Bell's theorem and contextuality show why quantum correlations are not classical hidden labels, while no-signalling explains why those correlations still cannot send messages faster than light.
This lesson connects the entanglement module to quantum networks, teleportation, cryptography, and foundational experiments. It is also the conceptual guardrail for the simulator: the Bell circuit can produce nonclassical correlations, but it cannot be used as a communication channel without classical comparison.
Follow this lesson into the surrounding principles, theorems, tools, and modules.
Entanglement is not just strong correlation. Bell's theorem turns the question into an experiment: local hidden-variable models obey Bell inequalities, while quantum mechanics predicts violations for suitable entangled states and measurement settings. Kochen-Specker contextuality pushes a related lesson: quantum measurement outcomes cannot always be treated as revealing pre-existing values independent of the full measurement context.
Build a Bell pair in the simulator. Each qubit alone looks random, but the joint results are structured. The important point is control: Alice can choose her measurement basis, but she cannot choose her random outcome. Without control over the local result, there is no message to send instantly.
Bell inequalities constrain theories that combine locality with hidden variables of the relevant classical type. Quantum mechanics violates those inequalities but does not thereby permit controllable faster-than-light signalling. The no-communication or no-signalling result says that local measurement statistics on one side do not depend on which measurement is chosen on the distant side. The correlations appear only when the two records are later compared through an ordinary classical channel.
Open the simulator and see this concept in action. Watch how the state changes and compare it to what you just learned.
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