entangling

Entangling or entanglement is a quantum phenomenon in which the quantum states of two or more objects become interdependent such that the state of each object cannot be described independently of the state of the others. An entangled system is described by a single joint state, even when its components are spatially separated. Measurement outcomes on entangled parts exhibit correlations that cannot be explained by classical properties assigned before measurement. The effect arises from quantum interactions and is a fundamental resource in quantum information science.

The concept emerged from debates over the completeness of quantum mechanics, notably the EPR paper in 1935 and Schrödinger’s later discussion of “entanglement.” Bell’s theorem and ensuing experiments demonstrated violations of local realism, showing correlations inconsistent with local hidden variables. Over the past decades, experiments with photons, ions, and other systems have closed various loopholes, reinforcing the reality of entanglement.

Entangling can be achieved in multiple physical platforms. Spontaneous parametric down-conversion in nonlinear crystals produces entangled photon pairs, while interactions between trapped ions, neutral atoms, superconducting qubits, or quantum dots can generate entangled states. Logical entangling gates, such as controlled-NOT or controlled-Z, are used to create entanglement in quantum processors.

Applications of entanglement include quantum computation, where it enables universal quantum gates and speedups; quantum communication, enabling quantum teleportation and superdense coding; and quantum metrology, where entangled states can improve measurement precision. A key feature is that entanglement creates nonlocal correlations, but it does not allow faster-than-light signaling, preserving causality in accordance with relativity.