What is Quantum Entanglement?
Quantum entanglement is a fundamental principle in quantum mechanics where two or more particles become interconnected such that the quantum state of each particle cannot be described independently, even when separated by vast distances. This phenomenon, first highlighted by Einstein, Podolsky, and Rosen in 1935 as 'spooky action at a distance,' implies that measuring one particle instantly determines the state of the other, defying classical intuitions of locality.
Key Principles of Quantum Entanglement
The core principles include superposition, where particles exist in multiple states simultaneously until measured, and non-locality, enabling correlated outcomes regardless of distance. Entanglement arises from quantum interactions, preserved during separation, and is formalized in quantum theory via wave functions that describe the joint system. Bell's theorem tests confirm these correlations exceed classical limits, ruling out local hidden variables.
Practical Example: Entangled Photons in Experiments
In a typical experiment, a laser splits a photon into two entangled photons via spontaneous parametric down-conversion. Measuring the polarization of one photon (e.g., horizontal) instantly sets the other to vertical, even if separated by kilometers. This setup, used in labs like those at NIST, demonstrates entanglement's reliability for quantum communication protocols.
Applications in Modern Physics
Quantum entanglement drives innovations in quantum computing, where entangled qubits enable parallel processing for complex simulations; quantum cryptography, securing data via unbreakable keys like in BB84 protocol; and precision measurements, such as gravitational wave detection with entangled atoms. These applications promise revolutions in secure networks, drug discovery, and fundamental research, underscoring entanglement's role in advancing physics beyond classical boundaries.