arXiv subjects

Jan Marcus Dahlström

Publications and source records attributed to Jan Marcus Dahlström.

At least 19 recordsLinked to original sources

arXiv2026

Coherent Control of Quantum and Classical Correlations in Photoionization

The ability to control quantum correlations in strongly driven systems is a central challenge across quantum science, with implications for ultrafast dynamics, quantum control, and information processing. In photoionization, the emitted electron and residual ion may form an entangled system whose correlations encode the underlying light-matter interaction, yet control of their generation and observable manifestation in continuum systems remains largely unexplored. Here we demonstrate phase-resolved control of electron-ion correlations using phase-locked pulse sequences in the strong-coupling regime. We show that entanglement can be halted and reshaped with attosecond precision, and that phase-dependent correlations can be redistributed into population-based correlations, leading to entanglement that is directly reflected in joint observables. These results establish a route to coherently shape entanglement in photoionization and open new possibilities for accessing and controlling quantum correlations in systems where measurements are intrinsically basis constrained.

quant-ph↗

arXiv2026

Quantitative analysis of resonant ionization by smooth laser pulses: Connection between effective Hamiltonian theory and strong-field dressed continua

Resonant photoionization in the intense high-frequency regime can exhibit extremely asymmetric Autler-Townes doublets whose origin remains debated. It has been attributed either to interference between perturbative ionization pathways or to a non-perturbative dressing of the continuum. Here we show that these interpretations arise from a common effective Hamiltonian framework, in which dressed-state stabilization is governed by the coherent interplay of resonant and nonresonant pathways. We demonstrate that further simplification, using the strong-field approximation, obscures this mechanism. In contrast, our time-dependent essential-state model, where electrons are rigorously coupled to the continuum, achieves excellent agreement with ab initio simulations of the time-dependent Schrödinger equation for helium.

physics.atom-ph↗

arXiv2026

Photoelectron spectroscopy with a resonant dichromatic field: Role of geometric phase

We investigate geometric-phase control in resonantly driven two-level atoms exposed to near-degenerate dichromatic laser pulses. In contrast to conventional two-color schemes based on widely separated frequencies, the closely spaced frequency components generate a slowly varying beating envelope that repeatedly reverses the pseudo-spin rotation on the Bloch sphere. This enables coherent control of the geometric phase accumulated during Rabi dynamics and strongly modifies the resulting photoelectron spectra. Using an exactly solvable model for flat-top pulse envelopes, we derive the essential-state dynamics analytically and analyze photoionization induced both by an auxiliary field and by the dichromatic driving field itself. We show that the formation of Autler--Townes doublets can be interpreted in terms of destructive interference associated with geometric phases acquired during completed pseudo-spin rotations. Beyond the canonical Autler--Townes regime, beat-induced reversal dynamics lead to qualitatively different spectral structures, including re-emergent central peaks and higher-order sidebands. These effects originate from the nonuniform temporal evolution induced by the beating envelope, which modifies the balance between positive and negative excitation amplitudes. Our results establish beating-field control as a route toward engineering geometric-phase interference in ultrafast light--matter interactions and suggest broader applications in coherent control of atoms, molecules, and x-ray-driven systems.

physics.atom-ph↗

arXiv2026

Reconciliation of effective Hamiltonians for intense light-matter interaction

Essential-state models are central for quantum control and technology in broad regimes of light-matter interaction. The canonical effective Hamiltonian is obtained equivalently from adiabatic elimination, the Markov approximation, and the pole approximation. These approximations are known to break down at high intensities, significantly limiting their applicability to moderate light-matter interaction. We show how this limitation can be addressed by applying quasi-degenerate Rayleigh-Schrödinger perturbation theory (QD-RSPT). We reconcile QD-RSPT with adiabatic elimination and propose a quasi-degenerate extension of adiabatic elimination that is robust when the detuning of the essential states is non-negligible. The accuracy of QD-RSPT is demonstrated in both the low- and high-frequency regime, showing excellent agreement with Floquet calculations at high intensities. The crucial corrections to adiabatic elimination make the eigenvectors of the effective Hamiltonian non-orthogonal. Physically, this allows us to account for the asymmetric strength with which different essential states couple to the non-essential states. We expect that our systematic approach to effective Hamiltonians from QD-RSPT will constitute a new state of the art in intense light-matter interaction and quantum optics with novel forms of strong coupling and quantum control phenomena being conceivable.

