  In all archetypical reported (001)-oriented perovskite heterostructures, it
has been deduced that the preferential occupation of two-dimensional electron
gases is in-plane $d_\textrm{xy}$ state. In sharp contrast to this, the
investigated electronic structure of a spinel-perovskite heterostructure
$\gamma$-Al$_2$O$_3$/SrTiO$_3$ by resonant soft X-ray linear dichroism,
demonstrates that the preferential occupation is out-of-plane
$d_\textrm{xz}$/$d_\textrm{yz}$ states for interfacial electrons. Moreover, the
impact of strain further corroborates that this anomalous orbital structure can
be linked to the altered crystal field at the interface and symmetry breaking
of the interfacial structural units. Our findings provide another interesting
route to engineer emergent quantum states with deterministic orbital symmetry.
