| quantum nonlocality | |
|---|---|
| Name | Quantum nonlocality |
| Field | Quantum mechanics |
| Discovered | 1935 (EPR); 1964 (Bell) |
| Notable experiment | Aspect experiments |
| Key people | Albert Einstein, Boris Podolsky, Nathan Rosen, John S. Bell, David Bohm, Alain Aspect, John Clauser |
quantum nonlocality
Quantum nonlocality is the phenomenon by which correlated outcomes of measurements on spatially separated quantum systems cannot be explained by local classical mechanisms. It manifests most sharply in violations of Bell's theorem and underpins practical protocols in quantum information science and emerging quantum technologies.
Quantum nonlocality denotes statistical correlations between measurement results on distinct subsystems that cannot be reproduced by any local hidden variable theory consistent with relativistic causality. It is distinct from faster‑than‑light signalling: nonlocal correlations respect the no‑signalling principle even when they cannot be modelled locally. The effect is most commonly probed using entangled states such as the singlet state of two spin-1/2 particles and is central to discussions of quantum entanglement and the foundations of Quantum mechanics.
The phenomenon traces to the 1935 Einstein–Podolsky–Rosen (EPR) paper by Albert Einstein, Boris Podolsky and Nathan Rosen, and to David Bohm's spin formulation. In 1964 John S. Bell derived inequalities distinguishing quantum predictions from any local realistic model (Bell's theorem). Experimental tests began with the pioneering experiments of John Clauser and Stuart Freedman (1972) and subsequent improvements by Alain Aspect (1982), who closed several practical loopholes. Modern experiments have used parametric down-conversion, superconducting qubits at IBM and Google devices, trapped ions at institutions such as University of Innsbruck and NIST, and photonic platforms to close the detection and locality loopholes in so‑called loophole-free Bell tests (e.g., experiments by Häffner, Ronald Hanson's group at Delft).
Quantum nonlocality is formalized through entangled states introduced by Erwin Schrödinger and characterized by tensor-product Hilbert spaces in quantum theory. Bell's theorem proves that no local hidden variable model can reproduce all quantum correlations. The Greenberger–Horne–Zeilinger (GHZ) argument provides a stronger, deterministic contradiction without inequalities for multi‑party entangled states. Related theoretical constructs include CHSH inequality (Clauser–Horne–Shimony–Holt), Hardy's paradox, and Leggett inequalities. Foundational work by Asher Peres and Nicolas Gisin further clarified operational and conceptual aspects of entanglement versus nonlocality.
Formally, nonlocality is expressed by probability distributions P(a,b,...|x,y,...) for outcomes a,b given measurement settings x,y on separated parties that violate Bell inequalities derived from local realistic models. The space of correlations is analyzed using convex geometry: the local polytope, the quantum set, and the no‑signalling polytope (studied by researchers like Sandu Popescu and Daniel Rohrlich). Measures include maximal Bell inequality violation, nonlocal fraction, and robustness to noise. Tools from operator theory, semidefinite programming (see NPA hierarchy), and entanglement measures such as entanglement entropy are used to quantify and bound nonlocal correlations in finite-dimensional Hilbert space.
Local hidden variable (LHV) models posit a shared classical variable λ distributed by a source; measurement outcomes are independent conditional on λ. Bell inequalities are necessary conditions for existence of an LHV model. The no‑signalling constraints enforce that marginal distributions for one party do not depend on distant measurement choices, preserving compatibility with special relativity. Hypothetical superquantum correlations like the PR box (Popescu–Rohrlich) maximally violate Bell inequalities while obeying no‑signalling, illustrating the separation between quantum limits and logical possibilities and motivating research on principles (e.g., information causality) that single out quantum correlations.
Nonlocality is a resource in quantum cryptography (notably in device‑independent quantum key distribution), randomness certification, and protocols for entanglement‑based quantum teleportation and entanglement swapping. Device‑independent protocols rely on Bell violation alone, reducing trust assumptions about hardware suppliers such as ID Quantique or cloud providers (e.g., Rigetti). Nonlocal correlations enable advantages in communication complexity tasks and underpin security proofs in protocols developed by research groups at University of Vienna and University of Geneva. Implementations span photonic quantum computing, trapped ions, and superconducting circuits, and influence standards in quantum communication networks and quantum repeaters.
Quantum nonlocality has deep implications for interpretations of quantum mechanics. It intensifies debates over realism, locality, and causation in schools such as the Copenhagen interpretation, Many‑worlds interpretation, and de Broglie–Bohm theory (pilot‑wave theory). Some approaches accept nonlocal influences (e.g., Bohmian mechanics), while others reinterpret measurement and probability to avoid explicit action‑at‑a‑distance. Philosophers and physicists at institutions like University of Oxford and Perimeter Institute have explored consequences for metaphysics and the ontology of the quantum state. Nonlocality also motivates research into reconciling quantum theory with general relativity and informs speculative programs such as the AdS/CFT correspondence's implications for entanglement and spacetime geometry.
Category:Quantum mechanics Category:Quantum information theory