently 10 billion times faster than the 53 superconducting bit quantum computer prototype "Plalanus" released by Google last year.
However, many readers, while marveling at this important scientific research achievement, are still unclear about the principles, the significance of the results, and the application prospects of quantum computers. Some readers even reported that "I know every Chinese character, but still don't understand." .
To this end, our reporter interviewed relevant experts to try to uncover the mystery of the "Nine Chapters" and understand the principles of quantum computers.
What is a quantum computer
"Quantum computers are computers manufactured using the principles of quantum mechanics. They are still at a very preliminary stage. Accordingly, the existing computers we are using are called classical computers." Deputy Deputy of the National Laboratory of Microscale Material Science, University of Science and Technology of China Researcher Yuan Lanfeng has been committed to popular science writing. He told reporters that the two calculation forms are different. “Computers perform calculations by turning on and off circuits, while quantum computers use quantum states as the form of calculation.”
The computer we use every day, whether it is the image on the screen or the input Chinese characters, the information will be converted into 1 and 0 in the hardware circuit. Each bit represents either 0 or 1, these bits are information, and then it is transmitted. , Operation and storage.
It is precisely because of this "calculation" process of 0s and 1s that computers are called "computers".
Quantum computing, on the other hand, uses the superposition nature of quantum to display the ability of parallel computing.
"Quantum mechanics allows an object to be in multiple states at the same time, 0 and 1 exist at the same time, which means that many tasks can be completed at the same time, so it has computing power beyond the computer." Professor Lu Chaoyang of the University of Science and Technology of China said that each qubit can not only It means 0 or 1, and it can also be expressed as 0 and 1 respectively multiplied by a coefficient and then superimposed. With different coefficients, the form of this superposition will have many possibilities.
"Current quantum computers use physical systems such as atoms, ions, and photons. Different types of quantum computers use different particles. This time the'Nine Chapters' use photons." Yuan Lanfeng said.
Yuan Lanfeng told reporters that quantum computers do not surpass classical computers for all problems, but only for certain 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 Photon Laboratory of the University of Science and Technology of China, 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 all prototypes of quantum computers. "Optical path", Professor Yuan Zhensheng of Pan Jianwei's research group said, "It is through my country's independent innovation of quantum light sources, quantum interference, single-photon detectors, etc., that we have constructed 76 photon quantum computing prototypes."
On another table, the receiver of "Nine Chapters" is placed.
"If you stand between two tables, it means you are in the'nine chapters'".
It turns out that the mysterious "Nine Chapters" are a bunch of light paths and receiving devices.
Yuan Lanfeng told reporters that optics is a means of realizing quantum computing, alongside many other means such as superconductivity, ion traps, and nuclear magnetic resonance.
"The University of Science and Technology of China has brought optics to the center of the world, and greatly expanded the academic community's estimate of the upper limit of this method. This is the technical significance of this achievement."
How powerful is the "superiority of quantum computing" established by "Nine Chapters"
The result of "Nine Chapters" is the realization of the superiority of quantum computing.
"Quantum computers surpass the strongest existing classical computers on a certain problem, and are called'quantum superiority' or'quantum hegemony'."
Yuan Lanfeng later explained, "Actually,'quantum hegemony' is a scientific term that has nothing to do with international politics. It means that quantum computers far surpass existing computers on a certain issue."
Based on the superposition of quantum, many quantum scientists believe that the computing power of quantum computers on specific tasks will far exceed any classical computer.
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 "Plana bell", and the problem they deal with can be roughly understood as: judging whether a quantum random number generator is truly random.
"Google’s Platanus chip contains 53 qubits. It took 200 seconds to sample a quantum circuit a million times, while the most powerful supercomputers in existence took 10,000 years to complete the same task. 200 seconds For ten thousand years, if this is the best performance of both parties, then it is indeed an overwhelming advantage." Yuan Lanfeng said, the difference between "Nine Chapters" and "Plane tree" is that they deal with different problems and that they are used to create quantum The physical system of the computer is different.
"Nine Chapters" use optics, and "Platanus" uses superconductivity.
"These two are not superior or inferior, but different technical routes."
"'Nine Chapters' are ten billion times faster than'Platanus' on the same track. This is equivalent speed, and it also means that my country has achieved'quantum hegemony' in quantum computing."
Yuan Lanfeng further explained that the achievements of the "Nine Chapters" firmly established my country's position as the "first square matrix" in international quantum computing research.
This is because there is "plane tree" in front, "Nine Chapters" is the second after all, so it just means that China is not far from the United States.
"In terms of quantum communication, we won't talk about the'first square matrix'. Because there is no square matrix there, China is clearly the most advanced in the world."
What is Gaussian Bose sampling
All reports mentioned that "the speed of'Nine Chapters' processing'Gaussian Bose sampling' is one trillion times faster than the current fastest supercomputer'Fuyue'."
