Behind Google's "100 million times faster": How is quantum computing achieved?
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Recently, Google launched the D-Wave quantum computer, which claims to be able to solve problems 100 million times faster than any other computer. Some people think that Google is hyping up its products to attract attention; others think that Google's creation of a quantum computer is a major technological advancement. On the same day, Russian media said, "China built the world's first quantum computer with a piece of diamond... The accuracy of quantum logic gates reached 99.99%, which represents the world's highest level of quantum manipulation accuracy of solid-state spin systems." So, which is the world's first quantum computer, Google's D-Wave or China's quantum computer based on diamond? The basic principle of quantum computing Quantum computing is a new type of computing based on quantum effects. The basic principle is to use quantum bits as the basic unit of information encoding and storage, and complete computing tasks through the controlled evolution of a large number of quantum bits. A qubit is a physical system with two quantum states, such as the two polarization states of photons, the two spin states of electrons, and the two energy levels of ions (atoms), which can all constitute the two states of a qubit. A transistor has only an on/off state, that is, it is either in the 0 state or the 1 state. Based on the principle of quantum superposition, a qubit can be in the 0 state and the 1 state at the same time. When the state of the quantum system changes, each superposition state can change. For example, because a qubit represents both the 0 and 1 states at the same time, two such quantum states can represent four states at the same time. N qubits can store 2^2N data at the same time, and the amount of data grows exponentially with N. At the same time, a quantum computer can transform 2^2N data at the same time in one operation. This ability to process data in parallel is equivalent to the effect of an electronic computer performing 2^2N operations... It is equivalent to completing the parallel processing of 2^2N data in one evolution, which is the advantage of quantum computers over classical computers. China has not built a quantum computer based on diamond Chinese scholars have not made a quantum computer with diamond. Instead, a carbon atom in the diamond is replaced by a nitrogen atom, and a vacancy next to the nitrogen atom is added to form an NV color center structure, which becomes the carrier of single-spin solid-state quantum computing. (NV color center) The unpaired electrons in the vacancy of the NV color center exhibit the property of spin 1. When the NV color center is excited by 532nm green light, the NV color center can emit red fluorescence, and the zero phonon line of the fluorescence is at 637nm. After being excited by 532nm laser, the NV color center can be prepared to the initial state with a probability greater than 90%, and the researchers can manipulate its state by microwave and radio frequency. (Crystal structure and energy level structure) Compared with quantum dots and superconducting systems, the coherence properties of NV color center electron spins are very good - quantum coherence is the premise of quantum computing, and quantum computing needs to use quantum coherence to perform calculations, but each quantum bit is very fragile and can easily be decohered by the environment, causing the quantum coherence to be lost, and the decoherence rate increases exponentially with the expansion of the system. The more quantum bits there are, the faster the decoherence rate. The use of NV color centers has the following advantages: First, the technology of injecting nitrogen ions into diamonds to form NV color center structures through ion beam implantation has matured. Second, it is a single spin quantum bit. Different from NMR, which uses the ensemble spin as the research object, the research object of the NV color center system is a single electron spin. Third, it is a stable fluorescence property and a single photon emission source. Under the action of strong light for a long time, the NV color center may continue to emit stable fluorescence. Fourth, it is a relatively ideal decoherence time. In ultra-pure samples, it can even reach the millisecond level. Fifth, the initialization and readout of single-spin quantum bits can be achieved using a confocal system, which is easy to achieve. Sixth, quantum state preparation, manipulation and readout can be achieved at room temperature. The experimental conditions of most other alternatives are relatively harsh (for example, Google D-Wave's low-temperature superconductors have very harsh temperature requirements), and the manipulation technical indicators are very high. The ability to conduct experiments at room temperature greatly improves the feasibility of the experiment. To achieve quantum computing, at least the following conditions must be met: Quantum computers must have identifiable and well-defined quantum bits. Quantum bits can be divided into physical bits and logical bits. Through the error correction code process, several physical bits are redundant, and finally a logical bit is generated, which has good fault tolerance. Quantum