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 Incoming links: Local indistinguishability of Bell-state phase

Past exam of the mathematics course of the University of Cambridge / 2023 / ii / Paper 2 / 10D / b / ii / Solution Created 2026-09-24 Updated 2026-10-03

No. Taking the partial trace over

B

gives

ρ_{A}^{+} = ρ_{A}^{-} = \frac{1}{2} (∣0 ⟩ ⟨ 0∣ + ∣1 ⟩ ⟨ 1∣) = \frac{I}{2} .

Every measurement on

A

alone therefore has identical outcome probabilities in the two cases. This is the local indistinguishability of Bell-state phase.

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