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Physics Physics Atomic Physics

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Wednesday 7 October

Atomic Physics · Cross-listed from quant-ph

Coherence-assisted quantum state preparation and thermal control through repeated interactions

Alessandro Prositto and Dvira Segal

We investigate the relaxation dynamics and steady state of a qubit interacting with a coherent environment through an energy-conserving flip-flop interaction within the Markovian repeated-interaction framework. We derive an exact analytical solution for the evolving and stationary state, nonperturbative in the environmental coherence. We show that environmental coherence provides a resource for dissipative state preparation by expanding the set of accessible steady states. In particular, coherent ancillas can increase the system ground- state population beyond that of the bath, corresponding to an effective temperature below the bath temperature, and enhance its steady-state purity, with these effects controlled by the interplay between the system energy scale and the system-ancilla coupling. Nevertheless, we prove that the purity of the system can never exceed the purity of the ancilla. We further analyze the thermodynamics of the protocol and show that environmental coherence can modify and even reverse the natural direction of heat transfer, giving rise to anomalous heat flow. Remarkably, for fixed interaction parameters, the direction of the coherence-induced heat transfer is governed by the relative phase between the system and ancilla coherence, whereas their magnitudes determine the amount of heat exchanged. Our results establish environmental coherence as a resource for controlling steady-state preparation and energy transfer in repeated-interaction dynamics.
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Atomic Physics · Cross-listed from cond-mat.quant-gas

Quantum simulation of material-specific Hubbard models with alkaline-earth atoms

Henning Schlömer, Reuben R. W. Wang, Bo Xing et al. (+2 more)

A long-term goal of quantum simulation is to faithfully reproduce the low-energy Hamiltonians of correlated quantum materials, establishing a direct connection between solid-state experiment, electronic-structure theory, and programmable many-body systems. Here, we introduce an integrated quantum-simulation architecture based on fermionic alkaline-earth atoms that enables programmable control over the key ingredients of multi-orbital material Hamiltonians. Long-lived clock states encode synthetic orbitals, their differential polarizabilities generate orbital-selective tunneling, and contact interactions provide multi-orbital Hubbard and Hund couplings. Spatially structured coherent driving on the ultra-narrow clock transition further realizes symmetry-selective inter-orbital hybridization, while the clock detuning independently controls orbital energy offsets. In this way, the spatial symmetries of solid-state orbitals can be encoded directly into the effective Hamiltonian, even though the atoms themselves remain in conventional $s$-wave Wannier orbitals of the lowest optical-lattice bands. We demonstrate the resulting integrated control using the four-band low-energy Hamiltonian of the bilayer nickelate La$_3$Ni$_2$O$_7$ as a demanding benchmark. Self-consistent calculations show that the microscopic parameters obtained from first-principles modeling can be reproduced within a single experimentally accessible lattice configuration and independently programmed over substantial parameter ranges. Our results establish a general route for translating material-derived multi-orbital Hamiltonians into programmable ultracold-atom quantum simulators.
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All authors Henning Schlömer, Reuben R. W. Wang, Bo Xing, Susanne F. Yelin, H. R. Sadeghpour

Atomic Physics · Cross-listed from quant-ph

Quantum simulation of the Heisenberg XXZ model on a Rydberg atom array

Matthias Werner and Artur García-Sáez

A major application of analog quantum hardware is quantum simulation, where a system evolves over time according to a given Hamiltonian. One challenge when implementing the Hamiltonian on a programmable quantum simulator is the availability of distinct coupling types, many of which go beyond the conventional ZZ-couplings found in Ising Hamiltonians that are naturally implemented by various platforms. In prior work, we have developed a method based on domain wall encoding to simulate one-dimensional spin chains using only Ising Hamiltonians, which would otherwise require XX+YY-couplings as well. Here, we implement the method on a commercially accessible Rydberg atom quantum device, and simulate the time evolution of the Heisenberg XXZ model with various anisotropies. We successfully probe the XY and Ising phases of the model and reproduce the qualitative behavior of several one- and two-body observables in both phases, as well as at the critical point. Our work augments the toolbox to simulate canonical quantum many-body systems on analog quantum simulators.
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Atomic Physics

A scanning cavity for large area coherent coupling in atom interferometry

D. O. Sabulsky, Q. Beaufils, A. Bertoldi et al. (+3 more)

Optical cavities can enhance atom-light coupling in atom interferometry, but free-fall geometries require large transverse modes. Marginally stable resonators provide such modes, yet aberrations and alignment imperfections turn their near-degenerate response into detuning-dependent transverse ring modes, strongly limiting the usable interaction volume under static excitation. Here, we turn this limitation into a resource. By chirping the interrogation frequency across the cavity resonance while dynamically shaping the input amplitude, we map laser detuning onto the transverse position and scan the cavity-enhanced coupling across the atomic cloud. In a cavity-enhanced Bragg diffraction experiment with $^{87}$Rb atoms, this produces a nearly uniform effective interaction region of about 15 mm and increases the transfer efficiency from about 5% to 28%. We also demonstrate phase-coherent operation of a three-pulse atom interferometer using chirped cavity-enhanced Bragg pulses. These results establish scanning-cavity interrogation as a route to large-area, cavity-enhanced atom interferometry.
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All authors D. O. Sabulsky, Q. Beaufils, A. Bertoldi, A. Landragin, P. Bouyer, B. Canuel

