Nosignallingperiaatetta

Nosignallingperiaatetta (no-signalling principle) is a foundational constraint in physics and information theory. It states that local measurements performed on one part of a spatially separated system do not enable faster-than-light communication with another part.

Formally, if two parties perform measurements A and B with settings x and y and obtain outcomes a and b, the joint probabilities P(a,b|x,y) must satisfy that the marginal distribution on a, P(a|x,y) = sum_b P(a,b|x,y), does not depend on y; similarly P(b|y) = sum_a P(a,b|x,y) does not depend on x. This enforces no-signalling from one location to another.

In quantum mechanics, no-signalling is ensured despite entanglement and nonlocal correlations: measurements on entangled states yield correlations that can violate Bell inequalities yet cannot be used to transmit information without a classical channel. The principle constrains the set of physically allowed correlations, ruling out conspiratorial correlations that would enable instant signaling.

The no-signalling principle is central to quantum information, underpinning device-independent protocols, quantum cryptography, and the study of nonlocal resources such as PR boxes. It also appears in generalized probabilistic theories and helps separate relativistic causality from other nonlocal features.