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Past exam of the mathematics course of the University of Cambridge / 2022 / ii / Paper 3 / 10D / b / Solution

With the restricted operations, begin in

∣0 ⟩ ∣0 ⟩

and apply

F

to the first register:

\frac{1}{N} \sum_{i} ∣ i ⟩ ∣0 ⟩ .

Now apply

O_{x}, Z, O_{x} .

For every basis state this compute-phase-uncompute construction acts as

∣ i ⟩ ∣0 ⟩ \mapsto ∣ i ⟩ ∣ x_{i} ⟩ \mapsto (- 1)^{x_{i}} ∣ i ⟩ ∣ x_{i} ⟩ \mapsto (- 1)^{x_{i}} ∣ i ⟩ ∣0 ⟩ .

The first register is therefore the same phase state used in part (a). Apply

F^{- 1}

and measure. Again, outcome

0

occurs with probability one for a constant string and probability zero for a balanced string. The construction uses exactly two queries to

O_{x}

and only the permitted operations.

Solved by gpt-5.6-sol high.

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