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This article comes from the official account of Wechat: ID:fanpu2019, author: innocence

How do ordinary people get access to real quantum computers? The answer is "quantum cloud"; how do we Chinese get access to real quantum computers? The answer is "our own quantum cloud".

Writing | innocence (Quantum Computing Practitioner)

Recently, "Quantum Cloud" has become a hot word in science and technology. In May 2023, IBM pushed the previously released 433 qubit "Osprey" processor to the quantum cloud platform. At the Zhongguancun Forum held in Beijing at the end of May, the Beijing Academy of Quantum Information Sciences ("Beijing Quantum Academy") officially launched the "Quafu" Quantum Computing Cloud platform, which was jointly developed by the Beijing Quantum Academy, the Institute of Physics of the Chinese Academy of Sciences and Tsinghua University.

Perhaps many people do not know what "quantum cloud" is, or it is easy to think of cloud computing, the two are actually two completely different things. Today we are going to talk about this closely watched quantum computing power sharing model-quantum cloud.

What is a superconducting quantum computing system quantum cloud? First of all, let's quickly introduce the difference between quantum computing and classical computing. The computing methods we use today, including computers, mobile phones, calculators, etc., are based on binary logic, and the lowest level of information storage and processing units are called bits. A bit can be in one of the two states of 0 or 1, connect a large number of bits through the circuit, and perform a series of logical operations on it, such as "and gate, and non-gate, XOR gate" and so on. finally, the state of the set of bits in which the calculation results are stored can be obtained, so that a variety of operations can be carried out. This kind of calculation is called classical calculation.

Here we see the four elements of computing: the first is the storage unit of information-bits; the second is a set of general logic gate operations acting on bits; the third is the algorithm, that is, how logic gates are organized and mapped to bits; the last element is reading. Quantum computing also needs these elements, and after making use of the basic principles of quantum mechanics such as superposition and entanglement, it can show the ability that many classical computing does not have.

The basic information processing unit of quantum computing is qubit, which is the simplest quantum system-two-level system. As an analogy, we can mark these two energy levels as 0 and 1, respectively. Because of the superposition of quantum states, such a system can be in the superposition state of 0 and 1, that is, the qubit can be part 0 and part 1. This superposition property gives qubits the ability to express multiple states at the same time, so it has a stronger ability to encode information. When multiple qubits are connected together, we can entangle them, which is also the ability that classical bits do not have.

It is difficult to explain the entanglement of qubits in detail, but we can understand it like this: in the entangled bits, the expression of information must be seen as a whole, and its dimension increases exponentially with the increase of the number of bits. this provides an exponentially growing coding space for computing, which can theoretically achieve exponential computing acceleration. If we can find such a pair of energy levels (that is, entangled qubits) and can continue to expand, perform accurate quantum gate operations on these qubits, and then accurately measure their quantum states, and finally design a good quantum algorithm, it will be possible for us to complete some incredibly efficient calculations. In fact, there is a real example-the famous Shor algorithm, which can reduce the complexity of the large number decomposition problem to the quasi-polynomial level; in theory, this algorithm may crack the RSA cryptography or elliptic curve cryptography commonly used on the Internet in a very short time, and the resulting threat degree can be said to be directly related to national security. This is one of the reasons why countries are investing heavily in the quantum computing / information industry.

In reality, there are many kinds of physical systems that can construct qubits, which can be based on photons, electrons, atoms, molecules, nuclei, lattice defects, etc. Readers who are familiar with quantum computing may have heard of superconducting quantum computing, ion trap quantum computing, semiconductor quantum computing, optical quantum computing and so on, which are essentially different technological routes developed based on different physical systems. The progress is also different. At present, superconductivity and ion trap are considered to be the two most promising technical solutions. IBM's Osprey processor and Quafu Quantum Computing Cloud platform are based on superconducting solutions.

Of course, whether it is superconductor or ion trap, the (quantum) hardware developed by it is still in the stage of application demonstration. The rapid development of industry requires the coordination of R & D and practical application. On the one hand, there is an urgent need for applications to put forward reasonable practical requirements, which can enable developers to plan future research directions and technical routes more accurately; applications also urgently need to test and optimize their algorithms on real quantum hardware. in order to produce practical benefits as soon as possible. On the other hand, the technical threshold and capital investment of quantum computing are very high, and high-quality quantum computing resources are very scarce, whether they are engaged in the theoretical research of quantum algorithms in university research institutes or in the exploration of quantum technology needs of enterprises. it's hard to get these quantum computing resources. Therefore, practitioners need an open sharing mechanism to closely connect the R & D side with the application side. At present, people think that the best way is the quantum cloud platform.

Performance and Connectivity of P136 Quantum Cloud View (Source: http://quafu.baqis.ac.cn/) how to build a quantum cloud platform? If there is a fine calibrated quantum chip measurement and control system, which opens a set of API, that is, application program interface, to the Internet, we can access the quantum measurement and control system through the Internet, send quantum circuits to it and obtain the returned measurement data. If we can further provide a relatively complete set of tools and scheduling system to help users compile and optimize quantum circuits, as well as bit mapping, and ensure that a large number of users can access it at the same time, we will build a quantum cloud platform.

