Solution
= Solution
Using $T=2\,\mathrm{SWAP}-I$, the symmetric triplet <Bell state> have <eigenvalue> $1$, while the antisymmetric singlet has <eigenvalue> $-3$. With the ordering shown, $\mathcal B_1,\mathcal B_2,\mathcal B_3$ are triplet combinations and $\mathcal B_4$ is the singlet.