So, what is Gaussian Bose sampling?
"Bose sampling is one of the specific tasks used to demonstrate the superiority of quantum computing. Scientists have always placed high hopes." Yuan Lanfeng said, "It can be roughly understood that there are many exits in a light path. Ask how much light each exit has. Get out."
Since quantum mechanics has endowed photons with many incredible properties, different paths of photons can not only superimpose each other, but also cancel 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 is useful. Since it is like a computer, it can solve specific mathematical problems with high accuracy, while at the same time applying the quantum mechanical properties of photons, so It can be called a kind of'light quantum computer'." Yuan Lanfeng said.
This time, the research team composed of Pan Jianwei and Lu Chaoyang of the University of Science and Technology of China, in cooperation with the Shanghai Institute of Microsystems of the Chinese Academy of Sciences, and the National Center for Parallel Computer Engineering and Technology, successfully developed the "Nine Chapters" and the main features of the previous Bose sampling machine. The difference lies in the input photon state, which is to upgrade the previous "Bose sampling" device.
"The input of the Bose sampling machine is individual photons, and the input of the'Nine Chapters' is a group of interconnected quantum light waves." Yuan Lanfeng said, therefore, the "Nine Chapters" are many times faster than the classical computer, which truly reflects "The advantage of quantum computing."
Do quantum computers need to be installed?
"Quantum computer itself is a set of'system'." Professor Lin Mei of the University of Science and Technology of China said that independent optical components provide the hardware, and the complex optical path structure determines its "algorithm."
"For example, a quantum computer using photons as qubits requires a single-photon source that can generate photons, a specific optical path structure that can change the state of the photon and complete the'algorithm', and it also requires a single-photon detector to observe the final state of the photon."
It is understood that the control of quantum computers still requires ordinary computers to input and output information.
The staff need to input the initial data on the ordinary computer, the data undergoes complex conversion and calculation in the quantum computer control system, and the final result will be transmitted back to the ordinary computer of the staff.
How far is quantum computer from practical
Can quantum computers deal with practical problems?
The answer is: yes.
"For example, factorization, quantum computers have fast algorithms. The difficulty of factorization is the basis of the most commonly used cryptosystem, RSA, so quantum computers can quickly factorize, which means that they can quickly crack the password." Yuan Lanfeng said, "The problem is that the existing quantum computers can only decompose very small numbers, not enough to crack practical codes. So after realizing quantum superiority, the next important goal is to create a practical value problem A quantum computer beyond classical computers."
"When the train was first invented, even the speed of the carriage could not be kept 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, quantum computing Today, the "Nine Chapters" we developed are not only fast and stable, but also have potential application value.
"No matter how elementary a quantum computer is now, one day, it will come to us step by step like trains and airplanes in the past. Perhaps in the future, we can use optics to realize truly powerful quantum computers, that is, programmable, A quantum computer that can handle many practical problems."
How does the scientific community evaluate the "Nine Chapters"
After the "Nine Chapters" came out, they won unanimous affirmation from the scientific community. The reviewers of "Science" magazine believed that this result was "a most advanced experiment" and "a major achievement."
Ignacio Cirac, Director of the Max Planck Institute in Germany and winner of the Wolf Prize and Franklin Medal, said, “In general, this is a major breakthrough in the field of quantum technology, and a step towards the development of quantum devices that have quantum advantages over classical computers. Stride forward." Anton Zeilinger, Dean of the Austrian Academy of Sciences, winner of the Wolf Prize, and academician of the American Academy of Sciences, commented, "The results of this work are very important because Pan Jianwei and his colleagues proved that photon-based quantum computers may also achieve'quantum computing'. Superiority'. I predict it is very likely that one day quantum computers will be widely used. Even everyone can use it."
Dirk Englund, a professor at the Massachusetts Institute of Technology, winner of the President's Award for Young Scientists in the United States, and Sloan Prize winner, called it "an epoch-making achievement."
He said, “This is a milestone in the development of a medium-sized quantum computer. It shows that we are at a very special moment in the frontier field of complex systems. Complex systems have complexity that we cannot predict on computers today. Therefore, this is An amazing 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 that has changed the current landscape. We have been working hard to prove that quantum information processing can defeat classical information processing. This The experiment is beyond the reach of classical computers. I think this is one of the most important results in the field of quantum computing. There is no controversy in this experiment. There is no doubt that the results of the experiment far exceed the simulation capabilities of traditional machines. What I want to say is , This experimental technical challenge is very huge. In order to obtain this result, they have to solve many very difficult technical problems. Only on the technical level, their achievements are impressive. This is the experiment that people dream of. They made Now, let the dream come into reality."
(Our reporter Chang He)