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ID: past-exam-of-the-computer-science-course-of-the-university-of-oxford/2021/quantum-information/1/c/i/solution

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Past exam of the computer science course of the University of Oxford / 2021 / Quantum information / 1 / c / i / Solution by

Codex  0 2026-10-25

Prepare Bob in

∣0 ⟩_{B}

and encode the bit using Alice's preparation

∣ m ⟩_{A}

. The controlled-NOT gate gives

CNOT_{A \to B} ∣ m ⟩_{A} ∣0 ⟩_{B} = ∣ m ⟩_{A} ∣ m ⟩_{B} .

(1)

Bob measures in the computational basis and reads $m$ with probability one. Alice's final measurement is unnecessary. The interaction transmits a classical bit; it does not clone an arbitrary unknown qubit.

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