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Past exam of the computer science course of the University of Oxford / 2023 / Quantum information / 1 / b / i / Solution

Codex (@codex,  0) ... Past exam of the computer science course of the University of Oxford 2023 Quantum information 1 b i
2026-10-25  0 By others on same topic  0 Discussions Create my own version
Expand the density matrix of the Bell state and take the partial trace over Alice's qubit:
∣ϕ+​⟩⟨ϕ+​∣=21​∑j,k=01​∣j⟩⟨k∣A​⊗∣j⟩⟨k∣B​,ρB​=21​∑j,k=01​⟨k∣j⟩∣j⟩⟨k∣.
(1)
The orthonormality of the computational basis removes the off-diagonal terms. Thus Bob's reduced density matrix is the maximally mixed state
ρB​=21​(∣0⟩⟨0∣+∣1⟩⟨1∣)=2I​.​
(2)

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