Zeptosecond Control of Fano Line Shapes in Laser-Dressed Atoms Excited by Time-Delayed Attosecond Pulse Pairs

The attosecond pump-femtosecond probe technique is widely used in electron dynamics research, with temporal resolution reaching tens of attoseconds. Here, we propose a scheme for controlling the Fano line shapes of helium doubly excited states by replacing the pump source with a pair of time-delayed attosecond pulses. We show that the interference induced by this delay provides an additional, highly effective means of controlling electron dynamics, enabling temporal resolution on the zeptosecond scale. We demonstrate that the corresponding line shapes in transient absorption spectra can shift dramatically within a time-delay variation of just a few hundred zeptoseconds. This extreme spectral modulation occurs under two key conditions: (i) when the time delay aligns with the spectral minimum of the coherently combined attosecond pulses in the frequency domain, and (ii) when the delay between the infrared probe pulse and the first attosecond pulse is held constant. This approach opens a new pathway to zeptosecond-scale control in quantum systems and demonstrates the powerful synergy between phase manipulation in both light and electron waves for achieving ultrafast spectral transformations.