Quantum interferences are central to the field of attosecond spectroscopy. For instance, the RABBIT (Reconstruction of Attosecond Beating by Interference of two-photon Transitions) technique, first used to reveal the attosecond structure of the high-harmonic emission, has recently been applied to the study of photoionization dynamics. It was shown to provide an advanced characterization (spectral amplitude and phase) of complex electron wavepackets (EWP) produced, e.g., in structured continua involving autoionizing states.
We will discuss how we may go one step further by performing a quantum state tomography of the EWP (i.e. access to the density matrix) using the recently-proposed MIXED-FROG technique [1,2]. This will provide a measurement of EWP decoherence due to entanglement with unobserved degrees of freedom: in atoms, due to electron-electron interactions, in molecules, due to entanglement with molecular vibrational modes, in aggregates, due to inelastic scattering.
 C. Bourassin-Bouchet C et al., Nature Communications 6, 6465 (2015)  C. Bourassin-Bouchet C et al., Phys. Rev. X, in print (2020)