  We report the synthesis of a novel isotope engineered $^{13}{\rm
C}$--$^{12}{\rm C}$ heteronuclear nanostructure: single-wall carbon nanotubes
made of $^{13}{\rm C}$ enriched clusters which are embedded in natural carbon
regions. The material is synthesized with a high temperature annealing from
$^{13}{\rm C}$ enriched benzene and natural ${\rm C}_{60}$, which are
co-encapsulated inside host SWCNTs in an alternating fashion. The Raman 2D line
indicates that the $^{13}{\rm C}$ isotopes are not distributed uniformly in the
inner tubes. A semi-empirical method based modeling of the Raman modes under
$^{13}{\rm C}$ isotope enrichment shows that experimental data is compatible
with the presence of $^{13}{\rm C}$ rich clusters which are embedded in a
natural carbon containing matrix. This material may find applications in
quantum information processing and storage using nuclear spins as qubits.
