Quantum Entanglement explained

Laws of humans rule the humanity and laws of physics rule the whole universe. In day to day live, there occurs many events which are quite explainable. Because, all events supports the laws of simple classical mechanics. For example, the projectile motion, laws of inertia, measurement of current and voltage in a circuit, boiling of water etc. The science behind such types of events can be explained with the simple laws of physics. We can explain what would happen when two basketballs are made to collide with each other. But what would happen when we collide two subatomic particles with each other? Will they behave like the two balls? Simple classical mechanics cannot explain this. So, welcome to the Quantum world.

Quantum mechanics is the study of the whole universe in a smaller scale. Every objects are made of some subatomic particles which are quite invisible with the naked eye. Quantum mechanics is a tool of studying the behaviors and properties of such particles. Because, the laws of classical mechanics do not work here. 

In here, the particles also behave like waves, the electrons can be different places at the same time, they show the tunneling phenomena (i.e the electrons do not move from one orbital to another, rather they vanish and reappear at the destination at the same moment. It seems that they use some kind of tunnels which are directly connected with the source and the destination). As, classical mechanics could not able to answer this, that's why new theories were developed for solving such types of problems. One of the greatest and most amazing theory is the Quantum Entanglement which even shocked Einstein.

Spooky Action At A Distance:

Quantum Entanglement is a quantum mechanical phenomenon where two particles are described with reference to each other. That means, their behaviors cannot be explained individually. Action of one of the two particles will affect the other entangled particle. The measurement of physical properties such as, position, momentum, spin and polarization on entangled particles are found to be correlated. That means, if a pair of entangled particles are generated so that the total spin is made to zero and one particle is spinning down, then the other entangled particle will be spinning up. That means, they are connected with each other.

It seemed that, this quantum mechanical phenomenon was violating the theory of special relativity; according to which the maximum velocity of any information should not exceed the speed of light. This made Einstein really unhappy. So, to prove this theory wrong; Albert Einstein, Boris Podolsky and Nathan Rosen published a paper on this topic which was later called as EPR Paradox. According to them, such behaviors of quantum particles are not possible, because they violate the laws of physics. That's why Einstein called it, "Spooky action at a distance."

How does Quantum Entanglement work?

In case of quantum particles, they show Superposition Principle. That means, these particles occupy different states at the same time. It is not fixed which results will be got by observation. But when an observation is made, the wave function gets collapsed and we get only one outcome. 

Let us take a photon having energy E=hf. According to Einstein's mass energy conservation theory, energy can be converted to mass and vice versa. We all know that photons do not have charge as well as mass. So, if photons are converted to electrons, then two particles will be formed where the net charge will be zero. That means, if electron is formed that an opposite version of electron should be formed which is called Positron. 

According to conservation of spin, the net spin in a photon should be zero. That means, the direction of spin of the entangled particles will be opposite with each other. It cannot be said which entangled particle will show which spin of direction. Because of the superposition principle, the entangled pairs will be existed every possible states at the same time.

After making an observation, the wave function gets collapsed and we will get a certain outcome; that means the electron may be spinning up and the positron is spinning down or the electron is spinning down and the positron is spinning up. That means, by observing the state of one entangled particle, we can safely say the state of the other particle. For example, a pair of entangled particle is taken and one particle is sent to the Mars. After observing, if the particle in the Earth shows spin up, then we can safely say that the particle in Mars has shown spin down. It seems that they are interlink with each other.

Why it was a paradox?

We have seen that, the entangled particle pairs are exchanging information greater than the speed of light. That means, the speed of exchanging information from one particle to other is completely independent with the distance between them. It seems that the laws of special theory of relativity are broken by this phenomenon. 

That is why, Einstein gave an explanation about this. According to him, the quantum theory was not complete at all and there should be some local hidden variables through which we can predict the state of the particles without even using superposition principle or wave function. He proposed that, if we have some local hidden variables and we know the state of one particle then we can definitely predict the state of the other particle. In this case, the information do not have to travel greater than the speed of light.

Let's make it understandable. For example, we have two boxes and two gloves; one is right handed and other one is left handed. We have put the two gloves in the two boxes randomly. Now we do not know which box contains which glove. You can imagine that they are in superposition state. After opening one of the two boxes, let us assume we got the right handed glove. Then we can obviously say that the other box contains the left handed glove.

