 Solution

For a block with nonzero typical dimension, write $P=P_{\epsilon }^{(n)}$ and $Q=I-P$. Let the compressed register be a copy of the typical space plus a one-dimensional orthogonal failure flag. Choose an isometry $V:P{\mathcal{H}}^{\otimes n}\to {\mathcal{K}}_n$ onto the successful code sector, extended by zero on $Q$, and a normalized flag $|f⟩$ orthogonal to its range. Choose any normalized vector $|\phi ⟩$ in the source Hilbert space. DefineThese are CPTP maps: the first performs the projective test and either encodes or prepares a flag; the second embeds the successful sector and prepares a fixed state on failure. Their trace identities follow from $V^{†}V=P$ and $V{V}^{†}+|f⟩⟨f|=I_{{\mathcal{K}}_n}$. Thus typical-subspace compression with a failure flag implementsFor a source vector $|{\Psi }_k⟩$, let $a_k=⟨{\Psi }_k|P|{\Psi }_k⟩={\alpha }_k^2$. Its overlap with the decoded state iswhere the final step is $(1-a_k{)}^2\ge 0$. Averaging proves average pure-source fidelity after typical projection:No orthogonality of the emitted vectors was used. The code dimension is $\dim {\mathcal{T}}_{\epsilon }^{(n)}+1$; the extra flag has vanishing asymptotic rate cost. Exceptional small block lengths with empty typical space can use any channel, since they do not affect reliability.