arXiv subjects

Marcelo F. Ciappina

Publications and source records attributed to Marcelo F. Ciappina.

At least 19 recordsLinked to original sources

arXiv2026

High-order correlations and ultrafast Wigner negativities in bright-squeezed-vacuum-driven high-harmonic generation

High-harmonic generation (HHG) is a prototypical strong-field process in which intense light drives matter to emit radiation at integer multiples of the driving frequency. Extending HHG into the quantum-optical regime offers new opportunities to probe and control strongly nonlinear light-matter interactions using nonclassical states of light. Yet describing this regime requires a fully quantum treatment of the correlated electron-photon dynamics, which becomes computationally challenging for broadband, strongly squeezed fields. Here we solve the quantum-electrodynamical dynamics of a two-level system driven by bright squeezed vacuum in a converged multimode Hilbert space. Both the driving field and emitted harmonics are fully quantized, with the light-matter interaction treated nonperturbatively. This enables direct access to the multimode quantum state and its higher-order correlations beyond semiclassical sampling or perturbative descriptions. We show that squeezed-vacuum driving produces harmonic emission with qualitatively distinct second- and third-order photon correlations compared with coherent excitation. Moreover, back-action from the driven emitter strongly reshapes the incident squeezed field, generating pronounced Wigner-function negativities that evolve on attosecond timescales. Our results establish a fully quantum framework for broadband strong-field dynamics with squeezed light and provide a route to predicting and interpreting quantum-HHG experiments and their extension to more complex emitters.

physics.optics↗

arXiv2026

Orbital-angular-momentum partition in hydrogen photoionization by a monochromatic vortex beam

Understanding how optical orbital angular momentum (OAM) is transferred to matter requires treating recoil and translational motion alongside the internal electronic dynamics. We develop a center-of-mass-resolved theory of one-photon ionization of hydrogen by a monochromatic Laguerre--Gaussian beam and show that the Bessel-vortex photoelectron predicted in fixed-target models is a preparation-dependent limit. For a sharply defined atomic center-of-mass momentum, the recoil records the photon-cone azimuth, and tracing over it generally destroys the coherence required for a pure electron vortex. In the small-transverse-retardation regime, the optical OAM is transferred predominantly to the center-of-mass motion and hence, in the laboratory frame, to the proton. Finite-retardation corrections redistribute angular momentum between center-of-mass and relative motion, while an additional correlation contribution to the electron and proton angular momenta can be tuned through the spatial uncertainty of the atomic center of mass. These results reveal atomic recoil as a key element of OAM transfer in photoionization.

physics.atom-ph↗

arXiv2026

Multidimensional attosecond clocking near Dirac cones in graphite

Two-color high-harmonic spectroscopy is widely used to access sub-cycle electron dynamics and to retrieve harmonic timing information, including harmonic phases and attochirp across gases, solids, and liquids. However, the dependence of such timing observables on additional laser-control parameters remains largely unexplored. Here, we introduce driving intensity as an additional dimension of two-color harmonic spectroscopy in highly oriented pyrolytic graphite (HOPG). The retrieved attosecond two-color delays maximizing the 4th and 5th harmonic yields evolve systematically and differently with driving intensity. Semiconductor Bloch-equation calculations reproduce these trends and reveal a pronounced sensitivity of the intensity-dependent delays to the electronic band dispersion. Our results demonstrate multidimensional attosecond clocking near Dirac cones and establish intensity-dependent two-color delays as a sensitive observable for band-dispersion in quantum materials.