computers must be able to prepare reliable initial states. Quantum computers must have weak decoherence effects. Quantum computers must be able to perform precise quantum gate operations. The operating accuracy of Google's low-temperature superconducting system is far from the single-bit gate and double-bit name accuracy required for quantum computing. Quantum computers must establish a very strong quantum measurement mechanism. The NV color center system meets all the above points - the NV color center system contains an electron spin system with spin 1 and multiple nuclear spins, all of which can be used as quantum bits; the electron spins in the NV color center system can be prepared to the initial state by laser excitation, and the nuclear spins around it can be prepared to the initial state with high fidelity by dynamic nuclear polarization or polarization transfer; in ultra-pure diamonds with a nitrogen content of less than 5×10^-9 (mass fraction), the decoherence time of the NV color center electron spin is as long as several hundred microseconds, and the decoherence time of the nuclear spin is as long as several hundred microseconds. The coherence time is as long as tens of milliseconds, and after purifying the nuclear spins around the NV color center electron spins, the decoherence time of the electron spins can be as long as several milliseconds, or even longer. Compared with the speed of a single quantum NOT gate operation that can reach GHz, these are enough to ensure the high precision of quantum computing; the current single quantum gate operation accuracy of the NV color center system can reach more than 99.99% (that is, the latest technological breakthrough reported by the Russian media); the single-shot measurement of single electrons and single nuclear spins by the NV color center system has been realized. If you don’t understand, just look at the conclusion - therefore, the NV color center system meets the most basic requirements for becoming a quantum computer carrier and becomes an excellent candidate for a quantum computer carrier. The latest scientific research results of Du Jiangfeng’s research group reported by the Russian media are that they have achieved very high operating accuracy under traditional error correction codes, and the quantum logic gate accuracy has reached 99.99%. Its single-bit gate accuracy has met the needs of fault-tolerant computing. To build a quantum computer, the next step is to expand the system and make hundreds or thousands of logical bits reach the accuracy that meets fault-tolerant computing, then quantum computing can be realized. However, the system expansion is very difficult, and there is still a long way to go to build a quantum computer. Therefore, although Du Jiangfeng's research group has indeed made key technological breakthroughs, the Russian media report that "Chinese scholars have built the world's first quantum computer based on diamond" is obviously an exaggeration. Is Google's "100 million times faster" reliable? Standard quantum computers are universal computers that can run a variety of algorithms, similar to classical computers, using circuits equivalent to the Turing machine model. The various functions of the Turing machine can be implemented using circuits, which are composed of basic gates. The standard quantum computer also has such a concept, which is a series of basic logic gates to realize quantum circuits, and then realize various algorithmic functions. This kind of quantum computer is called a universal quantum computer, and Google's quantum computer does not have the concept of gates corresponding to quantum computers. So what is the matter with Google's claim that "it can solve problems 100 million times faster than any other computer"? The concept of annealing originated from metal processing technology, which refers to heating the metal to a point above the recrystallization temperature and maintaining this temperature for a period of time, and then slowly cooling it. Google's D-Wave is a quantum annealing machine, which uses the similarity between quantum fields and an optimization algorithm space to directly simulate this optimization space with quantum and then at the lowest energy point of matter, rather than the lowest point in mathematics. Since quantum mechanics is different at the microscopic level and the macroscopic level, it can penetrate some barrier structures - for example, there is a wall 5 meters high and 0.1 meters thick, and to climb over it, you must have skills no less than PLA. But at the quantum mechanics level, there is a certain probability that ordinary people can directly pass through the wall. At the classical level, if you use a traditional computer to simulate, you must honestly climb the wall, and this is what causes the D-Wave quantum annealing algorithm to have a very large acceleration, and it is also the reason why Google claims to be 100 million times faster. In other words, Google's quantum computer is only a computer that does special algorithms for specific links. Google's annealing algorithm can surpass traditional computers in specific links and specific applications, but it is not universal. The so-called 100 million times faster than traditional computers is to compare a specially selected specific optimization