The 20th century gives us quantum theory, which explains the structure of matter, from the simple hydrogen atom to complex materials. This understanding led to the creation of computer chips and to the Information Age.

According to Moore’s Law (1965), the number of transistors on the same area of computer processors **increased** approximately twice every two years, ensuring the technological boom of modern society.

The size of computer components is rapidly declining and today they are of the order of the atomic dimensions, which are the natural limit at which the laws of the microworld, that is, of quantum physics, come into force. After this fundamental limit, **it is no longer possible** to develop traditional technologies, and therefore the basis of future technologies is *quantum physics.*

## What is Quantum Entanglement

**Quantum entanglement** is a specific feature of quantum systems, in the presence of which the quantum states of two or more objects turn out to be ** interdependent**.

Such interdependence persists even if these objects are spaced apart in the space beyond known interactions, which is in logical contradiction with Einstein’s locality principle, according to which if two systems **A** and **B** are spatially separated and do not interact, in a complete description of the physical reality, the actions performed on** system A** must not alter the properties of **system B.**

### Two subsystems are entangled, between which there are quantum correlations on a parameter that accepts at least two values for each of the subsystems. Measuring the state of one of the subsystems, unambiguously determines the state of the other. The joint state of the two subsystems is then called entanglement.

For three subsystems an unambiguous definition of the entangled states cannot be given, since when measuring the state of one of the subsystems, the other two may either take certain values.

## Brief History of Entanglement

The history of quantum entanglement began with the dispute between Bohr and Einstein at the Fifth Solvay Conference (1927) when the principles of the Copenhagen interpretation of quantum mechanics were discussed.

**Einstein **insisted on maintaining the principle of determinism in quantum physics and on interpreting the results of the measurement from the point of view of an “*unrelated observer*.” On the other hand, **Bohr **insists on the fundamentally non-deterministic (statistical) nature of quantum phenomena and the irreversible effect of state measurements.

“God does not play dice.”

— Einstein

Einstein’s dialogue with Bohr is often cited as the highlight of these disputes: “*God does not play dice*.”, *“Einstein, don’t tell God what to do*“, and Einstein’s sarcastic question, “*Do you really think the moon only exists when you look at it?*“.

As a continuation of these disputes in 1935. **Albert Einstein**, **Boris Podolski**, and **Nathan Rosen** formulate the **EPR paradox** – in the presence of two particles of common origin, by measuring the state of one particle, one can predict the state of the other without performing a measurement.

They offer two possible explanations for this interesting fact. According to them, either:

- (a) measuring the state of particle A has a nonlocal effect on particle B
- (b) quantum mechanics is an incomplete theory and it gives only partial information about the state of a system with classical correlations.

In the same year, analyzing such theoretically interconnected systems, **Schrödinger **called them “*entangled*” and he considered particles to be entangled only if they** physically interacted**.

## Why the Entanglement is Important for Quantum Computers

In quantum computers, changing the state of an entangled qubit will immediately change the state of the other entangled. Therefore, the quantum entanglement improves the processing speed. According to this research from the *University of Bristol*, quantum entanglement is necessary for a quantum algorithm in order to offer an exponential increase of speed over classical computers.

Some of the identified applications in quantum computing include:

– the process of obtaining 2 classical bits using only 1 entangled qubit.*Superdense coding*– quantum cryptography benefits from the**Quantum Cryptography***no-cloning*theorem which states that: “it is impossible to create an independent and identical copy of an arbitrary unknown quantum state”. Therefore, it is theoretically impossible to copy data encoded in a quantum state.– the process of exchanging quantum information between two parties*Quantum teleportation*“.**“instantly**