 # What is a quantum computer? How qubits and quantum computing work

A quantum computer is a means of computing where the laws of quantum mechanics are the basis for the operation of the central processor. Such a computer is fundamentally different from traditional PCs based on silicon chips. This device does not use classical algorithms, but rather processes of quantum nature – quantum algorithms that use the effects of quantum mechanics, such as quantum parallelism and quantum entanglement. The basis for calculations of this type is a cubit, a system in which the number of particles is analogous to momentum and the phase variable (energy state) to coordinate. The phase cubit was first realized in the laboratory of the University of Delft and has been actively studied ever since. Cubits can be as if in two states simultaneously: contain zero and one at once. Because of this, a quantum computer can perform specific mathematical tasks thousands of times faster than classical computers.

Quantum computer principle Quantum computers are often misunderstood because of the word “computer” in their name. When people hear the word “computer,” they think of laptops or phones, but the fact is that these devices and even the largest supercomputers in the world operate on the same fundamental circuitry. However, quantum computers have fundamental differences and cannot be called computers in the usual sense of the word.

Quantum computing systems are devices that use quantum superposition and quantum entanglement phenomena to transmit and process data. Such devices operate with qubits (quantum bits), which can simultaneously take a value of both logical zero and logical one. Therefore with increasing number of qubits used the number of simultaneously processed values increases exponentially. In a quantum computer the main element is a qubit – a quantum bit. Unlike a usual bit it is in a state of quantum superposition, i.e. it has a value of both 0 and 1, and any combination of them at any time. If there are multiple qubits in the system, changing one also entails changing all the other qubits.

This allows all possible variants to be computed simultaneously. A normal processor, with its binary computation, actually computes options sequentially. First one scenario, then another, then a third, etc. To speed it up, they started to use multithreading, running calculations in parallel, prefetching to anticipate possible variants of branching and calculate them in advance. In a quantum computer all this is done in parallel. The principle of computation also differs. In a sense quantum computer already contains all possible variants of problem solution, our task is only to count qubit states and… choose the right variant from them. And this is where the difficulties begin. This is the principle of a quantum computer.

For what tasks can a quantum computer be used? A quantum computer is not able to completely replace a classical computer. A usual computer can solve many problems, but nevertheless, there is a class of problems which a quantum computer can solve in an hour, while a classical computer would need the lifetime of the universe. The currently known problems of this type can be divided into 4 groups.

## Problems with the Fourier transform

These are mainly cryptography and encryption problems: the same Shor algorithm that can allow breaking RSA and Bitcoin. This is because the quantum Fourier transform is incredibly fast and, if you find the right application, it gives exponential acceleration. Optimization problems This includes combinatorial problems that can only be solved by enumerating all possible options, such as the maze discussed above. Another notorious quantum algorithm, Grover’s algorithm, allows to solve such problems faster than the usual brute force, however, it does not give such a strong acceleration as Shor’s algorithm. Combinatorial problems arise all the time in logistics, optimization, and economics.

## Quantum Machine Learning

A third quantum algorithm that gives a noticeable speedup is the HHL algorithm. It can solve a system of linear equations exponentially faster than any classical algorithm; as we know, linear equations occur everywhere, such as in machine learning problems. Quantum-assisted machine learning is one of the most useful applications of quantum computers. And in general, using quantum physics in artificial intelligence tasks is cool: you can, for example, use quantum samples that are in a state of superposition of several classical samples.

## Quantum system simulations

This is the most natural application of quantum computers. This approach was suggested by Feynman: to simulate a very complex quantum system you need another complex quantum system that you know everything about and know how to operate it. So a full-fledged quantum computer will help to create new materials, new drugs, high-temperature superconductors. This is a problem where you have to organize the interaction of atoms in a clever way, but to understand how to do this, classical computers need trillions of years of computation, while a large quantum computer takes a few hours.

## How is a quantum computer different from a conventional one?

Quantum computing and quantum entanglement – these very concepts were invented just 30 years ago, and the first papers by scientists were not even taken into scientific journals: they were said to be science fiction, not science. Today, quantum systems not only exist, but are sold for money, creating and solving new security problems, mainly in the field of cryptography. Quantum computers are machines based on a unique behavior described by quantum mechanics, and completely different from the behavior of classical systems. One of such differences is ability of a particle or a group of particles to be in some relation only in two discrete quantum ground states – let’s call them 0 and 1. Quantum computer is unsuitable for most daily business, but it can quickly solve mathematical problems, on which modern cryptography is based. The fundamental difference between a quantum computer and a conventional one is that its operating unit, the cubit (quantum bit), can be in a state of uncertainty, or, if you like, in several states simultaneously. It sounds confusing, even more complicated in practice, but as years of research have shown, it works. 