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New Journal of Physics

A field-quadrature perspective on strong-field ionization with squeezed light

Haoyu Liu, Jianmin Yuan, Xu Wang

Published Oct 1, 2026• 0 citations
Publisher

Abstract

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.

DOI
10.1088/1367-2630/aeb154

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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.
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.
The variance of bright squeezed vacuum (BSV) is not only strong but also phase dependent, yet recent strong-field experiments with BSV have recorded only its magnitude. A coherent field of the same color provides the phase reference that resolves this phase-dependent noise. Strong-field ionization of xenon by this light shows that the photoelectron yield follows the BSV quadrature along the coherent field, and that the nonlinearity of ionization amplifies the quadrature noise many times over in the electron yield. The noise is moreover distributed unevenly over the photoelectron momentum distribution, differing by an order of magnitude between ionization channels. Strong-field ionization thus offers a phase-sensitive, channel-resolved detector of quantum light, while the quadrature noise of quantum light offers a probe of the intensity dependence of individual ionization pathways.
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.
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.
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.
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.