algorithm on a specific problem with a simulated annealing algorithm on a traditional computer with unknown configuration. It is true that Google D-Wave has advantages over traditional computers in certain links and applications. However, D-Wave is not universal, and the operating accuracy of the superconducting system is far from meeting the requirements of quantum computing. Therefore, D-Wave is not a true quantum computer. In fact, on January 13, 2014, a research group led by Professor Martinis and Lidar of the University of California, including researchers from Google, officially announced that experimental data on the 503-qubit D-WaveTwo quantum computer showed that there was no evidence of quantum acceleration. MIT computer scientist and quantum computing expert Scott Aaronson believes that D-Wave is a hype master, and believes that even if D-Wave launches the so-called 1,000-bit quantum computer, it will not change anything, because in principle, it will no longer have the advantage of quantum acceleration. Although it is not ruled out that D-Wave can play its role in specific fields, it is inevitable that there is suspicion of commercial hype to compare a quantum annealing machine, which is not a quantum computer at all, with a traditional computer to simulate a specific algorithm, and then advertise that it is "100 million times faster than any other computer." Some domestic media that only seek to attract attention but not the truth have spread rumors that "Google has successfully manufactured a quantum computer", adding a bit of commercial hype to D-Wave. Is diamond too luxurious as a material? Regarding some netizens' view that diamond is too expensive as a material, in fact, the high price of diamonds is entirely due to the monopoly groups such as the De Beers Group artificially raising the price in order to obtain huge profits. In the beginning, by controlling the origin, channels, processing and other links, they firmly controlled the profit distribution and price of diamonds. When Russia began to sell diamonds in large quantities and could not control the origin, it continued to control the industrial chain through branding, marketing, channels, processing, certification and other means, so that Russian diamonds could not impact the interests of international monopoly groups and continued to raise the price of diamonds. When the technology of artificial diamonds became more mature and the quality was not inferior to or even better than natural diamonds, the international monopoly groups began to belittle artificial diamonds and enthusiastically support natural diamonds... In fact, China's artificial diamond production has long ranked first in the world, accounting for 90% of the global production. On the one hand, there is huge market demand; on the other hand, equipment and processes such as six-sided top presses and chemical vapor deposition with independent intellectual property rights are gradually maturing. According to statistics, in 2015, China's annual output of artificial diamonds was nearly 10 billion carats, with an annual growth rate of 21.45%. China's total export volume of artificial diamonds has increased by more than 14 times in the past decade, and the United States is the largest export market for Chinese artificial diamonds. According to this development trend, when the output is further expanded, the cost of artificial diamonds may fall further. Moreover, compared with low-temperature superconducting materials that have very strict temperature requirements, the material cost of artificial diamonds is no longer as "luxurious" as some people imagine. Conclusion The reason why Google studies quantum annealing instead of standard quantum computers is that the operating accuracy of superconducting systems is far from meeting the requirements of quantum computing. At the same time, with the relatively mature superconducting electronics technology of superconducting systems, Google can easily integrate a large number of quantum bits. But choosing this path also means that Google does not have the possibility of "upgrading" with quantum annealing machines to create quantum computers. The solid-state quantum computing based on the diamond system developed by Du Jiangfeng's research group is a new system, but its NV color center is far less integrable than quantum dot systems and superconducting systems, and there is still a long way to go before a real quantum computer can be built. Therefore, the former is not a standard quantum computer at all, and although the latter has achieved key technological breakthroughs, it will take some time before a real quantum computer can be built. As a winner of Toutiao's Qingyun Plan and Baijiahao's Bai+ Plan, the 2019 Baidu Digital Author of the Year, the Baijiahao's Most Popular Author in the Technology Field, the 2019 Sogou Technology and Culture Author, and the 2021 Baijiahao Quarterly Influential Creator, he has won many awards, including the 2013 Sohu Best Industry Media Person, the 2015 China New Media Entrepreneurship Competition Beijing Third Place, the 2015 Guangmang Experience Award, the 2015 China New Media Entrepreneurship Competition Finals Third Place, and the 2018 Baidu Dynamic Annual Powerful Celebrity. |
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