Quantum Computing - a distant term for the average person. Nowadays, you can also wear it and feel its magical charm.
On the 22nd of this month, at the 2018 annual work conference of the Institute of Quantum Information and Quantum Science and Technology Innovation of the Chinese Academy of Sciences held in Hefei, the Quantum Information and Quantum Technology Innovation Research Institute of the Chinese Academy of Sciences, Alibaba Cloud and other units The joint development of the 11-bit superconducting quantum computing cloud platform was formally announced. This is the second system in the world after IBM to provide more than 10 bits of quantum computing cloud services to the public. Users only need to access this quantum computer through a specific interface. You can upload quantum circuits that need to be run, and get a result given by quantum computing. The development of quantum computing cloud platform aims to verify the acceleration and stability of quantum computing from the hardware point of view while fostering the industry ecology and promoting quantum computing. Industrialization. For the vast number of theoretical researchers in quantum computing, this cloud platform can provide more support and simulation for their own research. For science lovers, it is also a close-range learning to understand quantum computing. Popular science positions.
Currently, 11-bit quantum computing services can be precisely manipulated. A complete back-end experience for classical computing simulation environments and real-world quantum processors has been implemented in the cloud. Users can not only run custom superconducting quantum processors that meet specific requirements. Various quantum circuits, and download the results, can also be compared with the quantum processor through classical computation simulation. The test results show that the average single-bit gate fidelity of the quantum processor is 99.7%, and the average of double-bit gates is 94.9. %. What is superconducting quantum computing?
Today, we have various types of computerized computers. In fact, the basic units are all integrated transistors. Each transistor is used to represent 0 or 1 information. Through various logical operations, we can help The desired result.
However, reintegration cannot keep up with the exponential growth of information computing requirements in the digital era. Because the integration density of chips always has physical limitations, especially when dealing with specific complex problems (such as large number decomposition), existing computers may take time to process them. It will take hundreds of thousands of years.
Thirty years ago, scientists proposed a new way of computing—embedding the traditional binary computational logic in a quantum system. Using the principle of quantum superposition, the qubits are placed in a superposition of 0 and 1 at the same time, in this way, with the number of bits. With the increase, the ability to store and calculate will increase exponentially!
How can we understand this kind of exponential acceleration? We can imagine that for classical computers, two bits can only represent one of the four possibilities 00, 01, 10, and 11 at a certain moment. In quantum computing, Two bit units can accommodate four values at the same time: 00, 01, 10, and 11. That is, we can manipulate 2^N states at the same time.
There is a Chinese word for 'died sheep', which means that you find it difficult to find the target in a complicated labyrinth where you can find a way to escape. Quantum computing is like playing a mysterious labyrinth game. It can use a lot less. The qubits, at the same time, illusioned out a lot of doublings, and at the same time looking for goals on many, many mile roads, completing missions in a very short time.
In all systems that are expected to realize this beautiful idea, superconducting has won the favor of most scientists because of its unique advantages.
When we mention superconductivity, everyone thinks of 'no resistance'. In fact, superconductors have many wonderful properties. Among them, the existence of a Josephson junction has greatly expanded the application of superconductivity and even developed a new one. Subject - superconductivity.
Josephson's knot was predicted by BD Josephson in 1962. What is the phenomenon? If two superconductors are weakly connected in some way, for example, there is an ultra-thin insulation layer between two superconductors that can be crossed by Cooper pairs. Then, a superconducting current will appear between the two superconductors; if a voltage is applied across the two ends, the current passing through the junction becomes an alternating oscillating superconducting current, and the frequency is proportional to the voltage; further, if The two Josephson junctions are connected in a closed loop. With a bias current of the appropriate size, the superconducting current will be modulated by the magnetic field in the interferometer. The rich electrical properties related to voltage and magnetic field regulation make scientists think To try to use it in a quantum circuit, play the role of a PN junction in a traditional circuit.
For example, if we use a Josephson junction as a quantum inductor, under certain conditions, the electrons behave in a quantized, unequal energy level system, just as electronics has a large villa with several floors— - It can live on the first floor, and can live on the second floor. If you are happy, you can live on two floors at the same time. As long as you apply a microwave with a frequency equal to the 0 and 1 energy levels, you can easily 'live' at the 0 level. With 1 level control, coupled with the lossless nature of the superconducting circuit, one can achieve quantum coherence with long coherence time.
The actual superconducting quantum circuit looks like this:
Next, the state of the qubit is read by a cavity decoupled from the bit (indicated by Lr and Cr in the figure). With the coupling of the bit, the resonant frequency of the cavity will move slightly, moving the size of the In relation to the state of the bit, we only need to measure the signal near the resonance frequency of the cavity to read the bit state.
Finally, multiple bits are coupled through adjacent capacitors, which is the sample used by the superconducting quantum processor.
As you can see, whether or not superconducting electrics are similar to classical electric circuits, in fact, it is realized that many processing technologies rely on existing semiconductor chip processing technology. This is also superconducting quantum computing. One of the reasons favored by scientists. Put the quantum computer into the 'big refrigerator' Superconducting quantum computer is an extremely fragile system, especially sensitive to thermal noise anomalies. As we said earlier, quantum computing is based on electronics. Living at 0 level and 1 level for manipulation, the energy level of this qubit is very small, which translates into a temperature of only about 48∼960mK. In addition, at a slightly higher temperature, there will be losses inside the superconductor. Therefore, our super The computer needs a dedicated large refrigerator to keep it at a very low temperature.