Atomic Physics

A solid-state nuclear clock based on VUV absorption spectroscopy of $^{229}$Th

Pengfei Wang, Xu-Fei Yin, Hanlin Wang et al. (+21 more)

We demonstrate sustained operation of a solid-state thorium-229 nuclear clock using continuous-wave absorption at 148.4 nm. In a $^{229}\mathrm{Th}:\mathrm{CaF}_2$ crystal, we resolve four quadrupole components of the D center and a broader O-center resonance. Feedback on the strongest narrow component is updated approximately every 10 s and remains active throughout a 30-h record. A hydrogen-maser-referenced frequency comb measures the output. Its fractional frequency instability follows approximately $1.29\times10^{-11}(\tau/\mathrm{s})^{-1/2}$ and reaches $1.24\times10^{-13}$ at $10^4$ s. A separate 6.6-h run demonstrates feedback on a second quadrupole component. These measurements connect the resolved absorption spectra with sustained nuclear-clock operation and frequency comparisons between crystal segments.
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All authors Pengfei Wang, Xu-Fei Yin, Hanlin Wang, Jinming Liu, Qichen Qin, Zhouzhi Tan, Chengrun Leng, Mingxuan Zhang, Zixuan Li, Wenxin Bu, Xuan Fan, Yihang Liu, Kjeld Beeks, Benedikt Gerstenecker, Sebastian Lahs, Meng Wang, Tung-Hsun Chung, Yong-Heng Huo, Yin Hang, Hanning Dai, Thorsten Schumm, Zhiqiang Zhang, Jian-Wei Pan, Yu-Ao Chen

Atomic Physics

Genuine Molecular Effects in Bound-State Quantum Electrodynamics

Kjell Janke, Konstantin Gaul, I. Agustín Aucar et al. (+3 more)

Quantum electrodynamics (QED) currently features prominently at the precision frontier in describing the structure of atoms and molecules. Dominant QED effects are governed by the electronic structure in the vicinity of the nucleus and are therefore generally regarded as atomic in origin, with molecular contributions largely arising from additive atomic contributions. By studying the nuclear magnetic resonance shielding tensors of thallium in Tl$X$, we reveal genuinely molecular QED effects that strongly depend on the bonding partner ($X$ = F, Cl, Br, I, At) and are absent in the Tl$^+$ ion.
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All authors Kjell Janke, Konstantin Gaul, I. Agustín Aucar, Karol Kozioł, Gustavo A. Aucar, Robert Berger

Atomic Physics

Full molecular dynamics simulations of a single trapped ion in a neutral bath

Saajid Chowdhury, Ruiren Shi and Jesús Pérez-Ríos

We present full molecular dynamics simulations that explicitly incorporate the simultaneous interactions between a trapped ion and a bath of neutral atoms. In contrast to conventional molecular dynamics treatments, this framework enables a systematic assessment of how the atomic-gas density influences both the ion's cooling dynamics and its steady-state mean kinetic energy. Our results show that the gas density measurably modifies the ion's average kinetic energy, albeit only weakly, in qualitative disagreement with predictions obtained from standard molecular dynamics simulations. In addition, the calculations indicate that short-range features of the atom-ion interaction potential become increasingly consequential as the atomic density increases. Simulations including many atoms yield higher mean kinetic energies than standard molecular dynamics, a trend we attribute to the transient formation of molecular-ion complexes. Finally, we observe that for a fixed ion, lighter atomic baths lead to a smaller final ion average kinetic energy than heavier atomic baths.
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Tuesday 6 October

Atomic Physics

Achieving a long lifetime synchronized state in a thermal vapor Rydberg time crystal

William J. Watterson, Dixith Manchaiah and Christopher L. Holloway

Time crystals are a unique and recently discovered state of matter in which time-translational symmetry is spontaneously broken. Although this state has been observed across a variety of quantum systems, understanding the nature between driving, dissipation, and noise and the effects of these forces on the state lifetime remains elusive. Here, we study phase diffusion and entrainment of the time crystalline oscillatory state of a thermal-vapor Rydberg atom ensemble. We apply a pulsed rf heterodyne driving field which can entrain the oscillation frequency during the on driving period. During the off period, the entrained ensemble phase diffuses on a 1/$e$ timescale up to 2.3 ms – significantly longer than the Rydberg state lifetime. We additionally study how the entrainment and phase diffusion timescales are affected by the strength and detuning of the rf driving field, and develop a classical Fokker-Planck model with an Adler-based driving term that reproduces the observed experimental findings. These results demonstrate the creation of a long-time synchronized time crystal Rydberg atom ensemble state, which could be utilized in applications requiring frequency and phase stability.
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Atomic Physics

