Unlike traditional computers that use bits of 0 or 1 to store information, quantum computing takes quantum bits as the basic unit of information coding and storage. Based on the superposition principle of quantum mechanics, a qubit can be coherently superposed in both states of 0 and 1 at the same time, that is, it can be used to represent 0 and 1. By extension, n qubits can represent the superposition of 2n numbers, so that a quantum operation can, in principle, perform parallel operations on 2n superposed numbers at the same time, which is equivalent to performing 2n operations on a classical computer. Therefore, quantum computing provides a way to fundamentally realize parallel computing, which may greatly exceed the computing power of classical computers.
Similar to classical computers, quantum computers can also follow the framework of Turing machine, and perform general quantum operations by performing programmable logic operations on quantum bits, thus achieving a substantial increase in computing power, even exponential acceleration. A typical example is the fast prime decomposition quantum algorithm (Shor algorithm) proposed by 1994. The computational complexity of prime number decomposition is the basis of the widely used RSA public key cryptosystem security. For example, it takes more than 654.38+ million years to decompose a 300-bit large number with a classical computer that operates trillions of times per second; However, if a quantum computer with the same operation speed is used to execute the Shor algorithm, only 1 second is needed. Therefore, once the quantum computer is successfully developed, it will have a great impact on the classical information security system.
The development stage of quantum computing
The computing power of quantum computer increases exponentially with the number of qubits, so the core task of quantum computing research is the coherent manipulation of multiple qubits. According to the scale of coherent manipulation of quantum bits, international academic circles recognize that quantum computing has the following development stages:
The first stage is to realize the superiority of quantum computing, that is, the computing power of quantum computers exceeds that of classical supercomputers, and it takes about 50 qubits to achieve this goal. In 20 19, Google took the lead in realizing the "quantum computing superiority" of superconducting circuit system. China has achieved the "quantum computing advantage" in the optical quantum system in 2020 and the superconducting circuit system in 202 1 respectively. At present, China is the only country in the world where both physical systems have reached this milestone.
The second stage is to realize a special quantum simulator, that is, to manipulate hundreds of quantum bits coherently and apply them to specific problems such as combinatorial optimization, quantum chemistry, machine learning, etc., to guide material design and drug research and development. It will take 5 to 10 years to reach this stage, which is the main research task at present.
The third stage is to realize a programmable universal quantum computer, that is, to manipulate at least millions of qubits coherently, which can play a great role in classical password cracking, big data search, artificial intelligence and so on. Because qubits are easily affected by environmental noise and make mistakes, it is an inevitable requirement and a major challenge for large-scale qubit systems to ensure the correct operation of the whole system through quantum error correction. Due to technical difficulties, it is not clear when the general quantum computer will be realized. International academic circles generally believe that it will take 15 years or even longer.
At present, various physical systems that are expected to realize scalable quantum computing are systematically studied. China has completed the research layout of all important quantum computing systems and become one of the three countries (regions) with complete layout, including the European Union and the United States.
Superconducting quantum computing to achieve catch-up
At present, Google, IBM and China University of Science and Technology rank the top three in the research of superconducting quantum computing in the world. 20 19, 10 In June, after investing heavily in quantum computing for more than 10 years, Google officially announced that the experiment proved the superiority of quantum computing. They built a quantum processor with 53 superconducting qubits and named it "Sycamore". In the specific task of random line sampling, "Platanus acerifolia" shows far more computing power than supercomputers. In May of 20021year, Zu Chongzhi, a 62-bit superconducting quantum computing prototype with the largest number of qubits in the world at that time, was built, and a programmable two-dimensional quantum walk was realized. On this basis, further realize the 66-bit "Zu Chongzhi II". "Zu Chongzhi II" has the programming ability to execute any quantum algorithm, and realizes the fast solution of quantum random line sampling. According to the classical optimization algorithm published at present, the processing speed of "Zu Chongzhi II" is 6.5438+million times faster than that of the fastest supercomputer at present, and the computational complexity is 6.5438+million times higher than that of Google "Platanus acerifolia".
Research on Quantum Computing of Other Systems
Physical systems such as ions and silicon-based quantum dots also have the potential of multi-bit expansion and fault tolerance, and are also the focus of international quantum computing research. The research on quantum calculation of ion system in China started late and is catching up on the whole. The dominant research units in China are Tsinghua University, China University of Science and Technology and National University of Defense Technology. , and accumulated a lot of key technologies in the basic elements of quantum computing, such as ion trap preparation, single ion coherent holding time, high-precision quantum logic gate and multi-bit quantum entanglement. The direction of quantum computing in silicon-based quantum dots in China is parallel to the main international research forces. In addition, due to the superiority of topological quantum computing in fault tolerance, it is an important long-term research goal in the world to realize general quantum computing by using topological systems. At present, efforts are being made at home and abroad to achieve the breakthrough of single topological qubit "0 to 1".
Future development of quantum computing
After realizing the phased goal of "the superiority of quantum computing", the future development of quantum computing will focus on two aspects: First, continue to improve the performance of quantum computing. In order to realize fault-tolerant quantum computing, the first thing to consider is how to expand the scale of quantum computing system with high precision. When realizing quantum bit expansion, the quantity and quality of bits are extremely important, and every step of the experiment (preparation, manipulation and measurement of quantum States) needs to maintain high precision and low noise. With the increase of quantum bits, the errors caused by noise and crosstalk also increase, which brings great challenges to the design, processing and regulation of quantum systems, and still requires a lot of collaborative efforts of science and engineering. The second is to explore the application of quantum computing. It is predicted that quantum computing is expected to exceed kilobits in the next five years. Although fault-tolerant general quantum computation can't be realized at present, scientists hope to explore the application of quantum computation in machine learning, quantum chemistry and other fields in the stage of noisy quantum computation (NISQ) and form a near future application.