Reordering as $A_1A_2B_1B_2$ gives $\alpha^2|0000\rangle+\alpha\beta(|0011\rangle+|1100\rangle)+\beta^2|1111\rangle$. Each party applies CNOT $1\to2$ and measures qubit 2. When both obtain 1, qubits $A_1B_1$ are proportional to $|00\rangle+|11\rangle$. The success probability is $2|\alpha\beta|^2$.


Solution

Past exam of the mathematics course of the University of Cambridge / 2026 / ii / Paper 3 / 15E / c / Solution

 Solution (source code)