quant-ph↗

arXiv2026

Coherent control of ionization via stabilization by resonant pulse pairs

We study the nonlinear and resonant process of two-photon ionization of atoms (He and H) in a pump-probe scheme. The pump pulse prepares the quantum system in a superposition of the ground state and an excited bound state. By varying the phase difference between the pulses, we show how it is possible to coherently control the dressed-state population during the probe pulse. Our main result is that for certain laser parameters, the control over the dressed state population leads to strong control of the ionization probability during the probe pulse. The effect arises due to one of the dressed states becoming stabilized against ionization. Contrasting effects from circular and linear polarized pulses demonstrate how such ``bound states in the continuum'' are sensitive to the degeneracy of the coupled continuum.

quant-ph↗

arXiv2026

Causality of ultrafast photoionization from argon 3s using an ab initio relativistic approach

We study real-time photoionization flux at the $3s$ Amusia-Cooper minimum (ACM) in argon using \textit{ab initio} simulations with the relativistic time-dependent configuration-interaction singles (RTDCIS) method in length (LG) and velocity (VG) gauges. A simple analytical model is used to interpret the results, and to construct Wigner delays and Wigner distributions for both gauges and relativistic channels of the photoelectron ($εp_j$ with $j=1/2$ and $3/2$). The two gauges are found to produce qualitatively different ionization dynamics, with LG having positive and VG having negative Wigner delays. The advancement of several femtoseconds, found for Wigner delays in VG, raises some concern for causality when atoms are ionized by attosecond pulses that are shorter than the absolute value of the Wigner delay. Reassuringly, numerical simulations of wave packets with RTDCIS show that the electrons behave in a causal way in both gauges. Weighted delays that take into account the temporal window of excitation (or the bandwidth of the pulses) are constructed from the Wigner distribution to reach agreement between the numerical simulations and our simple wave packet model. Furthermore, a strong effect of spin-orbit coupling of the photoelectron ($j$) is reported for ultrafast photoionization dynamics, and Schrödinger kitten and cat states are identified in the Wigner distributions as a result of the ACM. Our work paves the way for a deeper understanding of ultrafast photoionization and the role of causality in systems with strong electron-electron correlation effects.

physics.atom-ph↗

arXiv2025

Entanglement Transfer in a Composite Electron-Ion-Photon System

We study how entanglement in photoionization is transferred from an electron-ion pair to an electron-photon pair by fluorescence. Time-resolved von Neumann entropies are used to establish how information is shared between the particles. Multipartite entanglement, between electron, ion and photon, is found on intermediate timescales. Finally, it is shown how a phase-locked two-pulse sequence allows for the application of time symmetry, mediated by strong coupling, to reveal the entanglement transfer process by measuring the photon number and electron kinetic energy in coincidence.

quant-ph↗

arXiv2025

Mollow-like triplets in ultra-fast resonant absorption

We show that resonant absorption of smooth laser fields can yield Mollow-like triplet patterns. General conditions for such triplets are derived and illustrated with a super-Gaussian pulse sequence. Gaussian pulses can not exhibit triplets, super-Gaussian pulses can form triplets depending on the pulse area and flat-top pulses can produce absorption triplets after one Rabi cycle. Our results are compared side-by-side with resonance fluorescence to emphasize similarities and differences between these unlike observables. In the high-intensity limit, we show that the central absorption peak is asymmetric, which we attribute to non-linear photoionization, beyond two-level atomic physics.

