# Quantum Entanglement

Quantum entanglement is a phenomenon in quantum physics where two or more particles become correlated in such a way that the state of one particle cannot be described independently of the state of the other particles, even if they are separated by large distances. This correlation is maintained regardless of the distance between the particles and is often referred to as spooky action at a distance.

When particles become entangled, their quantum states become intertwined, and measuring the state of one particle instantaneously affects the state of the other particles, regardless of the distance between them. This instantaneous correlation between entangled particles is not explainable by classical physics and is a unique feature of quantum mechanics.

The concept of entanglement has been commented on by Albert Einstein, Boris Podolsky, and Nathan Rosen in a 1935 paper known as the EPR paper. They used entanglement to challenge the completeness of quantum mechanics. In the 1960s and 1970s an experimental tests of entanglement were conducted to confirm the existence of the phenomenon.

#### Here are a few notable experiments from that time period:

- Bell’s Theorem and Bell Inequality: In the 1960s, physicist John Stewart Bell developed a mathematical inequality, known as Bell’s inequality, to test the predictions of quantum mechanics against local hidden variable theories. Bell’s theorem and subsequent experiments, such as the ones conducted by John Clauser and Stuart Freedman in the 1970s, provided strong evidence in favor of quantum entanglement.
- Aspect Experiment: In the early 1980s, physicist Alain Aspect performed a series of experiments based on Bell’s theorem. These experiments involved measuring the correlations between entangled particles, such as photons, in different spatially separated locations. The results of the Aspect experiment strongly supported the predictions of quantum mechanics and ruled out local hidden variable theories.
- Clauser-Horne-Shimony-Holt (CHSH) Inequality: In the 1960s and 1970s, physicists John Clauser, Michael Horne, Abner Shimony, and Richard Holt developed an inequality, known as the CHSH inequality, to test the correlations between entangled particles. The violation of the CHSH inequality in experiments provided further evidence for the reality of quantum entanglement.
*The Clauser-Horne-Shimony-Holt (CHSH) inequality is a mathematical inequality that is used in the proof of Bell’s theorem. The violation of the CHSH inequality is seen as confirmation that nature cannot be described by such theories.*

Quantum entanglement has been observed and studied in various systems, including photons, electrons, ions, and even larger molecules. It has important implications for quantum information processing, quantum cryptography, and quantum communication. For example, entanglement is a crucial resource for quantum teleportation, a process that allows the transfer of quantum information from one location to another without physically moving the particles themselves.

The phenomenon of entanglement is still an active area of research, and scientists continue to explore its fundamental properties and potential applications. It remains one of the most intriguing and puzzling aspects of quantum mechanics, challenging our understanding of the nature of reality and the limits of classical physics.

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