Abstract
We devise a perturbation-immune version of Ramsey's method of separated
oscillatory fields. Spectroscopy of an atomic clock transition without
compromising the clock's accuracy is accomplished by actively balancing the
spectroscopic responses from phase-congruent Ramsey probe cycles of unequal
durations. Our simple and universal approach eliminates a wide variety of
interrogation-induced line shifts often encountered in high precision
spectroscopy, among them, in particular, light shifts, phase chirps, and
transient Zeeman shifts. We experimentally demonstrate auto-balanced Ramsey
spectroscopy on the light shift prone $^171$Yb$^+$ electric octupole
optical clock transition and show that interrogation defects are not turned
into clock errors. This opens up frequency accuracy perspectives below the
$10^-18$ level for the Yb$^+$ system and for other types of optical clocks.
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