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Multiple choice
In CHSH self-testing, observing the maximal Bell value
S
=
2
2
S = 2\sqrt{2}
S
=
2
2
on two untrusted black boxes certifies the underlying state and measurements. To what precision can the shared state be identified from the statistics alone?
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Exactly and uniquely, pinning down the state with no remaining freedom
Only up to its total entropy, with the actual state otherwise undetermined
Up to a local isometry (equivalent to a Bell pair plus a 'junk' register), since local unitaries and added ancillas leave all statistics unchanged
Not at all; the value
2
2
2\sqrt{2}
2
2
places no constraint on the internal state
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