Particle entanglement is a fundamental concept in quantum mechanics that describes a special correlation between particles, even when they are spatially separated. When particles become entangled, their individual properties, such as spin, position, or polarization, become intrinsically linked to each other.
The phenomenon of entanglement arises from the mathematical formalism of quantum mechanics. According to this theory, particles are described by wavefunctions that contain information about their possible states. When two or more particles interact in a way that their quantum states become entangled, the overall wavefunction of the system can no longer be described as a simple combination of individual particle states. Instead, the entangled particles form a composite state that cannot be expressed as a product of individual states.
The remarkable aspect of entanglement is that when a measurement is made on one of the entangled particles, it instantaneously affects the state of the other entangled particle, regardless of the spatial separation between them. This instantaneous correlation, which physicist Albert Einstein famously referred to as "spooky action at a distance," has been experimentally confirmed through a phenomenon known as Bell's theorem and subsequent experiments.
To prove the existence of entanglement and the non-local correlations it implies, experiments can be designed to violate Bell's inequalities. Bell's inequalities provide a mathematical test to distinguish between local realistic theories (which assume hidden variables and locality) and quantum mechanics (which allows for entanglement and non-local correlations).
Over the past few decades, numerous experiments have been conducted to test Bell's inequalities, and the results consistently support the predictions of quantum mechanics. These experiments involve generating pairs of entangled particles, such as photons or electrons, and measuring their correlated properties in different locations. The measured correlations violate the limits imposed by local realistic theories, providing strong evidence for the existence of entanglement and the non-local nature of quantum interactions.
The practical implications of entanglement are vast and include applications in quantum information processing, quantum communication, and quantum cryptography. For example, entanglement is at the heart of quantum teleportation, where the quantum state of one particle can be transferred to another distant particle without physically moving it.
It's important to note that while entanglement allows for instantaneous correlations between entangled particles, it does not permit the transmission of information faster than the speed of light. The entangled particles themselves cannot be used to transmit information directly. Instead, exploiting entanglement for communication purposes typically requires additional classical information transfer to complete the process.
In summary, particle entanglement refers to a profound correlation between particles that can persist even when they are separated in space. It has been experimentally verified and plays a crucial role in various applications within quantum information science.