Time delay in nonadiabatic tunneling ionization

Michael Klaiber, Karen Z. Hatsagortsyan and Christoph H. Keitel

The under-the-barrier recollision pathway during the tunneling ionization of an atom, despite its minuscule amplitude, leaves its signature on the observable photoelectron momentum distribution due to a phase shift relative to the direct ionization path, that can be interpreted as a tunneling time delay. We investigate how this time delay is modified in the nonadiabatic regime, where the tunneling barrier evolves dynamically during the ionization process. Our analysis is based on a simple model of a one-dimensional short-range potential driven by a strong laser pulse which remains amenable to a systematic analytical treatment with the strong field approximation (SFA). We show a significant contribution of the high-order SFA corrections, which describes the recollision channel with recombination and further ionization, and is additional to the rescattering. Whereas the recombination channel dominates over the rescattering for the tunneling time delay in the quasistatic regime at relatively large laser fields, their contributions are reversed in weak fields.
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Atomic Physics

Total-field atomic magnetometry using an elliptically-polarized frequency-modulated light beam for geomagnetic-field applications

D.V. Brazhnikov, A.O. Makarov, K.S. Kozlova et al. (+10 more)

We investigate an all-optical scheme for high-sensitivity measurements of the Earth's scale magnetic field ($B$). The scheme is based on the Bell-Bloom technique with frequency-modulated light. A single elliptically polarized light beam is used both for pumping the alkali-metal atoms and probing Larmor precession of their spins under the external magnetic field. In contrast to many other high-sensitivity magnetometry schemes based on nonlinear polarization rotation of the light, the proposed approach enables the observation of magnetic resonance via changes in the light-beam ellipticity parameter. The approach has been validated in experiments with a vertical-cavity surface-emitting laser, irradiating a $0.125$ cm$^3$ cesium vapor cell filled with a buffer gas. In the current experimental conditions, the achieved sensitivity is estimated at $220$ fT/$\surd$Hz under $B$$\,\approx\,$$50$ $\mu$T, while the shot-noise floor corresponds to $\approx\,$$20$ fT/$\surd$Hz. The bandwidth is estimated at $\approx\,$$1$ kHz. The proposed simple and robust single-beam scheme is well suited for miniaturization and is therefore particularly promising for the development of compact, highly sensitive, low-power-consumption atomic magnetometers for a wide range of applications in the geomagnetic field.
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All authors D.V. Brazhnikov, A.O. Makarov, K.S. Kozlova, M.A. Rusina, A.M. Mikhailov, V.A. Vasiliev, E.V. Tanakov, A.N. Goncharov, M.A. Bobrov, Ya.N. Kovach, A.A. Blokhin, N.A. Maleev, S.A. Blokhin

Atomic Physics · Cross-listed from cond-mat.quant-gas

Polaronic Response of a Supersonic Impurity Strongly Coupled to a Bose Condensate

Sooshin Kim, Yoonsoo Kim, Seokmin Jang et al. (+1 more)

How a mobile impurity exchanges momentum and energy with a many-body environment is a central question in nonequilibrium quantum physics. This exchange becomes particularly complex when the impurity moves rapidly and is strongly coupled to its bath. Here, we realize an interaction-tunable cold-atom collider with fermionic $^{40}$K impurities immersed in a $^{23}$Na Bose–Einstein condensate. A species-selective Raman pulse simultaneously launches the condensate and quenches the interspecies scattering length, producing an initial relative speed 36 times the condensate speed of sound. We track the ensuing relative motion as the interspecies interaction is tuned from weak coupling to resonance. At weak and intermediate coupling, the impurity dynamics are well described by a finite-energy two-body collision model. Near resonance, however, the early-time impurity acceleration is markedly suppressed relative to the two-body prediction, corresponding to a strong enhancement of the apparent dynamical inertia. The enhancement is observed near two distinct Feshbach resonances. These observations provide evidence for a polaronic response in the strongly coupled, supersonic regime of a degenerate Bose–Fermi mixture.
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All authors Sooshin Kim, Yoonsoo Kim, Seokmin Jang, Jee Woo Park

Atomic Physics

Kaonic Atom X-ray Spectroscopy: From SIDDHARTA-2 to the EXKALIBUR Program

F. Sgaramella, L. Abbene, F. Artibani et al. (+31 more)

X-ray spectroscopy of kaonic atoms offers a unique experimental window on the interplay between low-energy strong interaction and precision atomic physics studies. This article reviews the scientific progress achieved with SIDDHARTA-2 at DAFNE and discusses how it shapes the next generation measurements. Particular emphasis is given to EXKALIBUR, a multifacility program designed to perform measurements of kaon-nucleus interactions across the periodic table and to extend kaonic atom spectroscopy toward precision tests of bound-state QED and the charged-kaon mass extraction. The first proposed strong interaction campaign at J-PARC aims to investigate kaonic atoms with solid Li, Be, and B targets using new 1-mm-thick Silicon Drift Detectors and high-resolution TES microcalorimeters, exploiting isotope pairs to isolate nuclear-structure and kaon-multinucleon effects. Measurements of Mg, Al, Si, and S with CdZnTe detectors will broaden this study toward heavier nuclei. EXKALIBUR will establish a systematic experimental framework for testing low-energy strong interaction with strangeness and bound-state QED predictions across a broad range of kaonic atoms.
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All authors F. Sgaramella, L. Abbene, F. Artibani, M. Bazzi, G. Borghi, D. Bosnar, M. Bragadireanu, A. Buttacavoli, A. Clozza, F. Clozza, R. Del Grande, L. De Paolis, K. Dulski, C. Fiorini, I. Friščić, C. Guaraldo, T. Hashimoto, M. A. Iliescu, M. Iwasaki, A. Khreptak, S. Manti, J. Marton, P. Moskal, H. Ohnishi, F. Principato, A. Scordo, M. Silarski, D. Sirghi, F. Sirghi, M. Skurzok, A. Spallone, K. Toho, J. Zmeskal, C. Curceanu

