 2021

Source: original released examination PDF, bundle pages 93–96; paper pages 1–4. The printed sitting is 7 June 2021 despite the bundle filename.

Solutions to all three questions. We use the usual Pauli matrices, column state vectors, and the density operator convention $\rho \mapsto U\rho U^{†}$ for a gate. The CHSH conclusions require independent, uniformly chosen settings and no communication between the devices during a run. In Question 3, a reliable unknown-basis transformation means one deterministic operation that works for every common input basis.