Entanglement is a phenomenon in quantum mechanics where two or more particles become correlated in such a way that their quantum states are intrinsically connected. When particles are entangled, the state of one particle cannot be described independently of the other(s), regardless of the distance between them.
The concept of entanglement was first proposed by Albert Einstein, Boris Podolsky, and Nathan Rosen in their famous EPR (Einstein-Podolsky-Rosen) paper in 1935. They used a thought experiment to highlight what they perceived as a problem with the completeness of quantum mechanics. However, it was not until the 1960s and subsequent experiments that the phenomenon of entanglement was experimentally verified.
The experimental verification of entanglement involves performing measurements on entangled particles and comparing the results with the predictions of quantum theory. These experiments have been conducted using various systems, including photons, electrons, and atoms.
One common experiment is the Bell test, which tests the violation of Bell's inequalities. Bell's inequalities are a set of mathematical relations that describe the correlations that can arise in classical physics. If the measurements on entangled particles violate these inequalities, it suggests that the particles are genuinely entangled and their behavior cannot be explained by classical physics.
Many experiments have been performed to test Bell's inequalities, and the results consistently show violations that align with the predictions of quantum mechanics. These experiments provide strong evidence that entangled particles are not independent entities but are part of a single quantum system.
Furthermore, entanglement has practical applications in various fields, such as quantum cryptography, quantum teleportation, and quantum computing. These applications rely on the understanding and manipulation of entangled states.
While the experimental evidence strongly supports the existence of entanglement, it's important to note that it does not provide a direct explanation for the underlying mechanism or "spooky action at a distance" that occurs between entangled particles. The nature of entanglement and its implications for the fundamental nature of reality are still actively studied and researched in the field of quantum physics.