physics.optics↗

arXiv2026

Geometric Control of Cat States in High Harmonic Generation

High-harmonic generation (HHG) provides a powerful platform for exploring the interaction between intense laser fields and matter on ultrafast timescales. Beyond its conventional description in terms of emitted radiation and electron dynamics, a fully quantum treatment of HHG reveals that the nonlinear light-matter interaction can modify the quantum state of the driving field itself, establishing correlations between the fundamental and harmonic modes. This perspective opens new possibilities for using HHG as a tool to engineer and control nonclassical states of light. In this work, we investigate the geometric properties of optical Schroedinger cat states generated in HHG via conditioning and post-selection. By analyzing the coherent-state displacements induced by different structured driving fields, we characterize the resulting phase-space evolution and the associated geometric phases of the generated quantum states. Particular emphasis is placed on how the polarization and spatial-mode structure of the driving light influence the geometry and evolution of displaced coherent states. Our results demonstrate that structured light offers a versatile means of controlling the geometric dynamics of HHG-generated cat states across the parameter space. Furthermore, we discuss the prospects for realizing genuinely topological optical cat states by exploiting more complex structured light configurations.

physics.optics↗

arXiv2026

Electron Correlation Enables Phase-Coherent One-Attosecond Pulse Trains

Attosecond synthesis is ultimately a phase problem: a broad spectrum produces an ultrashort waveform only if its harmonics remain phase-locked. We show theoretically that correlated two-electron high-harmonic generation in helium driven by a long, multicycle laser pulse supports a train of soft-x-ray bursts with durations approaching 1 as. Using a two-electron strong-field approximation, we calculate the complex harmonic spectrum and reconstruct the temporal emission while retaining its full intrinsic spectral phase, rather than imposing a flat-phase approximation. Despite the trajectory-dependent phase accumulated by two continuum electrons, the correlation-extended plateau contains a broad phase-coherent region reaching the keV range. Its superposition produces reproducible bursts separated by one half-cycle of the driving field. These results identify electron correlation not only as a mechanism for extending the high-harmonic cutoff, but also as a route toward phase-coherent x-ray waveforms on the zeptosecond timescale.

physics.atom-ph↗

arXiv2026

Quantum Kramers-Henneberger Transformation

The classical Kramers-Henneberger transformation connects, via a series of unitary transformations, the dynamics of a quantum particle of mass $m$ located in a trap at position $α(t)$, with the dynamics of a charge $e$ moving in an electric field $eE(t)=-m\ddotα(t)$ within the dipole approximation. In this paper, we extend the classical Kramers-Henneberger transformation to the quantum electrodynamic and quantum optical realm, by explicitly treating the trap location quantum mechanically, thus taking into account the quantum fluctuations of the time-dependent displacement force. Compared to the classical case, we show that quantum electrodynamic corrections appear, and we propose an optomechanical realization for the quantized position of the trap to show that such corrections can manifest in state-of-the-art experiments. These results open the path to novel quantum simulation of quantum electrodynamics and quantum optics of attoscience and ultrafast physics by using ultracold trapped atoms and ions.

quant-ph↗

arXiv2026

Two-Electron Effects Extend High-Harmonic Generation into the keV Regime

Two-electron processes can generate high harmonics beyond the conventional single-active-electron cutoff. Motivated by recent experimental evidence of an extended secondary plateau in the helium high-harmonic spectrum [S. Wang et al, Optica, (2023); S. Wang et al, In Print in Nature Photon., (2026)], we present a two-electron generalisation of the strong-field approximation. We analyse the resulting expressions using the saddle-point method and determine the extended cutoff. We find good agreement with classical predictions of cutoff scalings of $4.7$ and $5.5$ times the ponderomotive energy, which significantly exceed the established single-electron scaling of 3.17. We calculate high-harmonic spectra generated via a two-electron process in helium atoms driven by an intense few-cycle infrared laser pulse. Our results demonstrate that the harmonic spectrum extends far beyond the water window, reaching photon energies up to $\approx 1.2\,\mathrm{keV}$ in the soft x-ray region. The large spectral bandwidth can support the generation of sub-attosecond soft x-ray pulses, which are of particular interest for probing ultrafast dynamics across matter, including applications in core-level spectroscopy and biological imaging.

physics.atom-ph↗

arXiv2026

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

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.

