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Highly charged ions (HCIs) are promising candidates for the next generationof atomic clocks, owing to their tightly bound electron cloud, whichsignificantly suppresses the common environmental disturbances to the quantumoscillator. Here we propose and pursue an experimental strategy that, whilefocusing on various HCIs of a single atomic element, keeps the number ofcandidate clock transitions as large as possible. Following this strategy, weidentify four adjacent charge states of nickel HCIs that offer as many as sixoptical transitions. Experimentally, we demonstrated the essential capabilityof producing these ions in the low-energy compact Shanghai-Wuhan Electron BeamIon Trap. We measured the wavelengths of four magnetic-dipole ($M$1) and oneelectric-quadrupole ($E$2) clock transitions with an accuracy of several ppmwith a novel calibration method; two of these lines were observed andcharacterized for the first time in controlled laboratory settings. Compared tothe earlier determinations, our measurements improved wavelength accuracy by anorder of magnitude. Such measurements are crucial for constraining the range oflaser wavelengths for finding the "needle in a haystack" narrow lines. Inaddition, we calculated frequencies and quality factors, evaluated sensitivityof these six transitions to the hypothetical variation of the electromagneticfine structure constant $\alpha$ needed for fundamental physics applications.We argue that all the six transitions in nickel HCIs offer intrinsic immunityto all common perturbations of quantum oscillators, and one of them has theprojected fractional frequency uncertainty down to the remarkable level of10$^{-19}$.

Probing Multiple Electric Dipole Forbidden Optical Transitions in Highly Charged Nickel Ions