Entanglement Toy — Full Derivation
Companion to /lab/entanglement. Two DANodes share one membrane patch — measuring one collapses the SAME phase, the other reflects it instantly. CHSH inequality is violated up to the Tsirelson bound 2√2; no faster-than-light signalling is possible.
This page is the mathematical companion to /lab/entanglement. The toy lets you adjust two detector angles and watch the CHSH sum S — this page explains why S can reach 2√2 in SPT, why no information actually travels between the nodes, and why every benchmark in the toy passes.
The state
Singlet (Bell pair):
|Ψ⁻⟩ = (|↑↓⟩ − |↓↑⟩) / √2
In SPT: one shared phase φ₁₂, two anchor cells in 3D space.
Either anchor can be 'queried' (measured); both queries read the same φ₁₂.The correlation function
When detectors A and B are oriented at angles θ_A and θ_B, the joint probability of opposite outcomes follows from rotating the singlet axis. The expectation of σ_A·σ_B is:
E(θ_A, θ_B) = -cos(θ_A − θ_B)
Maximum |E| = 1 when angles differ by 0 or π.
E = 0 when angles differ by π/2 (uncorrelated).CHSH inequality and Tsirelson bound
Bell-test experiments use 4 angle pairs (a, b), (a, b'), (a', b), (a', b'):
S = E(a,b) − E(a,b') + E(a',b) + E(a',b')
Classical (local-realist) bound: |S| ≤ 2
Quantum (Tsirelson) bound: |S| ≤ 2√2 ≈ 2.828
Optimal angles: a=0, a'=π/2, b=π/4, b'=3π/4 → S = 2√2 exactlyBenchmarks (why each passes)
Mathematical soundness
- No-signalling (causality) — marginal probability p(σ_A | θ_B) = p(σ_A) regardless of B's choice. Each side sees random outcomes; correlation only emerges when results are compared post-hoc through a classical channel. SPT respects this because the shared phase carries no information itself — only its measurement outcomes do.
- Unitarity — ⟨Ψ|Ψ⟩ = 1 preserved. Singlet state has unit norm; any rotation is unitary.
- Tsirelson respected — |S| can never exceed 2√2. SPT does not predict 'super-quantum' correlations (e.g. PR-box S = 4), which is consistent with all experimental evidence.
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