physics.atom-ph↗

arXiv2026

Static-Field Tunneling Ionization in Space-Fractional Quantum Mechanics

Tunneling ionization in static or slowly varying electric fields is a cornerstone of strong-field physics and provides the entry point for semiclassical descriptions of above-threshold ionization and high-harmonic generation. In conventional quantum mechanics, the Perelomov--Popov--Terent'ev (PPT) theory and its Ammosov--Delone--Krainov (ADK) form yield an ionization rate whose defining feature is an exponential dependence governed by an under-barrier (imaginary-time) action. Here we develop an analytical ADK-like tunneling model within \emph{space-fractional} quantum mechanics, where the quadratic kinetic energy is replaced by the Riesz fractional Laplacian of order $1<α\le2$. Working in a static electric field in the length gauge, we derive a closed-form tunneling exponent for a triangular exit barrier. The fractional kinetic operator deforms the conventional $I_p^{3/2}$ scaling to $I_p^{1+1/α}$ and introduces a characteristic $\sin(π/α)$ factor encoding the complex-phase structure associated with nonlocal dispersion. We position this benchmark relative to prior tunneling studies in fractional quantum mechanics (primarily scattering through model barriers and fractal potentials) and provide a validation protocol for testing the exponent in time-dependent simulations of the fractional Schrödinger equation under a constant field. The result establishes a transparent reference for static-field ionization in nonlocal quantum dynamics and a baseline for strong-field approaches extensions.

quant-ph↗

arXiv2026

Two-color harmonic spectroscopy of ultrafast Dirac electron dynamics

High-harmonic generation (HHG), the hallmark effect of attosecond science, is a nonperturbative nonlinear process leading to the emission of high-harmonic light from gases and solids. In gases, extreme driving laser pulse intensities can deplete the ground state, suppressing harmonic emission during the trailing edge of the pulse. Here, we report a similar effect, pronounced ultrafast carrier saturation dynamics and harmonic emission suppression during nonperturbative harmonic generation (NPHG) in a gapless Dirac semimetal -- highly oriented pyrolytic graphite (HOPG). Remarkably, HOPG supports NPHG at laser intensities as low as $\sim 10^{10}$ W cm$^{-2}$, facilitated by its vanishing bandgap. Ultrafast carrier saturation strongly modulates the interplay between interband and intraband currents, a key characteristic of NPHG in Dirac materials. Using two-color spectroscopy, we reveal the excitation dynamics of Dirac electron-hole pairs as it affects the emission of harmonics during the presence of the driving laser pulse. The excitation of out-of-equilibrium hot carriers and the concomitant saturation near the Dirac points leads to a marked suppression of interband harmonics and induces measurable temporal shifts. These observations are supported by simulations based on semiconductor Bloch equations. Our finding reveal that field-driven carrier saturation plays a critical role in gapless solid NPHG. We demonstrate the potential of NPHG and HHG as a sensitive, all-optical probe of ultrafast carrier dynamics, offering novel opportunities for ultrafast optoelectronics in Dirac materials.

physics.optics↗

arXiv2026

Single photon zeptosecond interferometry

We demonstrate the generation of a train of attosecond XUV pulses that are in a superposition of wavefront states. Such superposition yields a high precision, self-referencing, common path XUV interferometer setup to produce pairs of spatially separated and independently controllable XUV pulses that are locked in phase and time with a temporal jitter of 3.5 zs (zs = zeptoseconds = $10^{-21}$). In our approach, we can independently control the relative phase/delay of the two optical beams with a resolution of 52 zs. Since the jitter is on the order of the Compton time scale, we explore the level of correlation between the non-local photons by comparing different spatial mode superpositions. Further, thanks to the stability of the interferometer we can retrieve the interference pattern through photon counting. Through post-selection of different particle events we can analyze one, two or more photon events. We argue that this zeptosecond level of temporal precision will open the door for new dynamical QED tests at lower intensities while photon counting experiments can also have an impact on the emerging field of quantum light in strong fields. We also discuss the potential impact on other areas, such as time-dependent QED, imaging, measurements of non-locality, and molecular quantum tomography.

