## Appendix A Extraction of Delays  
Streaking delays \(\tau_{\textit{streaking}}\) can be determined by least-square fitting of IR vector potential \(A_{IR}\) (Equation (2)) to the momentum expectation value \(\langle p\rangle\) of the spectrum (Figure 11a). In order to obtain \(\langle p\rangle\), continuum wave packet \(\Phi(z,\ t)\) is evaluated for \(t\gg t_{IR}\). Keeping the streaking amplitude as a free fit-parameter accounts for squeezing the streaking amplitude due to a finite EUV pulse duration. The fit works reliably and gives a standard error for the streaking delay fit parameter of \(\Delta\tau_{\textit{streaking}}=\pm\)0.2 as. There are systematic deviations between model function and simulation results in the order of \(10^{-3}\) of the maximum streaking shift.  
**(a)** Photoelectron is generated by a single-attosecond pulse (streaking). IR vector potential (red line) is fitted to the simulated data points. The residuum between model function and data points (blue line) exhibits systematic deviations in the order of \(10^{-3}\) with respect to the streaking amplitude. **(b)** Photoelectron is generated by an attosecond pulse train (RABBITT). Equation (9) (red line) is fitted to the simulated data points. Here, the residuum between model function and data points (blue line) shows systematic deviations in the order of a few percent of maximum side-band intensity.  
In a RABBITT experiment using longer IR pulses (>20 fs), side-band delays are usually determined by fitting a cosine function to side-band intensity \(I_{SB}\)[2; 6; 23; 32]. However, in the simulations presented here, background-free few-cycle RABBITT spectra were calculated using a 5 fs IR pulse, i.e., its finite spectral width comes into play. The corresponding nonresonant two-photon transition matrix elements scale with \(1/\omega_{IR}\). Therefore, low-energy parts of the IR spectrum contribute more strongly to side-band intensity than the high-energy parts. This leads to a red-shift of side-band oscillation with respect to IR center frequency [41]. Determination of RABBITT delays becomes nontrivial, because neglecting the envelope of the EUV pulse train and IR pulses leads to systematic deviations between model function and results. The best agreement was obtained for fitting the square modulus of the convolution of the EUV pulse train envelope with the IR electric field (Equation (9)) to the integrated side-band intensity (Figure A1b):  
\[I_{\text{SB}}(\tau_{\text{IR-EUV}})=I_{0}\bigg{|}\int_{-\infty}^{+\infty}dtA_{ \text{EUV}}^{0}(\text{t}-(\tau_{\text{IR-EUV}}-\tau_{\text{RABBITT}}))\cdot E _{\text{IR}}(t)\bigg{|}^{2} \tag{9}\]  
Here, delay \(\tau_{\text{RABBITT}}\), IR oscillation frequency, IR pulse duration, and \(I_{0}\) are used as fit parameters to account for the red shift. The red shift of the side-band oscillation amounts to 60 meV at \(E_{\text{kin}}=50\) eV for a 5 fs pulse with 1.55 eV central energy. However, as shown in Figure A1b, there are systematic deviations for short IR pulses in the order of a few percent of the maximum side-band intensity, i.e., one order of magnitude larger than in the streaking case. These systematic errors cause instabilities in the fitting procedure that prevent convergence with respect to delay-axis sampling. For our analysis, we compared fit results for different distributions of IR-EUV delay sampling points. The scattering of results provides an accuracy estimate. Thus, RABBITT delays determined with this procedure are subject to a systematic error of \(\Delta\tau_{\text{RABBITT}}=\pm 2\) as.