Monday 5 October

Atomic Physics · Cross-listed from quant-ph

Out-of-control Hamiltonian Learning

Weiyuan Gong, Muzhou Ma, Sitan Chen et al. (+2 more)

Learning the Hamiltonian of a many-body system from its dynamics is a central task in quantum science, yet the algorithms with the strongest provable guarantees assume some level of quantum control–fast, arbitrary single-qubit gates interleaved with time evolution, and measurements in arbitrary bases–that is beyond the capabilities of near-term analog quantum simulators. Motivated by analog atom- and ion-based platforms, we study Hamiltonian learning under minimal access models. Uniform state preparation and measurements: We first consider the setting where in every experiment, one can rotate each qubit to the same state, perform short-time evolution, and measure every qubit in the same basis. Surprisingly, we show that for generic 2-local Hamiltonians on any interaction graph, all of the parameters can be reconstructed from such experiments. Computational basis state preparation and measurements: We then consider a similarly constrained setting, but where state preparation and measurement are restricted to the computational basis. For nearest-neighbor Hamiltonians with only Pauli $X/Z$ interactions, a class which captures contemporary Rydberg atom platforms, we show that over 1D and 2D rectangular lattices, all of the parameters can be reconstructed from such experiments up to unavoidable gauges. Our protocols introduce new techniques for solving structured polynomial systems over an extensive number of parameters. Taken together, our results suggest that one can learn a great deal from the dynamics of quantum many-body systems even under the most stringent experimental constraints.
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All authors Weiyuan Gong, Muzhou Ma, Sitan Chen, Jordan Cotler, Hsin-Yuan Huang

Atomic Physics

The influence of the binding energy on Compton-scattering-driven ionization

N. Melzer, M. Kircher, G. Kastirke et al. (+31 more)

We present a comprehensive study of Compton scattering of 20 keV photons by bound electrons over a wide range of binding energies (Ne 2p, Ne 2s, C 1s, O 1s, and Ne 1s orbitals). In particular, we have measured fully differential cross sections, i.e., electron angular distributions with respect to the direction of the photon momentum transfer for fixed electron energy. Our data show strong deviations from the predictions of the ”quasi-free” electron approximation. The binding energy and the shape of the Coulomb potential from which the electron escapes both play a significant role in its kinematics. We find that Compton electrons are scattered and even back-reflected by the ionic potential. The strength of this effect depends on the orbital from which the electron is ejected.
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All authors N. Melzer, M. Kircher, G. Kastirke, J. Rist, D. Tsitsonis, A. Pier, L. Kaiser, J. Kruse, N. Anders, J. Stindl, L. Sommerlad, O. D. McGinnis, M. Schmidt, L. Nowak, A. Kügler, I. Dwojak, P. Daum, P. Roth, M. Astaschov, J. Drnec, K. Lin, N. Wong, F. Trinter, M. S. Schöffler, L. Ph. H. Schmidt, D. M. Haubenreißer, N. M. Novikovskiy, N. Diehl, Ph. V. Demekhin, S. Brennecke, M. Lein, S. Eckart, T. Jahnke, R. Dörner

Atomic Physics

Sub-Doppler cooling in a compact conical hollow mirror integrated with anti-Helmholtz coils

Shintaro Nagase, Naoya Ozawa, Teruhito Nakashita et al. (+7 more)

We demonstrate sub-Doppler cooling of rubidium ($^{87}$Rb) atoms using a compact, single-beam configuration based on a conical hollow mirror integrated with anti-Helmholtz coils. The conical geometry converts a single incident beam into an effective six-beam magneto-optical trap (MOT) configuration while focusing the light towards the cone axis, thereby reducing the required laser power. The number of atoms in the conical MOT is maximized at an incident intensity of 6.5 $\mathrm{mW/cm^2}$ in our conical MOT, several times lower than in conventional six-beam MOTs. Under optimal conditions, we trap $\sim 1 \times 10^7$ $^{87}$Rb atoms. We obtain sub-Doppler temperature of $T_r=13.4\pm 1.2\ \mathrm{\mu K}$ and $T_z=106\pm 11\ \mathrm{\mu K}$ by polarization gradient cooling (PGC), revealing a strong anisotropy that we attribute to the polarization pattern created by the conical mirror. In addition, a modified release-and-recapture measurement in the MOT yields a radial temperature $T_r = 1.20\pm 0.02$ mK, approximately five times higher than that expected for comparable light-shift parameters in a six-beam MOT, in agreement with previous simulations. Our results establish conical hollow mirrors as a practical and power-efficient platform for compact cold-atom sources and integrated quantum sensors.
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All authors Shintaro Nagase, Naoya Ozawa, Teruhito Nakashita, Masaki Nakazawa, Kota Abe, Yu Nezu, Ryosuke Tsutsui, Hideki Ueno, Hiromitsu Haba, Yasuhiro Sakemi

