Quantum entanglement is a physical phenomenon that occurs when pairs or groups of particles generate or interact in ways such that the quantum state of each particle cannot be described independently of the state of the other(s), even when the particles are separated by a large distance. This feature of quantum mechanics is at the heart of much of quantum physics and quantum information science.
Here’s a simple example. Suppose we have two particles in an entangled state. If we measure one particle and find it in a particular state (like spinning up or down), the other particle will instantly be in the corresponding state (down or up, respectively) no matter how far apart they are.
This is not because information has traveled faster than light between the two particles. The state of the system was determined when the particles were entangled, it’s just that we only become aware of it upon measuring. This phenomenon is often referred to as “spooky action at a distance” — a term coined by Albert Einstein, who was skeptical of entanglement because it seemed to contradict his theory of relativity’s speed of light limit.
Despite Einstein’s reservations, quantum entanglement has been experimentally confirmed numerous times. It plays a key role in quantum computing and quantum cryptography, and continues to be a central topic of research in quantum mechanics.
Entanglement is not something that can be directly observed. Instead, it’s inferred from the results of measurements on quantum systems. The standard way to confirm that a pair of quantum particles is entangled is by conducting a test for violation of a Bell inequality, a mathematical expression that encapsulates the classical limit for correlations between measurements.
John Bell formulated the Bell inequalities in 1964 as a test of what’s known as local hidden variable theories, which attempt to explain quantum mechanics in classical terms. If a Bell inequality is violated, it means that the measurements can’t be explained by any classical local hidden variable theory. This is taken to imply that the particles are entangled.
Here’s a simplified example. Suppose we have two particles that we suspect might be entangled. We can measure a property of these particles, like their spin, along different axes. For entangled particles, the results of these measurements will be more highly correlated than any classical theory would predict. This high degree of correlation, if it exceeds the limit set by the Bell inequality, implies entanglement.
In practical terms, entanglement is typically created under controlled conditions where pairs of particles are generated in ways that are known to produce entanglement. For example, certain types of crystals can produce pairs of entangled photons when a single photon strikes the crystal. Another method involves using controlled interactions between particles in a quantum computer or similar device. By carefully manipulating the particles, one can entangle them and then verify the entanglement through a series of measurements.
Image by Uwe Dahlke from Pixabay
