Quantum Entanglement

Quantum entanglement is one of the most bizarre phenomena occurring in the quantum realm. When multiple particles are linked in a particular way, even if they are far away from each other, their states continue to be connected. In simple terms, they share an identical quantum state.

What is Quantum Entanglement?

Quantum entanglement is a quantum phenomenon whereby a group of particles is produced so that their quantum states are unclear until calculated as a whole. The act of calculating one decides the result of calculating the other even if they are far from each other. In other words, each particle’s quantum state cannot be sketched independently of the state of other particles (despite the spatial differences).

The subject of quantum entanglement is at the centre of the fundamental difference between quantum and classical mechanics. Entanglement is a unique quantum phenomenon that is completely absent in classical physics.

Entangled System

An entangled system can be defined as a system whose quantum state cannot be considered as a result of states of its individual constituents. In other words, they are not just independent particles but are inextricable ensembles. One particle cannot be fully explained in such systems without analysing other remaining particles. The condition of an entangled system is only definable as a superposition of states of individual constituents. Entanglement in quantum systems can occur through various interactions. Most of them are still unknown.

Discovery of Quantum Entanglement

In the early 20th century, theoretical physicists constructed the fundamental concepts behind quantum entanglement. This development was an obvious consequence of rigorous research in quantum mechanics. They realised that in order to efficiently explain subatomic systems, quantum states should be considered in every scenario.

In the quantum realm, everything is probabilistic. For example, no one can exactly know the position of an electron in an atom. We can only predict where it might be present. A quantum state sums up the probability of calculating a certain nature of a particle (angular momentum, position, etc.). In the case of an electron, the quantum state can let the observer know where the electron might be and the probability of detecting it at those locations.

Another attribute of a quantum state is that it can be mutually connected with other quantum states. This means that the calculation of one state can influence the other. In 1935, Albert Einstein, Nathan Rosen and Boris Podolsky analysed how heavily connected quantum states could interact with one another. They concluded that when multiple particles are firmly correlated, they will drop their discrete quantum states and share a unified, common state. An analogy of a vessel should describe the basics of quantum entanglement. A mathematical vessel could explain all particles concurrently despite their individual characteristics.

The word “entanglement” was first used by physicist Erwin Schrödinger (one of the pioneers of quantum mechanics). He explained quantum entanglement as one of the fundamental features of quantum mechanics. He said that its presence is an absolute deconstruction of classical mechanics or physical logic.

A video about quantum mechanical model

Creation of Quantum Entanglement

There are numerous methods to entangle particles. One way is to depend on some dynamic process such as nuclear decay and particle formations. According to many studies, entangled photon pairs can be created by splitting an individual photon (produces entangled pairs of photons). Entanglement can also be made by intermixing photon pairs in an optical fibre cable.

Let us get more into the technical side of quantum entanglement. Entanglement is generally produced by direct interaction between elementary particles. Such interactions can take many forms. One of the common methods is unconstrained parametric down-conversion to create a photon pair entangled in polarisation. Other techniques include the application of a fibre coupler to control and intermix photons. It can also be generated from the decay stream of the bi-excitons in quantum dots. In fact, quantum entanglement can also be caused by applying the Hong–Ou–Mandel effect. In the initial experiments of Bell’s theorem, the intertwined particles were created utilising atomic cascades.

Surprisingly, it is also viable to generate quantum entanglement between quantum particles or systems that have not directly interacted with one another. It can be done through entanglement swapping. Two identical particles from different sources could be entangled if their wave function just spatially overlapped (at least partly).

Applications of Quantum Entanglement

• Entanglement has numerous uses in quantum information science. With the use of entanglement, many impractical tasks can be achieved.
• Some of the important applications of quantum entanglement are quantum teleportation and superdense coding.
• Entanglement is considered to be necessary for the complete deployment of quantum computing.
• Quantum entanglement is used in a few protocols of quantum cryptography. However, under the standard assumption, entanglement is not required to confirm the security of quantum key distribution.

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