Atomic Physics

Bifurcation of laser-dressed band extrema in solids under intense low-frequency fields

Alexander Flegel, Liang-Wen Pi, Liang-You Peng et al. (+1 more)

We show that an intense low-frequency laser field can qualitatively modify the local extremal structure of an interband energy gap in a crystal. Using an instantaneous laser-dressed basis obtained from the nonperturbative diagonalization of the multiband crystal Hamiltonian, we derive a quartic normal form for the dressed gap near a high-symmetry point. At a critical field, the quadratic part of this normal form changes sign, and the gap experiences a bifurcation: a single symmetric minimum splits into two symmetry-related minima separated by a local maximum. We identify the microscopic origin of this bifurcation by decomposing the quadratic part into a homogeneous Stark contribution and a field-modified band-curvature contribution. The latter originates from the multiband redistribution of interband polarizations and provides the mechanism for the field-induced change of the gap extrema. A realization for the Kronig–Penney model demonstrates the same bifurcation at multiple high-symmetry points and for different interband gaps. As an illustration of its spectral consequences, we show that the bifurcation gives rise to an additional van Hove feature in the instantaneous interband susceptibility above the absorption edge. Our results establish a general mechanism by which strong-field dressing modifies the local geometry of the electronic spectrum.
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All authors Alexander Flegel, Liang-Wen Pi, Liang-You Peng, Mikhail Frolov

Sunday 4 October

Atomic Physics

Onset of Melting in Finite Ion Crystals: The Role of Structural Isomerization

Boris V. Pashinsky and Boris Blinov

We theoretically investigate the microscopic mechanism of melting in finite two-dimensional ion crystals confined in anisotropic traps. Building on our previous studies of melting probability as a function of temperature and anisotropy, we extend the analysis to crystal sizes ranging from $N=4$ to $N=101$ ions and examine how structural isomerization modifies the melting pathways. Using molecular dynamics simulations together with Metropolis-Hastings equilibrium sampling, we analyze radial fluctuations, angular disorder, Lindemann parameters, and isomer-dependent energy landscapes in these Coulomb crystals. We find that crystals with identical particle number and trap anisotropy can exhibit substantially different melting behavior depending on the metastable structural isomer they occupy. Contrary to a simple soft-mode instability picture, the observed melting process is characterized by a gradual thermally-driven accumulation of radial and angular disorder. Structural isomerization reshapes the fluctuation channels through which delocalization develops, leading to nonuniform melting probabilities across temperature and anisotropy space. These results provide a unified microscopic picture of melting in finite ion crystals and clarify the role of structural rearrangements in finite-size phase transitions.
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Atomic Physics

A cryogenic apparatus with a rotatable substrate for matrix isolation spectroscopy of BaF in neon crystals down to 3.2 K

Alessandro Lippi, Diego Bisero, Caterina Braggio et al. (+7 more)

We present a new experimental apparatus for matrix isolation spectroscopy capable of growing cryogenic crystal matrices doped with barium monofluoride (BaF), a candidate for measurements of the electron electric dipole moment (eEDM). \\ In this work, BaF molecules are produced in a cryogenic buffer-gas source and used to dope a neon matrix grown on a substrate, cooled by a cryocooler, reaching temperatures as low as 3.2 K. A rotatable flange allows for multiple substrate orientations, optimizing spectroscopic investigations and matrix crystal growth. \\ Fluorescence emission analysis of BaF in neon crystal show emission when exciting the $A\,^2\Pi_{1/2}$, $A\,^2\Pi_{3/2}$ and $B\,^2\Sigma^+$ states, along with the presence of fluorescence from barium atoms in the crystal. A first estimation of the column density of BaF in the crystal is given on the order of $5 \times 10^{-13} \, \text{cm}^{-2}$. \\ We also demonstrated the production of BaF using parahydrogen as the buffer gas of the cryogenic source, a first step toward BaF molecules embedded in a para-hydrogen matrix. \\ Finally, we also observed fast fluorescence decay, attributed to optical bleaching effects, whose amplitude arise at lower temperatures or higher laser intensities, introducing a systematic effect to be studied in future experiments.
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All authors Alessandro Lippi, Diego Bisero, Caterina Braggio, Giovanni Carugno, Roberto Calabrese, Federico Chiossi, Marco Guarise, Madiha M. Makhdoom, Giuseppe Messineo, Jacopo Pazzini

Saturday 3 October

Atomic Physics · Cross-listed from cond-mat.mtrl-sci

Quantitative near-atomic-resolution oxidation-state mapping in complex oxide heterostructures by electron energy-loss spectroscopy

Ioannis Iatrakis, Zitao Zhang, Ricardo Egoavil et al. (+3 more)

