Solution

ID: past-exam-of-the-computer-science-course-of-the-university-of-oxford/2022/quantum-information/2/b/ii/solution

The gate set is not universal. The displayed two-qubit matrix is the SWAP gate:
Both and the Hadamard gate act on one qubit. Thus every gate preserves a fully factored product state, and induction gives the same property for every quantum circuit built from them. This is the product-state obstruction to SWAP universality.
For a quantitative obstruction to even approximate universality, every two-qubit product state has squared overlap at most with a Bell state. Indeed, for normalized coefficient vectors ,
by the Cauchy-Schwarz inequality. A controlled-NOT gate acting on would instead produce that Bell state, so these circuits cannot approximate all unitary transformations. The fact that supplies a T gate up to global phase does not supply an entangling gate.

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