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arXiv:1503.08247 · Physical Review A

Complete photoionization experiments via ultrafast coherent control with polarization multiplexing. II. Numerics and analysis methodologies

Paul Hockett, M. Wollenhaupt, Christian Lux, Thomas Baumert

Published Jul 13, 2015• 11 citations
arXivHTMLPDF

Abstract

The feasibility of complete photoionization experiments, in which the full set of photoionization matrix elements is determined, using multiphoton ionization schemes with polarization-shaped pulses has recently been demonstrated [P. Hockett et al., Phys. Rev. Lett. 112, 223001 (2014)]. Here we extend our previous work to discuss further details of the numerics and analysis methodology utilized and compare the results directly to new tomographic photoelectron measurements, which provide a more sensitive test of the validity of the results. In so doing we discuss in detail the physics of the photoionization process and suggest various avenues and prospects for this coherent multiplexing methodology.

DOI
10.1103/physreva.92.013411

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Quantitative control and measurement of quantum entanglement are essential for advancing quantum technologies. Photoionization induced by ultrashort laser pulses provides a unique platform for studying entanglement between photoelectrons and residual ions, representing one of the most intriguing quantum phenomena in attosecond physics. Although extensive studies have focused on the coherence properties within either the emitted electrons or the ions individually, the electron-ion entanglement has remained largely unexplored. In this work, we bridge this gap by investigating the resonance-enhanced multiphoton ionization of argon atoms driven by two time-delayed ultrashort ultraviolet pulses. Employing state-of-the-art first-principles multi-electron simulations, we demonstrate the ability to reconstruct and precisely manipulate the purity of electron quantum states through detailed analysis of the photoelectron angular distributions. Our results reveal distinct scattering-phase differences among various electron configurations within the same partial wave channel, providing unequivocal evidence of electron-ion correlation and entanglement. With the fast development of free-electron lasers, this study establishes an experimentally feasible framework for directly controlling quantum entanglement in ultrafast ionization processes, offering new insights and powerful methodologies for exploring complex electron dynamics in many-electron systems.
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In this chapter, numerical methodologies/protocols for the complete quantum metrology of a photoionization event (a.k.a. a ‘complete’ photoionization experiment) are outlined. Following the theory outlined in chapter 3, this can be characterized as recovery of the photoionization matrix elements (including phase information) from the experimental measurements or, equivalently, the continuum density matrix. In general, such a recovery or reconstruction may be possible provided the channel functions are known, and the information content of the measurements is sufficient. These methods, specifically the generalised bootstrap retrieval protocol, are developed and illustrated, and they form the basis for the numerical reconstruction work for realistic cases presented in Part II (chapters 5–11).
Photoelectron interferograms, manifested in photoelectron angular distributions (PADs), are high-information, coherent observables. In order to obtain the maximum information from angle-resolved photoionization experiments it is desirable to record the full, three-dimensional (3D), photoelectron momentum distribution. Here we apply tomographic reconstruction techniques to obtain such 3D distributions from multiphoton ionization of potassium atoms, and fully analyze the energy and angular content of the 3D data. The PADs obtained as a function of energy indicate good agreement with previous 2D data and detailed analysis [Hockett et al., Phys. Rev. Lett. 112, 223001 (2014)] concerning the main spectral features, but also indicate unexpected symmetry breaking in certain regions of momentum space, thus revealing additional continuum interferences which cannot otherwise be observed. These observations reflect the presence of additional ionization pathways and, most generally, illustrate the power of maximum-information measurements of coherent observables for quantum metrology of complex systems.