  Several recent attoclock experiments have investigated the fundamental
question of a quantum mechanically induced time delay in tunneling ionization
via extremely precise photoelectron momentum spectroscopy. The interpretations
of those attoclock experimental results were controversially discussed, because
the entanglement of the laser and Coulomb field did not allow for theoretical
treatments without undisputed approximations. The method of semiclassical
propagation matched with the tunneled wavefunction, the quasistatic Wigner
theory, the analytical R-matrix theory, the backpropagation method, and the
under-the-barrier recollision theory are the leading conceptual approaches put
forward to treat this problem, however, with seemingly conflicting conclusions
on the existence of a tunneling time delay. To resolve the contradicting
conclusions of the different approaches, we consider a very simple tunneling
scenario which is not plagued with complications stemming from the Coulomb
potential of the atomic core, avoids consequent controversial approximations
and, therefore, allows us to unequivocally identify the origin of the tunneling
time delay.
