Today, our entire scientific society fully agrees that the quantum world is a mystery because nothing happens here based on our thinking. But when quantum theory was new, it disturbed many scientists of the time, where one of them is Albert Einstein. In fact, Albert Einstein's quantum theory did not digest, because its prediction was so unacceptable. Even, to prove it wrong, he along with his two companions took the help of a thought experiment called the EPR Paradox.

This thought experiment gave birth to a unique Phenomenon that has been the cause of our research today for its strange behavior. This phenomenon is named Quantum entanglement.

Usually, Einstein and his friends did the EPR Paradox Experiment to prove the quantum theory is wrong. But it was happened completely opposite to it.

In reality, when Einstein and his friends talked about the EPR Paradox, they didn't have the proper equipment to test it. So it was only tested as a thought experiment. For which scientists could not reach any conclusion.

But after some years later, when the appropriate equipment came to us and scientists tested this experiment again, then they found that Einstein and his friends were actually wrong.

Because the result of the experiment came just as the quantum theory was predicted. Since it has been found that quantum theory is correct, it has changed our view of Whole Reality.

Now scientists have been realized that if we want to know the exact nature of the world around us, it would be known through our quantum theory. If you do not know what is quantum entanglement then this article is for you. In this article, you will know -

1. What is Quantum entanglement?
2. History of Quantum entanglement
3. EPR Paradox in quantum entanglement
4. What are entangled particles?

# What is Quantum entanglement?

According to quantum entanglement, if we have two entangled particles and if we know the state of one of these two particles, then we can say at that time what the partner's state will be about.

No matter how far away they may be, it is easy to tell the state of another particle if we know about the first particle state. After the measurement, if we get the spin of the first particle in up-state, then we can say the second particle's state will be in down-state.

This indicates that an entangled particle has an effect on the other entangled particle and that happens instantaneously.

Since the condition of the second entangled particle is being determined depending on the state of the measured particle, it seemed that the two entangled particles were informing each other about their state that's also was many times higher than the speed of light.

But Einstein's Theory of Relativity says that the maximum speed for information spreading in the Universe will be the same as the speed of light, it will never more than the speed of light.

So how is it possible that two entangled particles exchange their information at the same time from despite being two endpoints of the Universe. Because even if the exchange of information between the two entangled particles is at the speed of light, the information will take a long time to reach from one particle to another.

For this, Einstein called it a paradox and for it, he added the word paradox in the EPR Paradox experiment. Although, Einstein was agreed in it that no matter how far away the two particles were, if we could know the state of one particle, then we can say the other particle's state.

But he was not agreed in that the exchange of information between the two particles could ever be more than the speed of light.

He believed if we have local hidden variable and if we know the state of one particle, then we can say about another particle's state. Here the speed of information does not need to go more than the speed of light.

Let's understand with an example that what he was saying -

Suppose we have two different colored balls, one red and the other green. Now we are closing these two balls in one box. And then the two balls are placed in two separate boxes.

Until we open the box we do not know what color ball is in which box. But whenever we open any of these two boxes then we will know what color ball is in the other box.

For example, if we find the red ball in the first box, then obviously we can say there is a green ball in the second box. And another way, if we find the green ball in the first box, then obviously we can say there is a red ball in the second box.

Here the information is not traveling as faster than light. Rather, from the very beginning, we knew that if there was a red ball in the first box, then there would be a green ball in the other box. We were able to say this because we already had local information.

But one of the major problems of Einstein's logic is that the quantum entanglement doesn't work this way. In this einstein example, although we don't know what color ball is in which box until we open the box. But here the ball is already present in that box.

But in quantum entanglement, nothing is fixed from its beginnings. Before the measurement, the quantum particle is present in both its state simultaneously. That means they are always in the superposition state.

Quantum entanglement is mysterious, the reason behind is that the state of the entangled particles is not fixed from its beginning.

Quantum entanglement was first experimented by John Stewart Bell where he got it right. After this, the test has been done several times where the distance between the two entangled partners is also increased. And every time the same results came.

Although, today we know how quantum entanglement works. But even today, it is mysterious that by what medium they use to exchange information faster than the speed of light.

Many of you are wondering if the exchange of information in quantum entanglement happens at a faster than light then isn't it violating the special theory of relativity?

So, the matter is that scientists are divided into two groups for this. Where the first group of scientists believes that Quantum entanglement violates the  Special Theory of Relativity, and on the other side the second group of scientists believes that quantum entanglement does not violate the Special Theory of Relativity. However, still today the Quantum entanglement is a mystery to us.

## History of Quantum entanglement

This happened in 1920 when the debate started between Einstein and Neil Bohr to take what is the Nature of our Reality.

On one hand, Einstein believed that our reality is always fixed. Whether we observe it or not, there is no effect will happen on the interval of its destiny.

He believed that the moon is always in its state, whether we see it or not. In fact, our entire classical physics is based on this concept.

So we easily believe that everything that we observe around us is always in presence at that point even when we are not observing it.

But the quantum theory does not agree on it. Neil Bohr's point was that in the absence of observation and measurement, our reality does not exist.

In the case of quantum particles, we cannot say anything about its characteristics unless we observe or measure it. Because before observation and measurement they do not stay in a well-defined form.

Rather than, they are always present in all possible states simultaneously. Which we call superposition state.