We present a theoretical framework for quantitative oxidation state mapping of transition-metal and rare-earth elements from near-atomic-resolution electron energy-loss spectroscopy (EELS) in complex oxide heterostructures. The total inelastic differential cross section of transition-metal L$_{2,3}$ and rare-earth M$_{4,5}$ edges is calculated by combining the relaxed Dirac-Hartree-Fock ionization cross section with ligand-field multiplet theory for the discrete transitions. We apply the approach to two correlated-oxide heterostructures, La$_{0.67}$Sr$_{0.33}$MnO$_3$/LaCr$_{0.2}$Mn$_{0.2}$Fe$_{0.2}$Co$_{0.2}$Ni$_{0.2}$O$_3$/SrTiO$_3$ and NdNiO$_3$/La$_{0.67}$Sr$_{0.33}$MnO$_3$/NdGaO$_3$, which include a wide range of transition-metal and rare-earth elements with overlapping features. The method resolves the oxidation states of Cr, Mn, Fe and Co and identifies the Ni contribution within the LaCr$_{0.2}$Mn$_{0.2}$Fe$_{0.2}$Co$_{0.2}$Ni$_{0.2}$O$_3$ layer at near-atomic resolution. We find predominantly Co$^{2+}$ together with mixed Mn$^{2+}$/Mn$^{3+}$/Mn$^{4+}$ character, indicating site-specific valence redistribution within the layer. We also separate overlapping spectral features, such as La-M$_{4,5}$ and Ni-L$_{2,3}$ edges in the NdNiO$_3$/La$_{0.67}$Sr$_{0.33}$MnO$_3$/NdGaO$_3$ heterostructure, thereby recovering the intrinsic Ni-L$_{2,3}$ excitation spectrum. We find that the effective Ni valence in NdNiO$_3$ deviates from the simple ionic picture. More broadly, this work establishes a generic method for quantitative valence mapping from near-atomic-resolution EELS, applicable to a wide range of experimental conditions for the study of local electronic structure of complex oxides.
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All authors Ioannis Iatrakis, Zitao Zhang, Ricardo Egoavil, Emma van der Minne, Christoph Baeumer, Frank de Groot

Atomic Physics · Cross-listed from physics.optics

Phase-matching in high-order harmonic generation with a plasma-reshaped driving field

Gaspard Beaufort, Venus Poulain, Melvin Redon et al. (+13 more)

The transition from titanium-sapphire to ytterbium-based laser technology enables turnkey implementations of high-order harmonic generation (HHG) setups at high repetition rates, opening up new applications, e.g., in semiconductor metrology. This, however, requires exploring and understanding the impact of new experimental conditions. Here, we study HHG in neon over the spectral range 8-17 nm with high spectral resolution, using a post-compressed ytterbium laser. Our measurements reveal a large pulse-energy- and pressure-dependent spectral shift, spanning more than two harmonic orders. The femto-chirp of the harmonic emission combined with the blueshift of the fundamental field allows us to relate the observed spectral shifts to the temporal window of harmonic generation, which is determined by phase-matching. To understand these dynamics, it is essential to consider the reshaping of the driving laser field inside the plasma, despite the low driving intensity and long pulse duration. We introduce an on-axis phase-matching description that captures these effects and reproduces the experimental and simulated trends. These results demonstrate the central role of plasma-induced laser reshaping in phase-matching of HHG, which should be accounted for when optimizing sources in a multi-parameter landscape.
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All authors Gaspard Beaufort, Venus Poulain, Melvin Redon, Robin Weissenbilder, Elisa Appi, Filip Salbrink, Saga Westerberg, Aimee J. Ross, Sylvianne Roscam Abbing, David P. O'Dwyer, Yin Tao, Per Eng-Johnsson, Cord L. Arnold, Anne L'Huillier, Peter Smorenburg, Anne-Lise Viotti

Atomic Physics

Origin of the inhomogeneous linewidth in a solid state $^{229}$Th:CaF$_2$ nuclear clock

Kai Li, Tian Ooi, Jack F. Doyle et al. (+7 more)

Narrow linewidth (high-Q) quantum discriminators are essential for realizing solid-state $^{229}$Th nuclear clocks, yet the observed transition linewidths remain at the tens-to-hundreds-of-kilohertz level, far exceeding the sub-Hertz linewidth set by the radiative lifetime of the nuclear isomer. Here, we investigate the origin of this broadening in $^{229}$Th:CaF$_2$ through systematic measurements of the linewidth as functions of temperature, thorium concentration, and quadrupole state, accompanied by theoretical analysis and first-principle simulations. We find that the linewidth is independent of temperature, increases linearly with thorium concentration, and depends strongly on the quadrupole state. Data analysis identifies variations of the local electric-field gradient (EFG) as the dominant source of inhomogeneous broadening and reveals that the EFG disorder increases linearly with thorium concentration. The observed Lorentzian line shapes and linewidth linear concentration dependence are explained naturally from the $1/r^3$ scaling of a random distribution of long-range dopant-induced perturbations. Complementary atomistic and ab initio density-functional-theory calculations reproduce these characteristic features and provide insights into the microscopic defect configurations responsible for the broadening. These results establish the dominant broadening mechanism in solid-state $^{229}$Th and provide guidance for engineering nuclear-clock materials with reduced inhomogeneous broadening.
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All authors Kai Li, Tian Ooi, Jack F. Doyle, Konstantin Herb, Emil Pellett, Martin Pimon, Thorsten Schumm, Steven M. Girvin, Leo Radzihovsky, Jun Ye

