Coherent control of interfering one- and two-photon processes has for decades been the subject of research to achieve the redirection of photocurrent. The present study develops two-pathway coherent control of ground-state helium atom above-threshold photoionization for energies up to the
N=2 threshold, based on a multichannel quantum defect and R-matrix calculation. Three parameters are controlled in our treatment: the optical interference phase $\mathrm{\ensuremath{\Delta}}\mathrm{\ensuremath{\Phi}}$, the reduced electric field strength $\ensuremath{\chi}={\mathcal{E}}_{\ensuremath{\omega}}^{2}/{\mathcal{E}}_{2\ensuremath{\omega}}$, and the final state energy $\ensuremath{\epsilon}$. A small energy change near a resonance is shown to flip the emission direction of photoelectrons with high efficiency, through an example where
90 of photoelectrons whose energy is near the
2p21Se resonance flip their emission direction. However, the large fraction of photoelectrons ionized at the intermediate state energy, which are not influenced by the optical control, make this control scheme challenging to realize experimentally.
Multiphoton light-matter interactions invoke a so-called "black box" in which the experimental observations contain the quantum interference between multiple pathways. Here, we employ polarization-controlled attosecond photoelectron metrology with a partial wave manipulator to deduce the pathway interference within this quantum 'black box" for the two-photon ionization of neon atoms. The angle-dependent and attosecond time-resolved photoelectron spectra are measured across a broad energy range. Two-photon phase shifts for each partial wave are reconstructed through the comprehensive analysis of these photoelectron spectra. We resolve the quantum interference between the degenerate p→d→p and p→s→p two-photon ionization pathways, in agreement with our theoretical simulations. Our approach thus provides an attosecond time-resolved microscope to look inside the "black box" of pathway interference in ultrafast dynamics of atoms, molecules, and condensed matter.
The controlled generation of nonclassical light beyond single photons remains a central challenge in quantum optics, due to the difficulty of enhancing multiphoton processes while suppressing lower-order excitations. Here we propose an interference-interaction-engineered scheme for programmable few-photon emission in a cavity-QED system of three atoms coupled to orthogonal cavity modes. By adiabatically eliminating an auxiliary Fabry-Pérot cavity, we generate a tunable cavity-mediated spin-exchange interaction
χ, which, combined with a controllable geometric phase
φ, reshapes the many-body dressed-state spectrum. This interplay enables selective addressing of excitation manifolds (
N=1,2,3), establishing a direct mapping between excitation structure and photon-emission channels. For
φ=0, constructive interference enhances the spectral anharmonicity of low-excitation manifolds, yielding tunable single- and two-photon emission associated with the
N=1 and
N=2 manifolds. In contrast, for
φ=2π/3, destructive interference suppresses lower-order excitation pathways and activates a resonant three-photon channel originating from the
N=3 manifold. Importantly, the cavity-mediated interaction
χ further enhances spectral separation between manifolds, enabling a substantial improvement in multiphoton purity while maintaining a sizable photon population. We demonstrate a three-order-of-magnitude enhancement in two-photon purity and more than two orders of magnitude improvement in three-photon emission. Our results establish a unified interference-interaction framework in which effective optical nonlinearities can be programmably engineered through phase and interaction, providing a scalable route toward high-purity multiphoton sources and programmable quantum photonic devices.