Shulou(Shulou.com)11/24 Report--

Beijing, June 15 (Xinhua) four years ago, Google claimed to have achieved "quantum hegemony", drawing people's attention to quantum computing. However, in practical applications, there is always a "reliability" problem in quantum computing. Repeat the same calculation many times, and it is likely to come up with a different answer each time. On Wednesday, Blue Giant IBM claimed that it had found a way to solve the reliability of quantum computing.

The quantum computer of the IBM Watson Research Center needs to explain that the basic binary units of traditional computers are 0 and 1, either 0 or 1. However, in quantum computing, its basic unit of measurement qubit can be either 0 or 1, or both 0 and 1. This phenomenon is called quantum superposition state. Quantum computer realizes the function of storing a large amount of information at the same time through quantum superposition. Therefore, they can quickly store a large amount of data when dealing with complex tasks, explore a variety of possibilities and choose the most effective solutions.

However, because it is very difficult to maintain the superposition state of qubits, the smallest environmental changes (vibration, electric field, magnetic field, cosmic radiation) may also lead to the collapse of the superposition state, resulting in computational errors. Therefore, at present, the world has not been able to build an error-free and widely used quantum computer.

On Wednesday, IBM researchers announced that they had devised a way to manage the unreliability of quantum computing to come up with reliable and useful answers. IBM scientists have published their research in the journal Nature entitled "evidence of the practicality of Quantum Computing before Fault tolerance". Fault-tolerant quantum computing refers to quantum computing with quantum error correction protection.

In a paper published by IBM in 2019, Google researchers claimed that they had achieved "quantum hegemony", in which quantum computing has more computing power than all classical computers. However, IBM attacked Google at the time, saying that Google exaggerated the performance of quantum computing and misled the public. On Wednesday, researchers at IBM said they had found some new and more useful methods, albeit with a lower profile.

"We are entering a phase of quantum computing that I call 'utility'," said Jay Jay Gambetta, vice president of quantum business at IBM. "the era of utility."

Dorit Aharonov, a professor of computer science at the Hebrew University of Jerusalem, who was not involved in the study, said: "it is surprising that what IBM is showing here is really an important step towards making progress in serious quantum algorithm design."

How to reduce the error? In the new study, IBM researchers performed a different task that attracted the interest of physicists. They used a quantum processor with 127 qubits to simulate the behavior of 127 atomic-scale magnets in a magnetic field. These magnet rods are small enough to be controlled by the strange rules of quantum mechanics. This is a simple system, called the Ising Model (Ising model), which is often used to study the ferromagnetism of matter.

The question of the quantum processor used by IBM in the experiment is too complex to calculate an accurate answer even on the largest and fastest supercomputer. But on quantum computers, the calculation can be done in less than 1/1000 seconds. However, every quantum calculation is unreliable because fluctuations in quantum noise (the fluctuation of any monochromatic light) inevitably interfere with the calculation and cause errors, but each calculation is fast and can be repeated.

In fact, in many calculations, the researchers deliberately added additional noise to make the answer more unreliable. But by changing the amount of noise, the researchers can infer the specific characteristics of the noise and its impact in each calculation step.

"We can amplify the noise very accurately, and then we can rerun the same circuit," said Abhinav Kandala, IBM's quantum capabilities and demonstration manager and co-author of the Nature paper. "once we get the results of these different noise levels, we can infer the results without noise."

In essence, researchers can remove the effects of noise from unreliable quantum computing, a process they call "error mitigation". "you have to bypass the noise by inventing very clever ways to mitigate the noise," Dr. Aharonov said. "that's exactly what they do."

What's the accuracy? To get the answer to the overall magnetization produced by the 127magnet rods, IBM's quantum computer performed a total of 600000 calculations. What is the accuracy of the answer?

For help, the IBM team approached physicists at the University of California, Berkeley. Although the Ising model with 127 magnets is too large and has too many possible configurations to apply to traditional computers, classical computer algorithms can produce approximate answers. This technique is similar to discarding less important data in JPEG image compression to reduce the file size while preserving most of the details of the image.

Michael Zaletel, a professor of physics at the University of California, Berkeley and one of the authors of the Nature paper, an IBM quantum computing researcher, said that when he started working with IBM, he thought his classical computer algorithms would do better than quantum algorithms. "the results are a little different from what I expected." Dr. Zaleter said.

The results show that the quantum computer can accurately solve some configurations of the Ising model. In a simpler example, the classical algorithm and the quantum algorithm have the same answer. For more complex but solvable examples, the quantum algorithm and the classical algorithm produce different answers, but the quantum algorithm gives the correct answer.

Sajant Anand, a graduate student at the IBM quantum experimental research laboratory at the University of California, Berkeley, has done a lot of work on classical approximations. Based on the above experimental results, he believes that for other cases where the results of quantum computation are inconsistent with those of classical computation and do not know the exact solution, "there is reason to believe that the results of quantum computing are more accurate."

It is not clear whether quantum computing can unquestionably outperform classical computing in the Ising model. Anand is currently trying to add an error-reducing version of the classical algorithm, which is likely to reach or exceed the performance of quantum computing.

"there is no clear sign that they have achieved quantum hegemony here." Zaleter said.

Temporary solution in the long run, quantum scientists expect a different approach, error correction, to detect and correct computational errors, which will open the door to many uses of quantum computers.

At present, error correction methods have been used to repair errors in traditional computers and data transmission. But for quantum computers, error correction may take several years, and better processors are needed to handle more qubits.

IBM scientists believe that error mitigation is a temporary solution that can now be used to solve increasingly complex problems outside the Ising model.

"this is one of the simplest natural science problems available," Dr. Gambetta said, "so it's a good start. But the question now is, how do you promote it to solve more interesting science problems?" These problems may include figuring out the properties of exotic materials, accelerating drug discovery and simulating fusion reactions.

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