Bad Qubits — Learn Quantum by Doing

Multiple choice

Tsirelson's bound (1980) gives the maximum value of the CHSH quantity |S| achievable by any quantum state and any +/-1-valued local observables. What is that maximum?

2 (the classical local-realism bound)
2*sqrt(2) (approximately 2.828)
1 (the same as the original Bell inequality bound)
4 (the most general no-signalling bound)