Achieving ultrafast control over chirality, the fundamental handedness of matter and light, remains a pivotal challenge in chiral photochemistry and information science. Current chiroptical switches are limited to picosecond or nanosecond timescales, constrained by the slow structural rearrangement of molecules. Here, we propose a femtosecond chiroptical switch in a hydrogen atom by exploiting the typically ignored global phase of a Rabi-oscillating system, which we map directly onto the chirality of a bound-electron wave packet. We show that a resonant $2\pi$-area pulse induces chirality inversion primarily via a geometric phase, i.e., the Aharonov-Anandan phase, accumulated during cyclic evolution on the Bloch sphere. This geometric origin endows the switch with high robustness against pulse area errors. By optimizing the initial-state composition, we enhance the geometric phase contribution, thereby reinforcing the switching stability. Our work provides a robust route for ultrafast chirality control, with implications for geometric-phase-based quantum sensing and chiroptical information processing.

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• Coherent control
• Single- and few-photon ionization & excitation
• Ultrafast optics
• Ultrafast phenomena