In this case, we had the local variable (in this case the local variables are right and left handed gloves). When we open one of the boxes we will know about the type of the glove. As we already know that there are only two types of gloves, i.e right and left handed glove (hidden variable), so we can definitely predict about the other box. In this case information has not traveled greater than the speed of light.

But the problem with this explanation is, quantum entanglement does not work like this. In case of the above example, we already knew that there are two types of gloves in the two boxes. If we get right handed glove in one box, then obviously we will get left handed glove in the other box. But, in case of quantum entanglement, there is not any fixed result. Every quantum particle exist in superposition state. 

After making an observation, unlike the two gloves if we get one particle spin up, it does not mean that the particle was also in that particular state before observation. This time we get spin up, but later we may get spin down for that same particle. In case of the two gloves, one of the two boxes we got right handed glove. This means that the right handed glove was actually in that box, we just could not see it. But in quantum world, bizarre things happen.

This was kind of nonsense for Einstein. Because, he actually knew that the entangled particles exchange their information greater than the speed of light. But, this breaks the laws of special theory of relativity. That is why he called it a Paradox.

How entangled particles are created?

To create entangled particles in laboratory, Spontaneous Parametric Down Conversion method is used, where a non linear crystal like Barium borate is used to incident unpolarized photons and emit two polarized photons or entangled particles. 

Is faster than light communication possible?

One might think that using quantum entanglement, we can make faster than light communication possible. But actually, it cannot be. Because, manipulation of such entangled pair breaks the connection of the two particles. That means, if one particle is forcibly turn to spin up, then at the instant the particles will not remain entangled pair any more. They will become individual particle at the instant.

Is quantum teleportation possible?

Quantum teleportation is a process of transferring quantum information from one place to another. It is not what you think actually. It is not about classical teleportation what you have seen in Star Trek movie. Rather the physical bodies, quantum information are sent from one place to another. As, the quantum information has been exchanged, it seems that the quantum particle has been teleported to another place. But, how can we do this actually? 

For example, two persons A and B are going to exchange their quantum information. Let A want to teleport a particle to B which are at a very long distance. To do this, they must have a pair of entangled particles. A and B each will keep one particle with them. Now, A will have two particles (one is the particle which will be teleported and the other one is entangled particle) and B will have one entangled particle. 

Before observing, each and every particle will remain in superposition state. So, to make the teleportation possible, A have to measure the combine state of the two particles (teleporting particle and entangled particle) at the same time rather measuring individually. This technique is called Bell Measurement. Due to the measurement, the wave function will get collapsed and A will get a certain outcome of his/her measurement. This information will be transferred to the entangled particle of B and he/she can clone the particle by manipulating. 

Now, A have to inform B about the outcome of the measurement so that he/she can manipulate it. After having the information of the combine state of the particles, B will manipulate it according to information what he/she got from A. After manipulation, the entanglement between the particles will be broken and the information of the particle of A will be teleported to B. As the properties of the particle of A has been transferred to the particle of B, it seems that the partcle has been teleported. 

To teleport any physical object from one place to another, we have to disintegrate it and again integrate in the destination. In this process the object has to be broken into millions of particles and again recombine where we want it to teleport. But, after teleportation, the object will just be a clone of previous one. Because, the object is not actually being teleported, rather the information is transferred. If you are teleported to a certain distance, your body will be broken to very very small millions of particles which all information will be transmitted and again reconstructed in the destination. 

But practically, it is quite difficult. Because, if you are disintegrated to millions of particles then it means you are dead. Then what if the information of yourself does not get recovered at the destination? So, it is quite risky. Till date, it is quite difficult. But who knows about the future?

How will quantum entanglement help us in real life?

1. Using quantum entanglement, we can teleport particles from one place to another.
2. The communication can be made quite secure and hacking proof. Because, quantum teleportation allows us to exchange information from one place to another without any physical medium.
3. May be in future, we will be able to teleport large things.
4. Quantum computing and many more.

Now, there may be a question,"Does quantum entanglement break the laws of special relativity?" This question does not have a specific answer. Because, we still do not know what type of medium is taken in use for entanglement. As, distance between the entangled particles does not matter and they affect each other at the instant, that is why some scientists say that they break the laws of special relativity. But according to some scientists, the entangled particles use some kind of bridges or worm holes to communicate with each other. Thus they do not break the laws of special relativity.

But whatever it is, this is a very very interesting phenomenon. The research on this field is still going on...