Physical Nature of Reality doesn't make any sense at this time. Because at that time these quantum particles have no physical form.

We can only represent the reality of this time with a wave function. But whenever we observe and measure these quantum particles their wave function becomes collapsed.

For which, in all of their possible states just one of them is selected and that's we get in the end. Since we get only one state after our observation and measurement, we think that is the actual form of that quantum particle.

But the fact is that we could get any of those states. This also means that it is not fixed from the beginning of what state we will get after observation and measurement.

This means that in the absence of observation and measurement our reality has no meaning. Copenhagen's interpretation is also based on this theory. Which is also still considered accurate in quantum mechanics.

Einstein's did not digest this theory, he believed that everything is real from its beginning. Whether we observe it or not, there is no effect will happen on the interval of its destiny.

In the Prediction of Quantum Theory, he states that quantum theory is still incomplete. It needs to have some local hidden variables that we can use them to predict everything accurately without using the concept of the superposition wave function.

Albert Einstein, along with Boris Podolsky and Nathan Rosen, announced the EPR paradox test to prove Neil Bahr's theory wrong. And it was one of the best examples of quantum entanglement.

In fact, at that time, there was no equipment that could test this theory. Thus the EPR Paradox was presented only as a Thought experiment.

## EPR Paradox in Quantum entanglement

In the EPR Paradox experiment, The word Paradox was present because, In this experiment, it seemed that the exchange of information between two particles was traveling faster than light.

Which was in violation of Einstein's Special Theory of Relativity. In which, he said that the maximum speed of information to spread in this universe must be the same as the speed of light. This means that information can never travel at a speed higher than the speed of light.

To understand quantum entanglement, we need to understand two important concepts -

1. Superposition principle
2. Measurement rule

Superposition Principle

As we have already known that, unless we observe and measure these quantum particles they do not remain in one state but they are always present in all possible state simultaneously. Which we call superposition state.

Electron-like subatomic particles have an intrinsic property that we call spin. The spin of such particles may be up or otherwise down. That means there are two possible states of such particles.

Now if we talk about superposition principals it says that unless we observe these particles they do not remain in one state of this two-state but they are always present in their two possible states simultaneously.

Which means they are present in both the up-state and the down-state simultaneously. We call this state as a superposition state.

But the point here is that the superposition state is not the same as raising a coin. If you talk about a coin, then it has two-phase a head and a tail.

This means that there are two possible states of a coin. When we raise a coin, we do not know whether it will give head or tail. As long as the coin is rotating, we do not know what position it is in.

It almost like the superposition state but it's not. Because from the beginning of the coin, the head and tail are always hidden on the two opposite sides.

But if we talked about superposition state, then there is nothing fixed in it from it's beginning.

Basically, superposition state means that quantum particles are not in one state. Rather, they are present in their all possible states simultaneously.

Which means they are in both the up-state and the down-state simultaneously.

I hope you understand the state of the superposition. Now another concept that you need to know about is measurement rule.

Measurement rule

The Measurement rule states that until we observe these quantum particles they remain in superposition.

If we assume up and down spin are the two quantum states, then they are present in this superposition state simultaneously.

Now whenever we measure them, their wave functions become collapsed. As a result, one of the states is selected in their all possible state and we get this one at the end.

## What are Entangled particles?

Quantum particles are used in quantum entanglement. So let's understand about quantum particles now. Suppose we have an Energetic Light particle such as photons. We can convert this energy into mass using Einstein's mass equivalent equation. For example, from a photon, we can create two separate particles.

$\LARGE&space;E=mc^{2}$

Where,
m = mass of the particle
c = speed of light

But the universe has some laws that determine how these particles will be. As we know, photons have no electrical charge, which means its charge is zero. For which the sum of all the particles created by this photon must always be zero.

For example, if an electron is created from a photon, then a positron will also be created. Because the charge of the electron is negative, a positive form of the electron needs to be created to cancel it out, which we call positron.

There is one more property associated with it which we know as Conservation of Spin. Photons have no spin but electrons and positrons have spin. This spin might be up-state that is positive and it may be down-state which is negative.

This means that if you add their spin, the sum of their spin will always be zero. Because these are made by photons that have no spin.

This means that it is certain that if one of them spin is up, then the other's spin is always in down. But we can't say whose spin will up and whose spin will be down.

If we look at it as superposition state of quantum theory, they remain in the up-state and the down-state simultaneously. unless we observe it.

Now if we talk about entangled state, then this is a state where the two particles are in such a way that they exist only in the combined state. This means we cannot say anything about the state of one without knowing the condition of these two particles.

If you don't understand it yet, let's understand it a lot easier -

Suppose we have an electron and a positron whose spin may be up or down. We don't know whos spin will up and whose spin will down. For which we can represent it in a superposition state like -

Now if we get the spin of the electron in the up-state, then we will definitely get the spin of the positron in the down-state. Similarly, if we get the spin of the electron in the down-state, then we will definitely get the spin of the positron in the up-state.

But since they are in the superposition state, so nothing is fixed here from the beginning. Here the electrons are in both the up-state and down-state simultaneously. Similarly, the positrons are also in both the up-state and down-state simultaneously.

For this, we cannot say the spin state of one without knowing the spin state of the other. This means in the entangled state particles can only be described in the join state.