quant-ph↗

arXiv2025

Photoionization time delays probe electron correlations

The photoelectric effect, explained by Einstein in 1905, is often regarded as a one-electron phenomenon. However, in multi-electron systems, the interaction of the escaping electron with other electrons, referred to as electron correlation, plays an important role. For example, electron correlations in photoionization of the outer $s$-subshells of rare gas atoms lead to a substantial minimum in the ionization probability, which was theoretically predicted in 1972 and experimentally confirmed using synchrotron radiation. However, recent attosecond photoionization time delay measurements in argon strongly disagree with theory, thus raising questions on the nature of electron correlations leading to this minimum. In this work, combining high-spectral resolution attosecond interferometry experiments and novel theoretical calculations allows us to identify the most essential electron correlations affecting the photoemission. The measurement of time delays gives unprecedented insight into the photoionization process, unraveling details of the atomic potential experienced by the escaping electron and capturing its dynamics.

physics.atom-ph↗

arXiv2025

Giant counter-rotating oscillations on the attosecond timescale

We predict an unexplored type of ultrastrong coupling between atoms and intense ultraviolet light that leads to giant population oscillations on the attosecond timescale. These counter-rotating oscillations can be of similar amplitude as the elementary femtosecond Rabi oscillations between the two strongly coupled states. The effect, which is beyond the two-level atom, is non-reciprocal: It only affects the excited state, while the ground state is unaffected. We propose that two-photon Rabi oscillations (1s$^2$-1s3d) in helium is suitable for the generation of this type of ultrastrong coupling with realistic pulses. We use a combination of Floquet theory and effective Hamiltonian theory to test our predictions against ab initio simulations.

quant-ph↗

arXiv2025

Harnessing Time Symmetry to Fundamentally Alter Entanglement in Photoionization

The Grobe--Eberly doublet phenomenon occurs in photoelectron distributions when a field dresses the remaining ion. Its manifestation is due to entanglement between a free electron and a hybrid state of light and matter. Direct detection of such entanglement is however not possible by coincidence schemes due to the dressing mechanism having an inconspicuous phase correlation effect on the ion. Here, it is shown that odd envelopes fundamentally alter the entanglement, such that channel-resolved photoelectron distributions become identifiable in coincidence with the internal state of the field-free ion. This constitutes a first usage of the parity of time symmetry in strong-field interactions.

quant-ph↗

arXiv2024

Spin-Polarized Photoelectrons in the Vicinity of Spectral Features

It has been shown by Fano (1969) https://doi.org/10.1103/PhysRev.178.131 that photoionization of a cæsium atom by a laser pulse tuned to the vicinity of a Cooper minimum generates spin-polarized electrons. Here we show that while photoionization of rare gases does not provide large spin polarization in the vicinity of the Cooper minimum, the Fano resonances yield much higher overall spin polarization ($\ge40\%$). The spin polarization increases in angle-resolved photoelectron spectra, and reaches $100\%$ when measured in coincidence with the photoion. We provide a general framework for achieving spin polarization in photoionization irrespective of the ionization regime.

physics.atom-ph↗

arXiv2024

Generation of entanglement using a short-wavelength seeded free-electron laser

Quantum entanglement between the degrees of freedom encountered in the classical world is challenging to observe due to the surrounding environment. To elucidate this issue, we investigate the entanglement generated over ultrafast timescales in a bipartite quantum system comprising two massive particles: a free-moving photoelectron, which expands to a mesoscopic length-scale, and a light-dressed atomic ion, which represents a hybrid state of light and matter. Although the photoelectron spectra are measured classically, the entanglement allows us to reveal information about the dressed-state dynamics of the ion and the femtosecond extreme ultraviolet pulses delivered by a seeded free-electron laser. The observed generation of entanglement is interpreted using the time-dependent von Neumann entropy. Our results unveil the potential for using short-wavelength coherent light pulses from free-electron lasers to generate entangled photoelectron and ion systems for studying spooky action at a distance.

