Suppressing a light shift with light in a single-ion ¹⁷¹Yb⁺ clock

Excitation of an optical transition is accompanied by electromagnetic radiation leading to an ac Stark shift. For the electric octupole (E3) transition in ¹⁷¹Yb⁺, its small oscillator strength leads to the need for high intensities of MW/m² to drive the transition. Consequently, the resonant frequency is shifted by about 100 Hz during the interrogation pulses, while the unperturbed transition is determined with an accuracy of a few mHz. This is currently achieved using interrogation schemes with nested servo loops, allowing to measure and correct the light shift in real time. However, an uncertainty resulting from intensity variations during the interrogation pulses and slow drifts of the light shift remains. In a new and complementary approach, we utilize the changing sign of the differential polarizability to suppress the light shift directly by combining the 467 nm light used to drive the E3 transition with 976 nm infrared light. Applying both wavelengths from a single photonic crystal fiber, leading to similar intensity profiles at the ion position, makes this method inherently insensitive to pointing instabilities. Controlling the intensity of the infrared light in a nested servo loop during clock operation combines active and passive suppression of the light shift and can reduce the corresponding uncertainty contribution. This approach is particularly promising for multi-ion clocks, where applying equal intensities to all ions is challenging.