It is necessary to understand what is "quantum measurement and control system" and "quantum circuit". In the previous article, we have roughly combed several elements of computing: information storage and processing unit (bits), complete general logic gate set, algorithm and reading. The quantum measurement and control system is to solve the physical realization of the general quantum gate and the reading of quantum states. The former corresponds to "control" and the latter corresponds to "measurement".

Taking the single-bit revolving gate as an example, if we want to rotate the state on a qubit QA 180 degrees around the X-axis, the actual operation is to apply an area-accurate resonant microwave pulse to the QA. Such a pulse signal is usually edited and generated by an arbitrary wave generator at room temperature, and the qubits are extremely low, so how can we accurately send this signal near the specified bits? This requires a cable to connect the pulse source to the control line of the QA. This sounds simple, but it is not easy to achieve. Scientists and engineers need to ensure that not only enough signals are transmitted, but also that the heat at room temperature is transmitted as little as possible. make sure that thermal noise and other noise sources do not sneak down the cable to cause damage near the qubits. For this reason, on the one hand, we need to use special low-temperature coaxial cable, on the other hand, we need to insert a variety of attenuators, filters and other devices step by step, the whole link is very exquisite. In the aspect of reading, we need to amplify the extremely weak quantum signal step by step and give it to the acquisition card at room temperature for acquisition and processing. All these are used to achieve accurate quantum gate manipulation, accurate quantum state reading related electronics, cables, various microwave devices, sample boxes, and so on, are the physical part of the complex and exquisite "quantum measurement and control system". In addition, the software part of measurement and control is also very important, which is responsible for efficient management of measurement and control equipment, waveform control, data processing and visualization, as well as related tool chains and so on.

Schematic diagram of quantum measurement and control system? picture source: IBM Quantum and quantum circuit is a logical sequence formed by organizing the above-mentioned quantum gates and readings according to specific purposes. The Shor algorithm mentioned earlier is essentially a quantum circuit. However, it is still quite abstract, and it generally needs to make some transformations to run on the actual quantum computer, which will use the quantum cloud compiler optimization tool. On an open quantum cloud, users can submit a variety of quantum circuits, as long as the basic rules are met. This openness makes the situation that quantum clouds have to deal with, to some extent, more complicated than conducting quantum experiments in the laboratory.

You can imagine the following scenario: users directly access real quantum computing resources through web pages and APP, and advanced users can even integrate into their own applications to build their own quantum applications. All this work can be done in the office without having to visit a quantum computing lab full of circuits, let alone build instruments and debug circuits-this is a quantum cloud platform that can actually be used. It can be seen that quantum cloud greatly reduces the threshold for users to use quantum computing resources, allowing more people to quickly verify and improve their ideas on real quantum computers. Getting more smart brains involved is a shortcut to the practical use of quantum computing.

Quantum cloud is highly competitive, independent research and development is the key to the earliest promotion of quantum computing in the form of quantum cloud is IBM,2016 year they launched the first 5-qubit quantum cloud. Today, IBM has launched as many as 25 quantum cloud computing platforms, and the publicly accessible systems currently contain up to 433 qubits. Since 2017, there have been teams in China to explore this model, the earliest being the 12-bit quantum computing cloud platform launched by the Institute of Quantum Information and Quantum Science and Technology Innovation of the Chinese Academy of Sciences. At present, many research institutions and enterprises have provided quantum cloud services. The Quafu quantum cloud platform released at the Zhongguancun Forum is the first time that China has launched a quantum cloud with a scale of more than 100 bits, and its significance is actually very great. Quantum computing on the scale of 100 qubits is much more complex than before, from chip design, extremely low temperature circuit layout, measurement and control electronics integration to measurement and control software system architecture. Combined with higher-level compilation, optimization and cloud front-end software system design and testing, it can be described as a systematic quantum engineering challenge.

The United States currently has the most advanced quantum cloud technology, and the ecological layout of quantum applications based on this is also more in-depth. It should be noted that foreign quantum clouds have imposed great restrictions on China, and we have long been unable to access the best and most advanced quantum resources (we can only visit IBM's earlier quantum cloud system with fewer bits). Therefore, the domestic independent research and development of high-quality quantum cloud platform is very important to the development of quantum computing technology and industrial ecological construction in China.

IBM's online quantum computing system, 433qubit Osprey processor launched for the first time (source: https://quantum-computing.ibm.com/) finally, we should also realize that there is a gap between China and IBM in the layout of quantum cloud, and the ecological construction of quantum computing applications around quantum cloud is just in its infancy, and there is still a long way to go. Along the way, more scientists, engineers and entrepreneurs from different fields are needed to join hands to move forward. Hope that China's quantum computing technology can be steady, scientific research and industry go hand in hand, in this subversive new track, to the top of the world!

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