physics.optics↗

arXiv2025

Optical Vortices: Revolutionizing the field of linear and nonlinear optics

Light is the fundamental medium through which we perceive the world around us. In the modern era, light can not only be used in its raw form but can also be used as a versatile tool. Generally, light fields carry energy and momentum (both linear and angular). Due to the transfer of linear momentum from light to matter, the radiation pressure is exerted, whereas, the intrinsic spin angular momentum (SAM) is associated with the polarization states of light. Light fields embedded with optical orbital angular momentum (OAM) -- also known as optical vortices or phase singular beams -- have truly revolutionized the field of optics and extended our basic understanding of the light-matter interaction process across various scales. Optical vortices -- spatially characterized by the presence of twisted phase fronts and a central intensity null -- have found a myriad of applications starting from microparticle trapping and manipulation to microscopy, optical communication, and quantum information science, among others. Here, we revisit some of the fundamental concepts on optical vortices and discuss extensively on how this new dimension of light i.e., the OAM, has been exploited in both linear and nonlinear optical regimes. We discuss the different types of vortex beams, the techniques used to generate and detect their OAM, and their propagation. Particularly, we put a special emphasis on the utilization of vortex beams in nonlinear regimes to explain different optical phenomena such as the second harmonic generation, parametric down-conversion, and high-order harmonic generation. The generation of vortex beams in the UV to XUV regimes, encoded with higher OAM values, could potentially extend their application range to areas such as high-capacity data transmission, stimulated emission depletion microscopy, phase-contrast imaging, and particle trapping in optical tweezers, among others.

physics.optics↗

arXiv2025

Solid-State High-Order Harmonic Generation: Emerging Frontiers in Ultrafast and Quantum Light Science

High-order harmonic generation (HHG) in solids has emerged as a versatile platform for exploring ultrafast and quantum-coherent phenomena in condensed matter. Recent advances reveal Berry-phase and topological effects in harmonic emission, strong-field control of excitons and lattice motion, the generation of nonclassical light states driven by quantum and squeezed fields, and the emergence of orbital-angular-momentum transfer in solid-state high-harmonic generation. Nanostructured and hybrid plasmonic-semiconductor platforms enable enhanced and spectrally tunable HHG, while interferometric and cryogenic setups allow attosecond-resolved phase measurements. On the theoretical side, multiband and topological models incorporating dephasing, propagation, and electron-hole coherence effects have deepened our understanding of the interplay between interband and intraband dynamics. These developments establish solid-state HHG as a bridge between ultrafast spectroscopy, quantum optics, and material science, paving the way toward quantum-engineered attosecond sources and coherent control of light-matter interactions in solids.

physics.optics↗

arXiv2025

Quantum Optics and Quantum Electrodynamics of Strong Field Processes

In its beginnings, the physics of intense laser-matter interactions was the physics of multiphoton processes. The theory was reduced then to high-order perturbation theory, while treating matter and light in a quantum manner. With the advent of chirped pulse amplification developed by D. Strickland and G. Mourou, which enabled generation of ultra-intense, ultra-short, coherent laser pulses, the need for a quantum electrodynamics description of electromagnetic (EM) fields practically ceased to exist and lost relevance. Contemporary attoscience (AS), and more generally ultrafast laser physics, awarded the Nobel Prize in 2023 to P. Agostini, F. Krausz, and A. L'Huillier, commonly uses the classical description of EM fields while keeping a fully quantum description of matter. The progress and successes of AS in the last 40 years have been spectacular, with an enormous amount of fascinating investigations in basic research and technology. Yet a central question remains: can ultrafast laser physics continue to advance without reintroducing quantum electrodynamics and quantum optics into its description of light-matter interactions? This article discusses future perspectives at the intersection of strong-field physics and quantum optics.