Atomic Physics

Frequency-Axis Calibration via Autler-Townes Splitting in an Electromagnetically Induced Transparency Feature

Stone B. Oliver, Noah Schlossberger, Tate McDonald et al. (+2 more)

Rydberg atom-based electrometry is a pivotal technology that enables radio-frequency (rf) field measurements traceable to the International System of Units. This is typically accomplished by converting the direct rf power measurement of classical probes to a frequency measurement referenced to two well-defined spectral features, a commercial wavemeter, or narrow cavity lines produced via clock disciplined electro-optic modulators, although there exist cases where none of these are available. In this article, we demonstrate that a known rf frequency scale can be used to infer a laser's frequency scan using Autler-Townes (AT) splitting. In other words, we show that frequency-axis calibration can be achieved using the avoided-crossing behavior of near-resonant rf radiation. The relative separation between AT-split features is measured as a function of an applied rf field's power and detuning from atomic resonance. These peak separations are then fit to a simple, two-level model using a weighted nonlinear least-squares regression analysis that utilizes three fitting parameters, one of which is the desired calibration factor. While we do not benchmark this method against an absolute frequency reference, we do show that this method is (1) consistent with calibration via known fine-structure separation to within approximately 1.11% or better, and (2) robust against substantial experimental deviation from the system's known rf power dependence.
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All authors Stone B. Oliver, Noah Schlossberger, Tate McDonald, Christopher L. Holloway, Nikunjkumar Prajapati

Friday 2 October

Atomic Physics

CDW-EIS calculations of atomic ionization by bare ions and antiprotons

Darío M. Mitnik, Jorge E. Miraglia and Claudia. C Montanari

We present ab initio calculations of atomic ionization by bare ions and antiprotons within the continuum distorted-wave eikonal initial-state (CDW-EIS) model. The dataset covers atomic targets from H to Kr, impacted by bare ions over a broad range of projectile charges and by antiprotons. Particular attention is given to elements in periods 3 and 4 of the periodic table, which are relevant to pigment analysis and technologically important materials. The resulting subshell-resolved total ionization cross sections are made openly available through an online repository covering a broad range of projectile–target systems, occupied initial subshells, and projectile velocities. We analyze systematic trends in the cross sections, including their dependence on projectile charge and velocity, binding-energy scaling, and charge-sign effects in proton and antiproton impact. To extend the practical coverage of the database, we also develop a neural-network surrogate, CDWEIS-NN, trained on the subset of the database with projectile charge $|Z_\mathrm{P}|\leq 8$. The network interpolates the CDW-EIS results to additional projectile–target systems and impact energies, reproducing both held-out CDW-EIS calculations and the experimental benchmarks considered in this work with satisfactory accuracy.
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Atomic Physics · Cross-listed from quant-ph

Rydberg-mediated microwave-to-optical transduction with tunable bandwidth

Iason Tsiamis, David Petrosyan and Anders S. Sørensen

Coherent microwave-to-optical transduction is essential for hybrid quantum networks. Typical transducers, however, have bandwidths set by fixed device parameters and face efficiency-bandwidth tradeoffs. We propose a cavity-based microwave-to-optical transducer mediated by a collective Rydberg excitation, where external driving fields tune the conversion bandwidth in situ from narrowband, spectrally selective operation to broadband operation without modifying cavity parameters. Under impedance-matching conditions, the system operates as a continuous-wave phase-preserving linear transducer, maintaining both near-unit efficiency and quantum-limited added noise throughout the tunable bandwidth range.
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Atomic Physics

Electrical and optical dipole polarizabilities of two lowest triplet states of the beryllium atom

Tymoteusz Mrozek and Krzysztof Strasburger

High-accuracy static and dynamic polarizabilities of the $2^3P_u$ and $^{3}S_{g}$ states of the neutral beryllium atom are reported. The calculations employed explicitly correlated Gaussian (ECG) basis sets, and first-order wave functions were obtained by minimization of the Hylleraas functional. The final basis sets used for the response-property calculations contained c.a. 7000-8000 functions, depending on the polarizability component and the chosen symmetry. The results cover frequencies from the static limit, \(\omega = 0\), up to the third resonance for the \(2^{3}P_{u}\) state and up to the second resonance for the \(3^{3}S_{g}\) state. To the best of our knowledge, these are the first calculations of dynamic and static polarizabilities for these excited states of the beryllium atom using explicitly correlated basis sets.
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Atomic Physics · Cross-listed from physics.plasm-ph

X-ray spectroscopy of hot dense plasmas using LLNL's upgraded Titan 2$\omega$ short-pulse laser

