Reconciling conflicting approaches for the tunneling time delay in strong field ionization.

Several recent attoclock experiments have investigated the time delay in strong-field tunneling ionization via extremely precise photoelectron momentum spectroscopy. The interpretation of the attoclock experimental results are intricate, due to the entanglement of the laser and Coulomb field-induced dynamics, and caused controversial theoretical discussions. The method of semiclassical propagation matched with the tunneled wave function, 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 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 within the strong-field approximation and to confirm it with the backpropagation method.