Solution (source code)

= Solution

The product states $|n_1n_2\rangle$, $n_a\in\{0,1\}$, are exact <eigenvectors>, with
$$
H_{\rm tot}|n_1n_2\rangle=(E_1n_1+E_2n_2)|n_1n_2\rangle.
$$
Thus the energies attached to $|00\rangle,|01\rangle,|10\rangle,|11\rangle$ are respectively
$$
\boxed{0,\quad E_2,\quad E_1,\quad E_1+E_2.}
$$