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arXiv:2606.07324

Anomalous Autler-Townes Splitting in Resonant Multiphoton Ionization Driven by Bright Squeezed Vacuum

Xu Zhang, Liding Li, Yutong Deng, Xinyou Lv, Yang Li, Marcelo F. Ciappina, Peixiang Lu, Yueming Zhou

Published Jun 5, 2026• 0 citations
arXivHTMLPDF

Abstract

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 $g^{(p+1)}$ limit to the $g^{(p)}$ 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.

DOI
10.48550/arxiv.2606.07324

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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.
Autler–Townes splitting (ATS) is a hallmark phenomenon of strong light–matter interaction, in which the resonant photoelectron energy spectrum splits into distinct peaks when driven by a strong electromagnetic field. We investigate the Autler–Townes Splitting (ATS) in K 2 molecules with quantum light. The triple splitting structures originating from three-photon ionization are analyzed under different types of driving fields, including bright squeezed vacuum (BSV), phase-squeezed coherent (PSC) light and amplitude-squeezed coherent (ASC) light by numerically solving the time-dependent Schrödinger equation. Compared with the results with the coherent-state (classical) light drive, we show that the ATS dynamics is sensitive with the squeezing parameter of quantum light, leading to modifications of the subband structures and splitting magnitudes. In particular, a BSV pulse exhibits a higher splitting efficiency than a classical pulse of the same mean intensity. This work has implications on ultrafast coherent control of molecular reactions with quantum light.
Strong-field ionization is the cornerstone of attosecond physics, which has been extensively studied under coherent-state driving. Recently, the interface between attosecond physics and quantum optics has emerged as a new frontier. Yet, owing to experimental limitations, the role of the quantum nature of light in atomic strong-field ionization has remained unexplored. Here, we demonstrate strong-field ionization of xenon atoms driven by bright squeezed vacuum (BSV) with average pulse energy up to 10 \textmu J. We show that, as a nonclassical state with zero mean field and strong intensity fluctuations, BSV selectively enhances the spider-like holographic structures in the photoelectron momentum distributions. Using a quantum-light-corrected quantum-trajectory Monte Carlo (q-QTMC) model, we attribute this effect to the intrinsic coherence of trajectory pairs emitted within the same subcycle field fluctuation. These dynamically correlated paths exhibit enhanced phase stability and remain robust against dephasing, whereas asynchronous paths are filtered out by field noise. Our results reveal a quantum-fluctuation-induced mechanism for coherence protection in strong-field processes, positioning BSV as an effective coherence filter and establishing a new regime of quantum-enabled noise-resilient ultrafast dynamics.
Nonlinear optical processes, mediated by multiphoton interactions rather than single-photon response, are routinely exploited to enable a range of light-based functionalities in devices and applications. Nonlinear effects are enhanced through higher intensity fields, which is a limiting strategy owing to potential radiation damage. An alternative strategy relies on the fluctuation redistribution typical of quantum light, but experimental demonstrations at the most fundamental level have been limited. Here we report experimental nonlinear tunnelling ionization of isolated atoms, a pivotal nonlinear process that drives high-harmonic generation and forms the basis of attosecond science, boosted by quantum light -- bright squeezed vacuum (BSV). A BSV light with an average pulse energy of 300 nJ achieves an effective intensity equivalent to that of a coherent light with 7.1 {\textmu}J, demonstrating a more than 20-fold quantum boost in nonlinear effect from BSV light. This boost is revealed by matching the peaks of the photoelectron momentum spectra produced by the BSV and coherent light using angular streaking. Furthermore, we demonstrate control of the effective intensity of the BSV by tuning the correlation function at fixed average pulse energy, establishing a robust method to tailor nonlinear processes via quantum statistics rather than classical intensity scaling. These findings may facilitate the development of quantum-controlled strong-field dynamics using tailored quantum light sources.
Abstract Bright squeezed vacuum (BSV) has emerged as a macroscopic nonclassical light source capable of driving strong-field processes. Yet its broad, long-tailed photon-number distribution bears little resemblance to the narrow poissonian distribution of a coherent field, obscuring how the corresponding strong-field responses are connected. We show that field-quadrature space supplies the missing continuous description: coherent, squeezed coherent, and BSV states are Gaussian wave packets distinguished only by their centers and widths. This representation provides a unified framework for tracing the coherent-to-BSV transition at fixed mean photon number and for analyzing both ionization yields and photoelectron spectra. Crucially, BSV is a symmetry-protected endpoint in quadrature space. Near the BSV limit, we identify a simple scaling law of the ionization response associated with a residual coherent component. We show that this scaling arises from parity-induced stationarity with respect to the Gaussian center, together with the accompanying width variation near the BSV limit. The result provides a symmetry-based criterion for approaching the BSV regime and a potential probe of a mode-matched coherent displacement through the strong-field response itself.
Strong-field ionization provides a fundamental route for probing nonlinear electron dynamics on ultrafast timescales, but it has been explored almost exclusively with classical coherent driving fields. Intense quantum light sources, particularly bright squeezed vacuum (BSV), now make it possible to ask how macroscopic photon-number fluctuations affect nonperturbative electron emission. Here, we address this question by performing shot-resolved BSV-photoelectron coincidence measurements in strong-field ionization of xenon atoms, with simultaneous access to the single-shot driving pulse intensity and the energy- and momentum-resolved photoelectron response. By calibrating the intensity-dependent ionization probability with a pulse-integrated Ammosov-Delone-Krainov response, we reconstruct the single-pulse ionization-yield distribution and quantify its fluctuations in electron momentum space. We find that intensity-conditioned BSV-driven spectra recover coherent-field-like cutoff scaling, while the reconstructed yield fluctuations increase toward higher electron energies and momenta, showing that high-energy electrons preferentially sample rare high-intensity BSV shots. Beyond ensemble-averaged spectra or photoelectron-number statistics, this approach resolves the momentum- and energy-dependent transfer of quantum-light fluctuations into atomic ionization yields. These results establish yield fluctuations as observables for strong-field quantum optics and benchmarks for theories beyond the classical-field approximation.
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.