  Quantum coherence plays a fundamental role in the study and control of
ultrafast dynamics in matter. In the case of photoionization, entanglement of
the photoelectron with the ion is a well known source of decoherence when only
one of the particles is measured. Here we investigate decoherence due to
entanglement of the radial and angular degrees of freedom of the photoelectron.
We study two-photon ionization via the 2s2p autoionizing state in He using high
spectral resolution photoelectron interferometry. Combining experiment and
theory, we show that the strong dipole coupling of the 2s2p and 2p$^2$ states
results in the entanglement of the angular and radial degrees of freedom. This
translates, in angle integrated measurements, into a dynamic loss of coherence
during autoionization.
