There are many prominent scientists who have worked on the idea for several decades. A Hungarian mathematician, J. Von Neumann, was the first who paid attention to the possibility of developing quantum logic. Also, Richard Feynman, a winner of the Nobel Prize in Physics, believed that quantum computing is a very promising area of science and inspired a great number of specialists to explore this method of calculations. David Deutsch, a British physicist from Oxford University, developed a theoretical model of the quantum computer. Peter Shor, an eminent American scientist, proposed a quantum algorithm that has generated great practical interest for various intelligence services. Similarly, L. Grover, an American mathematician, created a quantum algorithm for fast search in an unregulated database.

How does the quantum computer work? A quantum computer is a computing device that stores and processes information on the level of individual atoms and elementary particles. Quantum physics is the best way to describe mathematically a physical phenomenon on this subatomic level. Elementary particles constantly interact and influence each other; their positions and conditions are changed, so before observation, we can determine only the probability of their potential states. When this subatomic physical system is exposed to external influence, each of elementary particles and whole system take on different characteristics. The probability collapses, when each element of this system chooses a certain state . There are mathematical formulas that describe this process with high precision. The world of elementary particles is subject to the quantum uncertainty principle and can be described, in particular, by the wave function and the principle of superposition. According to this principle, a subatomic particle can be perceived as existing in several states at the same time.

It is difficult to perceive and understand quantum physics using conventional thinking. But one analogy may help to illustrate this theory in a simple way. Let’s imagine a person who has two gloves: right and left. He puts each glove in a separate box: the right glove goes to the first box and the left glove goes to second box, without showing you which glove goes to which box, in other words, you don’t know which box contains which glove. Then this person asks you to determine which glove lies in which of boxes. The probability that either the left or the right glove is in the first box equals 0.5.The same is true for the second box. The sum of the probabilities always equals to one. Hence, for you, both left and right gloves exist in two states simultaneously in each of the boxes. Opening only one of two boxes, you can collapse this uncertainty by observation (or as physicists say “by measurement”). If you find the left glove in the first box, it means that the right glove must be in the second box. In other words, you don’t need to open the second box in order to determine what it contains. Making an observation (measurement) of the first box, you can know which glove is in the second box, as if you opened the latter box yourself.

Let’s move on to computers. Conventional computers store information in specific locations; each of them either has or not an electrical charge. Each location corresponds to a minimum amount of information. The bit may take the meaning of either 0 or 1. A good example of a bit is an electric lamp switch, whose value is either 0 (lamp is off) or 1 (lamp is on).

The quantum computer stores information in a qubit (quantum bit). As explained earlier, the quantum computer is subjected to quantum theory. The qubit may exist in two states simultaneously: 0 and 1 with a certain degree of probability before measurement. Remember the gloves and the boxes? Drawing analogies we can assume that the qubit is the box. The two possible states of qubit are similar to the two possible states of the gloves.

Both in classical and in quantum computers, the bits and qubits are combined in large units of memory or registers. The usual two-bit register can take four values – 00, 01, 10 or 11, but only one of them at any given time. But in the two-qubit register, all four possible values exist simultaneously. Actually, a register with the size of N qubits may simultaneously have 2^{n }values.

It is important to understand why the quantum computes is more efficient for specific calculation, while simultaneously processing large amounts of information. Let’s imagine that we need to perform 100 logical operations with 100 bits of information. In a conventional computer, we will need one unit of time to perform each logical operation. So, the whole process will take 100 units of time to complete the work. However, in a quantum computer, all 100 qubits of information can be processed in one unit of time because all qubits interact with each other simultaneously, not discretely. So, this example illustrates how quantum computing can increase the speed of calculation in by a hundred times. Such operations will be useful for certain types tasks, for example, the search for required data in a very large database containing uniform records.

The prototypes of quantum computers exist today. However, they still only have small registers, which consist of a few quantum bits. Unfortunately, existing systems are not capable of providing reliable calculations since they are either poorly controlled or very susceptible to noise. However, there are no physical restrictions on the construction of the quantum computer. Developers need only to overcome some technological difficulties.

**Source:**

http://video.mit.edu/watch/explained-quantum-computing-26355/

https://www.youtube.com/watch?v=g_IaVepNDT4

Svetlana Stroganova, Nikolai Shmelev

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