This special large refrigerator, like our domestic refrigerator, is also a heat pump. Instead, it uses the endothermic phase of the He3/He4 mixture liquid to dilute the phase diffusion He3 to cool it. We can use one. The pump continuously extracts the He3 in the dilute phase and simultaneously returns the extracted He3 to the concentrated phase, so that a continuously absorbing heat-reducing refrigeration cycle can be produced, keeping the temperature at a low temperature of 10mk.
Superconducting Quantum Computation
For the prospect of quantum computing, the industry generally believes that it is necessary to achieve the following conditions to achieve universal quantum computing:
1. An extensible and well-defined qubit system; 2. The ability to initialize the qubit state to a simple reference state, such as the |000...> state; 3. A much longer decoherence time than the gate operation time; 4. Group common operation gate set; 5. Can perform efficient measurement of bits.
For superconducting quantum computing, the above threshold has been initially achieved. Due to its compatibility with traditional semiconductor processes, Google, IBM, NASA and other commercial giants have laid out this complex and esoteric computing field.
With respect to the prospect of quantum computing, the mainstream view is that when the number of qubits that can be precisely manipulated exceeds a certain number, quantum computers can make classical computers inaccessible to specific tasks, that is, under the constraints of limited time or space, more than any other Classical Taiwanese computers do a lot better. This is quantum supremacy proposed by physicist John Preskil of the California Institute of Technology.
Google has the best accumulation of superconducting quantum technology. As a practitioner of the 'quantum hegemony' concept, it proposes an algorithm for quantum random lines. Google estimates that the number of quantum bits for realizing quantum hegemony is about 49'1'.
In October last year, Google’s competitor, IBM of America, published an article on arXiv, claiming a classical simulation '2' of a 49-bit line with a depth of 27 and a 56-bit line with a depth of 23, which is intended to prove 49 bits away. Quantum hegemony cannot be realized, and then Google's plans to use 49 bits to realize quantum hegemony are overturned.
IBM's provocation was quickly attacked by Google. Two months later, Google also followed up an article in arXiv, pointing out that the number of qubits is only one aspect of quantum hegemony, and that the same important quantum circuit depth needs to be considered simultaneously. Google claims that Their first proposed quantum hegemonic roadmap '1' was not attacked by IBM, because the text clearly stated that the number of qubits reached 49, and the depth of the quantum circuit also reached 49. Google further gave the number of different qubits The detailed test results at different line depths show that if the quantum circuit depth is only 19, even if it reaches 100 qubits, it can be classically simulated as '3'.
The "Nature" report cited Martinis's own response and Scott Aaronson's evaluation of MIT's computer science authority that the paper did not weaken the rationality of the 'quantum hegemony' experiment, but instead proved the premise of Google's planned 'quantum hegemony' test'4' .
According to data provided by Google in December 2017, the time required for a classical computer to simulate quantum bits is closely related to both the number of bits and the depth. From the test results, the classic simulation has 100 qubits and a depth of 20 The quantum circuit is similar to the difficulty of having 25 qubits with a depth of 45. Therefore, at the time of the announcement, it is not possible to talk about the number of qubits. This is like if the entanglement of N qubits cannot be achieved, only the number of qubits on the chip is said. It is meaningless. In the classical simulation of quantum circuits, there are many groups of follow-up studies in China. The team led by Academician Guo Guangcan of China National University of Science and Technology used a classical computer to theoretically simulate complex quantum bit lines and achieved a depth of 22 Classical simulation of a 64-bit line '5'. This result was also published at the annual work conference of the Quantum Innovation Institute of the Chinese Academy of Sciences on February 22.
In other words, the existing experimental data all show the prospect of quantum hegemony, and any current classical computer cannot compete with a quantum computer with more than 50 qubits with reasonable quantum circuit depth. In this fierce battle In the competition, everyone wants to take the lead in putting their own flags on the high ground. The Chinese team is no exception.
Last year, Professor Pan Jianwei of the University of Science and Technology of China and his colleagues Zhu Xiaobo and Lu Chaoyang, together with Professor Wang Haohua's research group of Zhejiang University, realized the entanglement of six superconducting qubits for the first time in a superconducting system, and implemented them on this basis. Quantum algorithm '7' for fast solution of linear equations.
The on-line quantum computer is the first cloud computing platform independently developed by China Quantum to provide the public with 11 superconducting qubit quantum computing services. It can implement single-bit operation, double-bit operation and multi-bit reading. Quantum circuits composed of a variety of operations. The test results show that the calculation time, fidelity and other important performance indicators are equivalent to the IBM quantum cloud.
It is reported that the team has increased the fidelity of the two-bit gate to more than 99% in the laboratory. At the same time, 24 superconducting qubit processors have been delivered and preliminary tested. In the future, researchers will continue to improve cloud platform performance. , To provide users with more bits, more precise quantum computing services. Pan Jianwei, academician of the Chinese Academy of Sciences and the executive vice president of the University of Science and Technology of China believes that quantum computing has epoch-making significance. Quantum mechanics gave birth to the third industrial revolution, and now it To prepare for solving the bottleneck of computing capacity brought by big data.
Quantum computing, with its ultra-fast computing power, is expected to truly surpass the 'supercomputer' in solving capabilities for special problems such as artificial intelligence, quantum simulation, drug development, and quantum optimization. China has its own route map in this race. , Chinese scientists hope that in the next few years, realize the manipulation of 50 qubits to achieve 'quantum hegemony'.