Entanglement and Coherence in Attosecond Science Experiments

• Applicant: Professor Dr. Marc Vrakking
• Subject Area: Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 523041401

Attosecond science is a branch of ultrafast laser physics that aims to investigate and possibly control electronic motion on its natural timescale by means of pump-probe experiments. Attosecond pulses formed by the process of high-harmonic generation have wavelengths in the extreme ultra-violet (XUV) to soft X-ray spectral range. Accordingly, attosecond pulses are ionizing radiation for any medium (solid, liquid or gaseous) that is placed in its path. As my research group has recently shown in preliminary theoretical and experimental work, the ion and photoelectron will commonly display quantum-mechanical entanglement, which influences the coherence that attosecond pump-probe experiments rely on. Here I propose to perform a project that will clarify the relation between entanglement and coherence in attosecond molecular science, by performing a series of experiments that combine attosecond pump-probe spectroscopy utilizing isolated attosecond laser pulses with kinematically complete ion-photoelectron coincidence detection. To the best of my knowledge, my laboratory at MBI is the only laboratory worldwide where experiments combining isolated attosecond pulses and coincidence detection can currently be performed. More specifically, two experiments will be performed within the proposal. On the one hand, experiments will be performed in H2, where the isolated attosecond laser pulse forms a maximally entangled ion+photoelectron state. I will investigate how interaction of the ion or the photoelectron with a near-infrared probe laser induces electronic coherence in the ion, which experimentally manifests as an asymmetry in the laboratory-frame H+ ejection when the cation dissociates. I will explore how the electronic coherence depends on the alignment of the molecule with respect to the laser polarization axis and the ejection angle of the photoelectron, and will experimentally investigate how averaging over the photoelectron degrees of freedom leads to a transition from a “closed” to an “open” quantum system. On the other hand, experiments will be performed in N2, where ionization by the isolated attosecond laser pulse will lead to the formation of several different cationic states. The degree to which these states are in a coherent superposition will be probed using dissociation by an ultrashort (<2fs) UV laser pulse, both as a function of the alignment of the molecule with respect to the laser polarization axis and the ejection angle of the photoelectron. It is expected that the degree of electronic coherence in the cation, or, conversely, the degree of entanglement between the ion and the photoelectron, will strongly depend on these parameters. I expect that this proposal will lead to significant clarification of the role of entanglement in attosecond science, and thus will contribute to the further development of attosecond science towards full realization of its promise.

• DFG Programme: Research Grants