Naukovi Novyny, Vol 2, Issue 10, October 29, 2020


Quantum computers


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Kateryna Hnatiuk

PhD student, Faculty of Physics, Kyiv National T.G. Shevchenko University


Computers play important role in our lives and are used almost everywhere from the usual editing of photos on a PC to computerized production control systems and the simulation of physical systems on computer clusters in research institutions. The tasks of the computers are obvious and consist in operations with data, data analysis and control of various devices using software.

We will call a computer principally based on the operations of a logic gate, a system that performs logical operation (AND, OR, NOT, etc.), a classic computer.

The basis of the logical element of the computer is a trigger. A trigger is an electrical system that can be in two stable states: "0" or "1".These states are called bits. The sequence of these states transmits information. On practice, the role of the trigger is played by a transistor.

To better understand the principle of its operation, we will draw an analogy of a transistor with a conventional toggle switch, which can be mechanically switched to "on" or "off" positions. The activation of such states in the field-effect transistors used in processors is controlled electrically, namely by the voltage at the input of the transistor. If the input voltage is less than a certain threshold value, then there is no current at the output of the transistor, which in the informational interpretation transmits bit "0"; otherwise, when the input voltage is greater than a certain value, the current passes through the transistor and transmits bit "1". The method of transmitting information is ideologically similar to the Morse code, where the text is transmitted using a combination of short and long signals (dots and dashes in writing).

Logic elements, which consist of transistors, in turn, are also combined, forming a logic that can perform a certain operation (addition, subtraction, etc.). Combining logical elements, we move to the level of the microprocessor, which will perform not one, but a certain set of instructions (operations).

Why quantum? Because some types of calculations, even the most modern conventional (classic) computers do not perform or perform quite slowly.

The practical sense of using quantum computers was first theoretically demonstrated in 1994 by P. Shore [1]. He proposed a quantum algorithm for decomposing integers into prime factors. A quantum computer can complete the above task in minutes. But the classical computer cannot perform this task effectively for numbers with more than 200 decimal points. This fact, by the way, is used in the development of the cryptographic protocol RSA, which is used to protect information [2].

Thus, the benefits of using quantum systems, primarily related to higher speed performance of certain types of calculations. The speed of these certain types of calculations by a quantum computer increases exponentially, that is, the larger the array of information which is processed, the faster it works.

The principle of operation of a quantum computer is based on the basic principles of quantum mechanics [3]. This section of physics is used to describe systems that belong to the micro world. The main provisions of this theory (discreteness, wave-particle duality, principle of uncertainty) seem paradoxical, but are confirmed experimentally. In quantum computers, the function of the trigger is performed by quantum (elementary) particles. It’s realization is possible due to certain features of such systems. For example, an electron can be in two states: spin (direction of rotation around its own axis) up, or spin down, which respectively transmit bits "0" and "1". The direction of the spin can be determined by its ability to interact with the magnetic field. Different spin orientation - different interaction. The photon enters the recording device at certain points in time: if it arrives earlier than the expected time - transmits the state "0", later - the state "1".The bits transmitted in this way are called quantum bits, or qubits [4]. Logic gates built using qubits are called quantum gates.

The construction of such mechanisms must be consistent with quantum entanglement and the principle of quantum superposition.

In quantum physics, quantum entanglement is a correlation (mutual influence) of the quantum states of the subjects of interaction, even at a great distance, while in classical physics it is believed that the particle is affected only by its immediate environment [4].

The principle of quantum superposition assumes that a system or particle can be in all possible states until it is measured.

Now, understanding the features of quantum systems, we outline the current state of development of this field and the main difficulties and challenges the developers are facing [1].

The creation of technical support for quantum computing devices requires an individual approach, as the state of each qubit may undergo certain deviations due to the above mentioned properties. This so-called "noise" leads to errors. The smaller the number of qubits, the smaller the error rate of the device. On the other hand, the greater the number of qubits, the faster the operations are performed. In 2018, for a system with more than 5 qubits, the error rate was more than a several percent. This is unacceptably high value for the modern computers.

Today, quantum computers can be divided into three general categories: analog quantum computers, digital NISQ (Noisy Intermediate-Scale Quantum) computers, and fully error corrected quantum computers. The first two types of devices have "noise", which means that the resulting calculation error limits the range of tasks that these machines can perform. NISQ computers are computers that have "incomplete" qubits (they have "noise") and are characterized by an average number of qubits. These devices became available in 2017, and had by dozens of qubits. By the end of 2017, the number of qubits in these devices had increased to 2000. This can be considered an achievement of the certain level and transition to a new stage of development. As of today, this type of devices is used to demonstrate the advantages of the quantum computer over the classical in a number of tasks and to test the basic principles of quantum devices. A fully error corrected quantum computer is a version of the quantum computer that has become more reliable through the development of quantum error correction (QEC). This technique allows "noisy" physical qubits (see examples above) to simulate stable logical qubits, which significantly reduces the likelihood of error and causes more reliable computer behavior in calculations. Further development, implementation and improvement of this technique is an important area of development of quantum computers.

Because the process of transmitting information using quantum mechanisms differs from this process in the classical computer, due to the peculiarities of quantum systems, quantum devices require their own software, which will have a new approach to implementation, in contrast to conventional computer software. And since they are likely to act as additional devices for other types of computing, such as an accelerator or coprocessor specializing in a particular task, this, in turn, creates a new urgent task for inventors - to create a mechanism for converting information from a classic computer to information that will be perceived by the quantum computer and vice versa, and the method of its transmission.

The predicted speed of certain types of calculations by quantum computers has become an activator of new developments in the field of cryptography. In future, the use of quantum computers may call into question the reliability of current methods of information security and encryption in general, as it will significantly reduce the computation time of the access key to asymmetric ciphers used to protect almost all Internet traffic and stored encrypted data. Because the implementation of such a process is highly undesirable and dangerous, there is great commercial interest in creating post-quantum cryptography long before a quantum computer with such capabilities is built.

Operating with publicly available information on progress in the field of quantum computing, there are no theoretical reasons that would not allow creating a powerful, reliable quantum computer. However, there are serious technical problems (stabilization of the quantum processor operation, development of approaches for its programming and interaction with conventional computers, etc.) on the way to creating such a system and to its practical usage for solving important tasks. Whether they will be solved and how long it will take is unknown, but the continuation of research and development in the field of quantum computing and quantum technology will expand the limits of human knowledge about the world around us.



Used sources:

1. Quantum computing: progress and prospects, National Academies Press, 2019 (https://www.nap.edu/catalog/25196/quantum-computing-progress-and-prospects).

2. I.D. Voitovych, V.М. Korsunsky, Prospects for quantum computing using superconductivity, Mathematical Machines and Systems.1 (2008).

3. І.О.Vakarchuk, Quantum Mechanics, Lviv LNU.(2004).

4. Wikipedia, Qubit --- Wikipedia, (2020). (https://en.wikipedia.org/wiki/Qubit).



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