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Attempts are made to unify gravity with the other three fundamental forces ofnature. As suggested by higher dimensional models, this unification may requirespace and time variation of some dimensionless fundamental constants. In thisscenario, probing temporal variation of the electromagnetic fine structureconstant ($\alpha= \frac{e^2} {\hbar c}$) in low energy regimes at thecosmological time scale is of immense interest. Atomic clocks are idealcandidates for probing $\alpha$ variation because their transition frequenciesare measured to ultra-high precision accuracy. Since atomic transitionfrequencies are functions of $\alpha$, measurements of clock frequencies atdifferent temporal and spatial locations can yield signatures to ascertain suchconjecture. Electrons in highly charged ions (HCIs) experience unusuallyenhanced relativistic effects. Hence level-crossings can be observed often inthese ions compared to their isoelectronic neutral or singly charged atomicsystems. Such a process features by their more significant relativisticsensitive coefficients ($q$) of atomic transitions. For unambiguous detectionof subtle changes in the transition frequencies due to $\alpha$ variation, itwould be judicious to contemplate transitions for which $q$ values areenormous. HCIs are considered one of the most suitable candidates for makingatomic clocks as they are the least sensitive to external electromagneticfields owing to their exceptionally contracted orbitals. The first HCI clockhas been realized, but its accuracy is much less than the counter opticalclocks based on neutral atoms and singly charged ions. The realization of HCIclocks can add an extra dimension to investigating fundamental physics. In thiswork, we survey HCIs suitable for clock candidates on the grounds of generalfeatures, including their potential to probe temporal variation of $\alpha$.

Highly Charged Ion (HCI) Clocks: Frontier candidates for testing variation of fine-structure constant