Tsirelson's 2√2

Plate 39 speaks for itself: “Nature beats the classical limit by √2.”

The Physics

In a CHSH Bell test, local hidden-variable theories keep the correlation sum S within 2. Quantum mechanics allows up to 2√2 ≈ 2.83, Tsirelson's bound, and experiments land above 2.

|S| ≤ 2√2

• S: CHSH combination of four correlation coefficients, dimensionless

Local hidden variables: |S| ≤ 2. Aspect et al., 1982: S = 2.697 ± 0.015.

Sources

• B. S. Cirel'son, Lett. Math. Phys. 4, 93 (1980).
• A. Aspect, P. Grangier, G. Roger, Phys. Rev. Lett. 49, 91 (1982).

On the poster

Plate 039, “Tsirelson's 2√2”: the pixel-art plate with its slogan “Nature beats the classical limit by √2.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.