Measurement of the absolute timing of attosecond XUV bursts with respect to the driving field

We demonstrate that a simple two-color ionization measurement can be used to extract the time of birth of attosecond extreme ultraviolet pulses. A high-order-harmonic attosecond pulse train generated in xenon gas is used to excite a laser-dressed helium atom, which we model using the Floquet formalism. The interference between ionization paths from different Fourier components of the Floquet states results in the oscillation of ion yield with time delay. Using two IR pulses to create a reference intensity modulation, we obtain the phase of ion-yield oscillations, which provides the absolute timing of attosecond bursts with respect to the driving IR field.