Autobalanced Ramsey Spectroscopy Improving Clock Accuracy and Testing Fundamental Physics

Everyone is trying to make frequency measurements as accurate as possible. But what if the measurement process itself becomes a major source of error? This issue is particularly relevant in the context of state-of-the-art atomic clocks where errors at the 18th decimal place are not negligible anymore. In order to spectroscopically interrogate an atomic clock transition without compromising the clock's accuracy, we devise a perturbation-immune version of Ramsey's method of separated oscillatory fields. By actively balancing the spectroscopic responses from phase-congruent Ramsey probe cycles of unequal durations, we eliminate 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 autobalanced 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. Further applications of this simple and universal approach include next generation coherent population trapping clocks and perturbation-immune readout of quantum sensors. Future long-term clock comparisons searching for physics beyond the standard model will also benefit from this technique.