What is a quantum computer?

Quantum computers are being researched and developed as the next generation of high-speed computers. Modern computers cannot solve all difficult problems, but if some of them can be solved, they are expected to have a major impact on society.

Let us first briefly look at what a quantum computer is. A quantum computer is defined in this paper as “a computer that achieves high computational speed by actively exploiting the unique physical states of quantum mechanics”. Quantum mechanics is one of the physics subjects studied at university and is a theory developed to explain the motion of very small things such as atoms and electrons. According to quantum mechanics, a mysterious phenomenon occurs in tiny things such as atoms, electrons, photons as particles of light, and matter cooled to very low temperatures that we do not normally see, namely superconductivity. It is known to have been experimentally demonstrated

Quantum computers and classical computers

 Classical physics deals with the motion and action of forces and the properties of electromagnetism, which are taught in middle and high school physics classes. Quantum mechanics, on the other hand, is the physics that deals with the “properties of atoms and electrons” and is learned at the university science level. It can be said that there are two types of calculations that correspond to these two types of physics. The difference between classical and quantum computing is explained and understood. Devices that work with quantum computers are called “quantum computers”, while devices that work with classical computers are called “classical computers”. Therefore, in this book, ordinary computers are called “classical computers”.

 On the other hand, quantum computing is compatible with classical computing; any problem that can be solved by a classical computer can also be solved by a quantum computer. This is equivalent to the fact that all phenomena that can be handled by classical mechanics can (in principle) also be handled by quantum mechanics (i.e. classical physics is an approximation of quantum physics).

Types of quantum computers

 There are several types of so-called quantum computers. For the purposes of this document, quantum computers are divided into the following three categories.

General purpose quantum computers

 A type of quantum computer that can perform general quantum computations. Explained in a little more detail, it means “a computer that can perform transitions from any quantum state to another arbitrary quantum state with sufficient accuracy”. An arbitrary quantum state here is a state of any number of quantum bits that can be converted to the desired state with a general-purpose quantum computer with sufficiently high precision (since it is absolutely difficult to do so). It can be said to be available.

 As the number of quantum bits increases and the conversions to be performed become more complex, the influence of noise also increases, so you need to be able to correct for errors during computation (error tolerance). You need. A fault-tolerant quantum computer is called a “fault-tolerant quantum computer”.

Non-universal quantum computer

 Although it cannot perform universal quantum computations, it is a quantum computer that can perform some quantum computations and has proven its superiority over classical computers. The quantum computers currently in use, which are not (or not sufficiently) fault-tolerant and are called “noisy mesoscale quantum computers” (NISQ), belong to this type.

Non-classical computer

 This is a computer that uses physical states unique to quantum mechanics to perform calculations or a quantum mechanics-oriented computer that has not yet been shown to be superior to a classical computer. Quantum annealing computers currently under development fall into this category.

Types of quantum computer models

 In the previous section, we explained the classification of quantum computers as hardware. On the other hand, there are different types of computation, and in this book, we distinguish between “generic types” and “dedicated types” as types of quantum computer models. A computational model is a model that describes how computation is performed.

Generic types

 You can write about any quantum computation. A typical example is the quantum circuit model. There are also several equivalent computational models, such as measurement-based quantum computation, adiabatic quantum computation and topological quantum computation, that are currently being studied. In this paper, quantum circuit models are described in detail.

Specialized types

 You can write specific computations. This paper describes a computational model called quantum annealing. Quantum annealing is a computational model for calculating the ground state of the Ising model, and problems can be solved by mapping to this Ising model.

Current state of quantum computing

 The current state of development of quantum computers is on par with the non-classical computers described above that are currently being tested for cloud computing. Several companies have created these non-classical test computing environments. However, the available functionality is rather limited and does not reach the level of some practically useful computations beyond the limits of classical computers.

How can quantum computers used?

Imagine a future where non-generic quantum computers are implemented and quantum acceleration is possible. Quantum computers play a role in solving problems that classical computers are not suited for. Quantum computers will also be integrated into the system.

 The important thing is that it is part of the system. For now, the quantum computer is considered a special machine. In other words, it will be used as a “machine for solving certain fixed problems at high speed”.

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