Entanglement is a fundamental concept in quantum mechanics that describes a unique correlation between two or more particles, even when they are physically separated. It is a phenomenon that goes beyond classical physics and is characterized by a strong correlation that cannot be explained by classical means.
When two particles are entangled, their quantum states become intertwined, so that the state of one particle cannot be described independently of the other. This means that measuring the properties of one particle instantaneously affects the properties of the other, regardless of the spatial separation between them. This instantaneous correlation is what Albert Einstein famously referred to as "spooky action at a distance."
The entangled particles exhibit what is known as quantum superposition, where they exist in a combined state of multiple possibilities until a measurement is made. The act of measuring one particle's property, such as its position or spin, collapses the superposition, and the state of the other entangled particle is immediately determined, even if it is far away.
The remarkable aspect of entanglement is that it persists regardless of the spatial separation between the entangled particles. They can be separated by vast distances, and yet their states remain connected. This property has been experimentally verified in numerous studies, including the famous Bell's theorem experiments, which demonstrated the violation of local realism and provided evidence for the non-local nature of entanglement.
To summarize, entanglement refers to the strong and inseparable correlation between quantum states of two or more particles, even if they are not in the same place. The entangled particles share a unique connection that allows for instantaneous information transfer and non-local correlations, which is a distinct feature of quantum mechanics.