### Quantum Entanglement

With the coming of quantum physics, the laws of classical physics were challenged as these could not explain phenomena occurring at a quantum, i.e., subatomic level.

Coined by many as the weirdest physics phenomenon they have come across, “Quantum Entanglement” continues to bizarre everybody. Broadly speaking, quantum mechanics is the theory that describes the behaviour of atoms and even smaller particles. The behaviour of this subatomic particle like a photon or an electron is described in a wave function. The wave function gives us the probability of finding the particle in a particular configuration. This configuration can also be described as the ‘spin’ of the subatomic particle. Now to check the spin of a subatomic particle, one has to take an axis along which they want to check the spin. For e.g. checking along the vertical axis the spin can be either up or down and checking along the horizontal axis the spin can be either left or right. The outcome will always be in the direction of the axis and its opposite. However, according to the wave function, the spin axis of the particle can be in any direction not just vertical and horizontal. In other words, spin direction is actually in all directions allowed by the wave function.

Let us consider two subatomic particles now. One will naturally assume that these particles would be governed by individual probabilistic wave function and they are totally independent, with the spin direction of one particle having no effect on the other particle. However, scientists tell us that it is possible to prepare two subatomic particles so that both of them are described by a single wave function. That is what is actually meant by entangled, i.e. two particles and one wave function. This is done by taking a parent particle which has zero spin. We let this decay into two particles. Since spin is a conserved quantity, it never changes. Therefore if the parent particle has zero spin then the two daughter particles have to have opposite spins. Hence we can say that in Quantum Physics entangled particles are those particles with opposite spins and a single wave function that governs them. Moreover, this entanglement doesn’t depend on the proximity of these particles with each other. Even at huge distances from each other, two entangled particles will have opposite spins and will be governed by a single wave function.

One should keep in mind that the study of quantum mechanics in highly probabilistic. This means that we can’t know the outcome of a measurement before we make it even in principle. Suppose we take the measurement of one of the entangled particles along the horizontal axis and it comes out to be left, we are a 100% sure that the spin of the other entangled particle will be right. This proves that the information that a measurement was made on one particle was transferred to the other particle. One can make these observations on the particles in very quick succession and the result will still be the same. The speed with which information seems to travel from one particle to another is even greater than light. This baffled up Einstein and in 1935 he co-authored a paper and called this transfer of quantum information, “spooky action at a distance”. Einstein explained this phenomenon using the concept of ‘hidden variables’ and argued that the particles were carrying information from before. However, this theory was subsequently ruled out and it was accepted that quantum information is capable of being transferred at speeds greater than that of light.

We have merely set the tip of our toe in the realm of quantum physics and its wonders have just started to amaze us. With the ever changing dynamics of physics, there is much more to be learned and experimented with before coming at definite results.

Quantum Entanglement and
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