  An emergent $\mathrm{SU}(4)$ symmetry discovered in the microscopic model for
$d^1$ honeycomb materials [M. G. Yamada, M. Oshikawa, and G. Jackeli, Phys.
Rev. Lett. $\textbf{121}$, 097201 (2018).] has enabled us to tailor exotic
$\mathrm{SU}(4)$ models in real materials. In the honeycomb structure, the
emergent $\mathrm{SU}(4)$ Heisenberg model would potentially have a quantum
spin-orbital liquid ground state due to the $\textit{multicomponent
frustration}$, and we can expect similar spin-orbital liquids even in
three-dimensinal versions of the honeycomb lattice. In such quantum
spin-orbital liquids, both the spin and orbital degrees of freedom become
fractionalized and entangled together due to the strong frustrated interactions
between them. Similarly to spinons in pure quantum spin liquids, quantum
spin-orbital liquids can host not only spinon excitations, but also fermionic
$\textit{orbitalon}$ excitations at low temperature.
