2021 (source code)
= 2021
{scope}
{title2=C7.4}
{title2=A14687W1}
Source: https://www.whatdotheyknow.com/request/oxford_physics_past_papers_2/response/2340696/attach/9/Physics%20Papers%202020.pdf?cookie_passthrough=1#page=93[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 $\rho\mapsto U\rho U^\dagger$ 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.
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