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arXiv:2508.01621 · Physical Review A

Microscopic analysis of above-threshold ionization driven by squeezed light

Javier Rivera-Dean, Philipp Stammer, Carla Figueira de Morisson Faria, Maciej Lewenstein

Published Oct 28, 2025• 7 citations
arXivHTMLPDF

Abstract

Above-threshold ionization (ATI) is a strong-field-driven process where electrons absorb more photons than required for ionization. While ATI dynamics and outputs are well-understood when driven by classical, perfectly coherent light, the recent development of nonclassical light sources for strong-field phenomena has spurred interest in their effect on the involved electron dynamics. In this work, we present a microscopic quantum optical theory describing ATI under the influence of strong squeezed light. We observe that squeezed light significantly enhances the coupling between light and matter, making their mutual backaction more important than under classical driving. This backaction profoundly impacts the electronic ionization times, as well as the nonclassical properties of the joint electron-light state. This results in pronounced entanglement features, both immediately after ionization and at later times. These entanglement features are reflected in the properties of the quantum optical state of the driving field, revealing notable non-Gaussian features that depend on both the amount of squeezing and the number of ionization events occurring during the interaction.

DOI
10.1103/hb3n-h2zy

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Above-threshold ionization (ATI) is a strong-field-driven process where electrons absorb more photons than required for ionization. While ATI dynamics and outputs are well-understood when driven by classical, perfectly coherent light, the recent development of nonclassical light sources for strong-field phenomena has spurred interest in their effect on the involved electron dynamics. In this work, we present a microscopic quantum optical theory describing ATI under the influence of strong squeezed light. We observe that squeezed light significantly enhances the coupling between light and matter, making their mutual backaction more important than under classical driving. This backaction profoundly impacts the electronic ionization times, as well as the nonclassical properties of the joint electron-light state. This results in pronounced entanglement features, both immediately after ionization and at later times. These entanglement features are reflected in the properties of the quantum optical state of the driving field, revealing notable non-Gaussian features that depend on both the amount of squeezing and the number of ionization events occurring during the interaction.
Squeezed coherent quantum light is described by the complex squeeze parameter ζ = s e i ϑ and the coherent-state displacement parameter β = | β | e i ϕ β . Our study of strong-field ionization [D. Habibović and D. B. Milošević, ], which focused on bright squeezed vacuum quantum light with β = 0 and s ≫ 1 , is extended to ionization by squeezed coherent light. It is shown that the amplitudes s and | β | strongly influence the ionization process, whereas phases ϑ and ϕ β play a less significant role. The present work provides a unified treatment of coherent, squeezed vacuum, and squeezed coherent light within a single theoretical framework. The quantum nature of the driving field is incorporated by averaging the strong-field ionization probability over the Husimi-Kano phase-space distribution. Using neon as a representative target, we analyze how the parameters characterizing squeezed coherent light modify photoelectron spectra and intensity-dependent enhancements. We show that nonclassical photon statistics can lead to a pronounced steepening of the rescattering plateau and, at low mean intensities, to an effective suppression of the nominal high-energy cutoff. These effects originate from the reduced weight of high-intensity field components inherent to squeezed states and have no analog in strong-field ionization driven by a coherent light. Our results highlight the potential for quantum-statistical control of strong-field ionization and provide a foundation for exploring laser-matter interactions driven by quantum light.
Light-matter entanglement plays a fundamental role in many applications of quantum information science. Thus, finding processes where it can be observed is an important task. Here, using a one-dimensional model, we address this matter by investigating theoretically the entanglement between light and electrons generated in the above-threshold ionization (ATI) process. The study is based on the backaction of the ATI process on the quantum optical state of the system, and its dependence on the kinetic energy and direction of the emitted photoelectrons. Taking into account the dynamics of the process, we demonstrate the creation of hybrid entangled states. The amount of entanglement has been studied in terms of the entropy of entanglement. Additionally, we use the Wigner function of the driving field mode to motivate the entanglement characterization when considering electrons propagating in opposite directions.
Bright squeezed vacuum (BSV) light has a vanishing mean optical electric field yet can strongly enhance strong-field nonlinear responses beyond the conventional semiclassical paradigm. Here we examine this scenario in the light-matter strong-coupling regime by investigating resonant multiphoton ionization of atoms driven by BSV, using a fully quantum treatment of both the electron and the field. Our results show that the photoelectron energy spectrum exhibits an anomalous Autler-Townes splitting whose magnitude grows with the Above-threshold-ionization (ATI) order, rather than remaining essentially ATI-order independent as in the case of coherent driving. This behavior reflects a general scaling with the number of absorbed photons and originates from the broad photon-number fluctuations of the driving field together with the resulting electron-field entanglement. We further show that the BSV-induced enhancement of ionization yields evolves with intensity, crossing over from the g(p+1)g^{(p+1)}g(p+1) limit to the g(p)g^{(p)}g(p) limit as Rabi oscillations become established. These results identify a quantum regime of strong-field ionization governed by the interplay of photon statistics, nonlinear transitions, strong coupling, and nonseparable light-matter dynamics.
Strong field physics and quantum optics, two prosperous fields in modern science, have recently been connected with each other. Here, we study the quantum effect of bright squeezed vacuum light and thermal light on above-threshold ionization. With quantum strong-field approximation theory, we reveal a decoherence phenomenon caused by the statistics of photons, in which above-threshold ionization could be degenerated into a classical-like picture under the effect of quantum light. This represents a direct transition from the wavelike nature to the particlelike behavior of photoelectrons. We also reveal the ionization enhancement effect of the bunched light in photoionization from the perspective of autocorrelation function g ( n ) . The number distributions of electrons ionized by different quantum light fields are studied, and it is shown that the photon number distribution can be mapped onto the photoelectron number distribution. This finding enables the production of bunched electrons with strong-field ionization with quantum light.
The ionization of an atom by a nonclassical electromagnetic field has been studied by a method developed to obtain results for an arbitrary initial state of the field, including squeezed vacuum with a large average number of photons. Characteristics of the ionization of atomic systems by nonclassical squeezed light have been demonstrated for the first time. In contrast to the case of a coherent field, a significantly smoother decrease in the contributions of different above-threshold channels to ionization has been found. Features of photo-electron spectra in continuum have been revealed for the first time as a significant overlapping of peaks from different ionization channels.
In recent years, strong-field processes such as high-order harmonic generation (HHG) and above-threshold ionization driven by nonclassical states of light have become an increasingly popular field of study. The theoretical modeling of these processes often applies an approximate phase-space expansion of the nonclassical driving field in terms of coherent states, which has been shown to accurately predict the harmonic spectrum. However, its accuracy for the computation of quantum optical observables like the degree of squeezing and photon statistics has not been thoroughly considered. In this work, we introduce this approximative phase-space description and discuss its accuracy, and we find that it mischaracterizes the quantum optical properties of the driving laser by making it an incoherent mixture of classical states. We further show that this error in the driving field description maps onto the light emitted from HHG, as neither sub-Poissonian photon statistics nor quadrature squeezing below vacuum fluctuations can be captured by the approximative phase-space description. Lastly, to benchmark the approximative phase-space description, we consider the quantum HHG from a one-band model, which yields an exact analytical solution. Using the approximative phase-space representation with this specific model, we find a small quantitative error in the quadrature variance of the emitted field that scales with pulse duration and emitter density. Our results show that using this approximative phase-space description can mischaracterize quantum optical observables. Attributing physical meaning to such results should therefore be accompanied by a quantitative analysis of the error.