quant-ph↗

arXiv2025

Weak measurement in strong laser field physics

The advantage of attosecond measurements is the possibility of time-resolving ultrafast quantum phenomena of electron dynamics. Many such measurements are of interferometric nature, and therefore give access to the phase. Likewise, weak measurements are intrinsically interferometric and specifically take advantage of interfering probability amplitudes, therefore encoding the phase information of the process. In this work, we show that attosecond interferometry experiments can be seen as a weak measurement, which unveils how this notion is connected to strong field physics and attosecond science. In particular, we show how the electron trajectory picks up a new phase, which occurs due to the weak measurement of the process. This phase can show significant contributions in the presence of spectral features of the measured system. Furthermore, extending this approach to include non-classical driving fields shows that the generated harmonics exhibit non-trivial features in their quantum state and photon statistics. This opens the path towards investigations of attosecond quantum interferometry experiments.

quant-ph↗

arXiv2025

Free temporal evolution of a superbandwidth wave packet: The quantum tale of the hare and the tortoise

Can the interplay between quantum mechanics and classical optics offer new perspectives on wavepacket dynamics? Building on this connection, we show that local momenta with both super-oscillatory and suboscillatory characteristics can arise during the free propagation of a quantum particle. This behavior is mathematically analogous to the propagation of a superbandwidth laser pulse in a dispersive medium, where the instantaneous frequencies of the electric field exhibit similar sub- and super-oscillatory features. This analogy is rooted in the equivalence between the wave description of light in classical optics and the formalism of quantum mechanics. In addition, we explore the phenomenon of wavepacket localization during free propagation within a bounded region. This localization is directly linked to the distribution of local momenta within the confined wavepacket region. To complement our quantum mechanical analysis, we also perform a classical analysis to provide further insights into this phenomenon. Our findings reveal that both the emergence of local momenta with sub- and super-oscillatory features and the wavepacket localization occur within a distinct timescale, which we define as the interference time.

quant-ph↗

arXiv2024

High-Order Harmonic Generation Driven by Perfect Optical Vortex Beams: Exploring the Orbital Angular Momentum Upscaling Law

Orbital angular momentum (OAM) light beams for high-order harmonic generation (HHG) provide an additional degree of freedom to study the light-matter interaction at ultrafast timescales. A more sophisticated configuration is a perfect optical vortex (POV) beam, a light beam with a helical wavefront characterized by a phase singularity at its center and an azimuthal phase variation. POV beams are characterized by a radial profile which is independent of the OAM. Here we study the non-perturbative process of gas-phase HHG using a linearly polarized POV beam. We observe that the harmonics are emitted with similar divergence due the perfectness of the POV-driven harmonics. Furthermore, the topological charge upscaling is rigorously followed. We show that a POV beam is more advantageous than that of the Laguerre-Gaussian beam for cases where a large topological charge with a small core size is required. Our research establishes a pathway for producing bright structured extreme ultraviolet (XUV) coherent radiation sources--a pivotal tool with multifaceted applications across various technological domains.

physics.optics↗

arXiv2024

Analog simulation of high harmonic generation in atoms

The demanding experimental access to the ultrafast dynamics of materials challenges our understanding of their electronic response to applied strong laser fields. For this purpose, trapped ultracold atoms with highly controllable potentials have become an enabling tool to describe phenomena in a scenario where some effects are more easily accessible and twelve orders of magnitude slower. In this work, we introduce a mapping between the parameters of attoscience platform and atomic cloud simulators, and propose an experimental protocol to access the emission spectrum of high harmonic generation, a regime that has so far been elusive to cold atom simulation. As we illustrate, the benchmark offered by these simulators can provide new insights on the conversion efficiency of extended and short nuclear potentials, as well as the response to applied elliptical polarized fields or ultrashort few-cycle pulses.

physics.atom-ph↗