quant-ph↗

arXiv2024

Circularly polarized RABBIT applied to a Rabi-cycling atom

We utilize the reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) technique to study the phase of a Rabi-cycling atom using circularly polarized extreme ultraviolet and infrared (IR) fields, where the IR field induces Rabi oscillations between the 2s and 2p states of lithium. Autler-Townes splittings are observed in sidebands of the photoelectron spectra and the relative phases of outgoing electron wave packets are retrieved from the azimuthal angle. In this RABBIT scheme, more ionization pathways beyond the usual two-photon pathways are required. Our results show that the polar-angle-integrated and polar-angle-resolved RABBIT phases have different behaviors when the XUV and IR fields have co- and counter-rotating circular polarizations, which are traced back to the different ionization channels according to the selection rules in these two cases and their competition relying on the propensity rule in laser-assisted photoionization.

physics.atom-ph↗

arXiv2023

Control of Spin Polarization through Recollisions

Using only linearly polarized light, we study the possibility of generating spin-polarized photoelectrons from xenon atoms. No net spin polarization is possible, since the xenon ground state is spin-less, but when the photoelectron are measured in coincidence with the residual ion, spin polarization emerges. Furthermore, we show that ultrafast dynamics of the recolliding photoelectrons contribute to an apparent flipping of the spin of the photoelectron, a process that has been completely neglected so far in all analyses of recollision-based processes. We link this phenomenon to the ``spin--orbit clock'' of the remaining ion. These effects arise already in dipole approximation.

physics.atom-ph↗

arXiv2023

Photoelectron signature of dressed-atom stabilization in intense XUV field

Non-perturbative resonant multiphoton ionization $(1+1)$ is studied using the resolvent operator technique. Scaling parameters for effective two-level Hamiltonians are computed for hydrogen and helium atoms to provide a quantitative description of Rabi oscillations at XUV wavelengths, which were recently observed using a seeded Free-Electron Laser [S. Nandi et al., Nature 608, 488-493 (2022)]. The resulting photoelectron spectra exhibit a range of Autler-Townes doublets, which are studied for different intensities, detunings and interaction times. We identify a photoelectron signature that originates from stabilization against ionization of helium atoms interacting with intense circularly polarized XUV light. Thus, our work shows how it is possible to test the prediction of dressed-atom stabilization by Beers and Armstrong [B. L. Beers and L. Armstrong, Phys. Rev. A 12, 2447 (1975)], without the demanding requirement of atomic saturation in the time domain.

physics.atom-ph↗

arXiv2023

Gauge-invariant absorption of light from a coherent superposition of states

Absorption and emission of light is studied theoretically for excited atoms in coherent superposition of states subjected to isolated attosecond pulses in the extreme ultraviolet range. A gauge invariant formulation of transient absorption theory is motivated using the energy operator from Yang's gauge theory. The interaction, which simultaneously couples both bound and continuum states, is simulated by solving the time dependent Schrödinger equation for hydrogen and neon atoms. A strong dependence on the angular momentum and the relative phase of the states in the superposition is observed. Perturbation theory is used to disentangle the fundamental absorption processes and a rule is established to interpret the complex absorption behaviour. It is found that non-resonant transitions are the source of asymmetry in energy and phase, while resonant transitions to the continuum contribute symmetrically to absorption of light from coherent superpositions of states.

physics.atom-ph↗

arXiv2022

Rydberg Atomic Antenna in Strongly Driven Multi-Electron Atoms

We study the role of intermediate excitations of Rydberg states as an example of Kuchiev's "atomic antenna" in above-threshold ionization of xenon, in particular their effect on the coherence between the spin-orbit-split states of the ion. We focus on the case of a laser frequency close to resonant with the spin-orbit splitting, where a symmetry (parity) argument would preclude any coherence being directly generated by strong-field ionization. Using ab initio simulations of coupled multielectron spin-orbit dynamics in strong laser fields, we show how field-driven rescattering of the trapped Rydberg electrons introduces efficient coupling between the spin-orbit-split channels, leading to substantial coherences, exceeding 10 % for some photon energies.

physics.atom-ph↗