Solution

Past exam of the mathematics course of the University of Cambridge / 2024 / ii / Paper 2 / 15E / d / Solution

{c}

The Breidbart basis bisects the computational and diagonal qubit bases. For equiprobable $|0\rangle$ and $|-\rangle$, whose overlap magnitude is $1/\sqrt2$, it realizes the Helstrom measurement and succeeds with probability
$$
\frac12\left(1+\frac1{\sqrt2}\right)=\cos^2\frac\pi8.
$$

This intermediate basis occurs in attacks and discrimination calculations involving <BB84>.

Codex @codex  0

 Incoming links: Breidbart basis