The phenomenon of entanglement in quantum mechanics has been extensively studied and confirmed through numerous experiments. The experimental evidence for entanglement supports the view that particles can be correlated in ways that go beyond classical explanations. Here are a few key points about how we know particles are genuinely entangled:
Bell's Theorem: In 1964, physicist John Bell formulated a theorem that provided a way to experimentally test the predictions of quantum mechanics against classical explanations. Bell's inequality states that if particles have pre-existing information that determines their future states, their correlations should satisfy certain constraints. However, experimental tests of Bell's inequality have consistently shown violations, indicating that the correlations between entangled particles cannot be explained by classical hidden variables.
Violation of Local Realism: Entanglement violates the principle of local realism, which states that physical processes occurring in one location cannot instantaneously affect events in another distant location. The phenomenon of quantum entanglement has been demonstrated through experiments like the Aspect experiment (or Bell test experiments), where entangled particles, such as photons, exhibit correlated behaviors even when separated by large distances, instantaneously responding to changes in their quantum states.
Non-Separability: Entangled particles exhibit a property known as non-separability, where the state of a system cannot be described by the independent states of its individual parts. When particles are entangled, their combined state becomes inseparable, and any measurement or change in one particle instantaneously affects the state of the other particle, regardless of the distance between them. This non-separable behavior has been experimentally verified through tests such as the violation of Bell's inequality mentioned earlier.
Quantum Information Processing: The principles of entanglement have been harnessed in various technologies related to quantum information processing, such as quantum cryptography and quantum computing. These technologies rely on the ability to generate and manipulate entangled states to perform tasks that are not achievable with classical systems, further demonstrating the reality and usefulness of entanglement.
The accumulated experimental evidence, along with the successful practical applications of entanglement, strongly supports the understanding that particles are genuinely entangled and do not carry hidden information that determines their future states at the moment of entanglement.