How do Quantum Computers work?

Now people talk a lot about new computing technologies. In particular, the words “quantum computing”, “quantum Internet” and even “quantum cryptography”. Let’s see what it is and whether we need it. Let’s start with a quantum computer.

Bits and qubits

In a conventional computer, all calculations are based on the concept of “bit”. This is an element that can take the values ​​0 or 1. Physically, this is implemented as follows:

  1.  The computer has a part called a transistor. Imagine that it is a crane on a pipe: if you turn it on, water will flow, if you turn it off, it will stop.
  2. In a transistor, water is electricity, and turning the tap on and off also depends on electricity. Imagine that the taps are interconnected so that the water from one tap turns on or off the other tap, and so in a cascade along the chain.
  3. Transistors are connected in such a functional way that when they turn on and off, they can perform mathematical calculations.
  4. Due to the fact that there are a lot of transistors (billions), and they work very quickly (close to the speed of light), transistor computers can perform mathematical calculations very quickly.
  5. All that you see on the computer is a derivative of the calculations. You see a window, letters, pictures, and somewhere in the very very depths. It’s just addition and subtraction, and even deeper – turning on and off the taps with electricity at the speed of light.

Concept of superposition

The transistor in the computer can take the value 1 or 0, that is, “on” or “off”. In terms of computer logic, this transistor is called a bit. This is the smallest unit of information in a computer. Physically, the bit can be in the processor, on the memory chip, on the magnetic disk, but the essence is the same: it is some kind of physical space that is definitely either on or off.

The keyword here is “definitely”. A programmer and engineer can know exactly what state a particular bit is in. The charge in it is either there or not, no intermediate states exist there.

In a quantum computer, instead of bits, qubits. Qubits are quantum particles that have an interesting feature: in addition to the standard 0 and 1, a qubit can be between zero and one – this is called a superposition.

All solutions are already known

Another feature of qubits is the dependence of the value on the measurement. This means that the programmer does not recognize the value of the qubit until it measures it, and the fact of measurement also affects the value of the qubit. It sounds strange, but this is a feature of quantum particles.

But, It is due to the fact that the qubit is in all states at the same time until it is measured, the computer instantly goes through all possible solutions, because the qubits are interconnected. It turns out that the solution becomes known as soon as all the data is entered. Superposition also gives that parallelism in calculations, which speeds up the work of algorithms by several times.

The whole difficulty is that the result of a quantum computer is the correct answer with some degree of probability. And you need to build algorithms in such a way that as close as possible the probability of the correct answer to unity.

How do qubits and what is the complexity

As simple as possible: to get a working qubit, you need to take one atom, fix it as much as possible, protect it from extraneous radiation and connect it with another atom by a special quantum bond.

The more such qubits are interconnected, the less stable they work. To achieve “quantum superiority” over an ordinary computer, at least 49 qubits are needed. And this is a very unstable system.

The main difficulty is decoherence. This is when many qubits depend on each other and anything can influence them: cosmic rays, radiation, temperature fluctuations and all other phenomena of the world around.

Such a “phase noise” is a disaster for a quantum computer, because it destroys the superposition and forces the qubits to take limited values. A quantum computer is turning into a regular one – and very slow.

Decoherence can be fought in many ways. For example, the D-Wave company, which manufactures quantum computers, cools atoms to almost absolute zero to cut off all external processes. Therefore, they are so large – almost the entire place is occupied by protection for the quantum processor.

Why do we need quantum computers

One of the most important applications of a quantum computer right now is prime decomposition. The fact is that all modern cryptography is based on the fact that no one can quickly decompose a number of 30–40 characters (or more) into prime factors. On a regular computer, it will take billions of years. A quantum computer can do this in about 18 seconds.

This means that there will be no more secrets. Because any encryption algorithms can be immediately hacked and accessed to anything. This applies to everything – from bank transfers to messages in the messenger. Perhaps an interesting moment will come when conventional encryption will stop working and quantum encryption will not be invented yet.

Quantum computers are also excellent for modeling complex situations, for example, calculating the physical properties of new elements at the molecular level. This, perhaps, will allow you to quickly find new drugs or solve complex resource-intensive tasks.


Now quantum computers do not know all this – they are too complicated in production and very unstable in operation. The maximum that can be done so far is to sharpen a quantum computer under a single algorithm in order to get a huge performance gain on it. It is for these purposes that the largest companies are buying them. In order to quickly solve one or two of the most important tasks for themselves.

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