A. J. Fairchild, D. T. Bishel, T. Cordova et al. (+10 more)

We are developing an experimental platform at Lawrence Livermore National Laboratory's Jupiter Laser Facility to characterize X-ray emission from highly charged ions in the high-density ($n_e > 10^{23}$ cm$^{-3}$), high-temperature ($T_e \sim 1$ keV) regime, leveraging the recently upgraded frequency-doubled short-pulse Titan laser. These measurements will provide experimental benchmarks for high electron density effects on atomic structure, including Stark broadening, ionization potential depression, density-dependent line shifts, and suppression of dielectronic recombination, effects that are relevant to spectral modeling packages used by both the high-energy-density and astrophysics communities. Here, we describe the current status of the platform and present results from first experiments using the upgraded frequency-doubling capability, including calibrated K-shell spectra acquired from five time-integrated X-ray channels. Comparisons with MERL line shape calculations and with spectra recorded at AWE's Orion Laser Facility indicate that the upgraded Titan laser reliably produces the necessary plasma conditions, establishing a foundation for systematic benchmarking of high-density atomic physics in future campaigns.
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All authors A. J. Fairchild, D. T. Bishel, T. Cordova, H. Flores-Alimboyoguen, M. F. Gu, P. Hamilton, N. Hell, M. J. MacDonald, E. W. Magee, R. C. Mancini, E. V. Marley, R. Shepherd, G. V. Brown

Atomic Physics · Cross-listed from cond-mat.mtrl-sci

Floquet density response in laser-assisted fast-electron scattering from solids

Ferdinand Evers, Sascha Schäfer and Richard Korytár

We extend the Bethe–Floquet formalism of Joachain and coworkers, originally developed for laser-assisted electron–atom collisions, to inelastic scattering of fast electrons from a many-body condensed-matter target in a time-periodic light field. At first order in the projectile–target interaction, the general Floquet–Fourier cross section separates into exact laser-dressed projectile kernels and a matrix-valued Floquet density response of the target. For spatially structured or translationally non-invariant systems, the latter is the bi-momentum Floquet structure factor; its momentum diagonal defines the Floquet generalization of the dynamic structure factor, constrained by Hermiticity, positivity, and sum rules. As a controlled realization of the general projectile kernel, an eikonal–Volkov approximation for slowly varying inhomogeneous fields yields a finite-momentum-resolution convolution of the target response. In the homogeneous-field dipole limit, the projectile kernel reduces to Bessel-function sidebands and the cross section takes the familiar Bethe–Floquet form. The Floquet density correlator separates exactly into the outer product of the pump-induced coherent mean density and connected fluctuations. In the straight-trajectory nonrecoil limit, the coherent, target-elastic sector defines the weak-coupling PINEM amplitude; repeated coherent insertions generate the PINEM Bessel ladder, with an explicit no-double-counting prescription for combining this channel with connected losses. As worked examples, we evaluate the connected, target-changing cross section analytically for a metal in the Drude and diffusive limits, recovering Bessel-weighted plasmon-loss and diffusive combs, and for a driven two-level model that exhibits off-diagonal Floquet coherences through Bessel-channel interference.
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Atomic Physics

Interaction-induced period shift of the Kohn oscillation in a parity-time-symmetric harmonic trap

Gaoqing Meng and Mingshu Zhao

The center of mass of a Bose–Einstein condensate in a harmonic trap oscillates at the trap frequency independently of the interactions. Without interactions a parity-time-symmetric gain-loss gradient leaves this frequency unchanged, and a cloud of the ground-state width is translated rigidly while the atom number is periodically modulated. With interactions the protection is lost, because the gain and the loss change the number of atoms; the modulated atom number drives the width of the cloud, and the width acts back on the center of mass. At small gain the period grows quadratically with the gain strength, with a coefficient that in the Thomas–Fermi regime is set by the size of the cloud alone. The family of periodic orbits continued from the Kohn oscillation can be followed up to a critical gain, where it comes into resonance with highly excited states of the trap, and over the computed families this critical gain decreases with the amplitude of the oscillation and with the size of the cloud. A five-variable moment model reproduces the period and the width of the cloud along the orbits nearly up to the critical gain.
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Thursday 1 October

Atomic Physics

On hyperfine structure of the rotationally excited $1 s_{\mu} n\ell_{e}$-states in helium muonic atoms

Alexei M. Frolov

The new approach is develop to determine the hyperfine structure of the rotationally excited $1 s_{\mu} n \ell_{e}$-states in the three-body helium muonic ${}^{3}$He $\mu \; e$ and ${}^{4}$He $\mu \; e$ atoms. The formulas derived in this study allow one to evaluate the hyperfine structure splittings for an arbitrary rotationally excited (bound) $1 s_{\mu} n \ell_{e}$-state in the three-body helium muonic atoms. Similar formulas have never been produced in earlier studies and this explains why these hyperfine structure splittings are still unknown for these bound states.
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Atomic Physics

Stark effect of hydrogenic ions as a test of the indefinite-metric formalism of the eight-component relativistic wave equation for spin-$\frac{1}{2}$ particles

Paulus C. Tjiang, Sylvia H. Sutanto and Vincentius E. W. Tjia

The eight-component relativistic wave equation for spin-$\frac{1}{2}$ particles of Robson and Staudte (FV$\frac{1}{2}$), a Feshbach–Villars linearization of the Feynman–Gell-Mann equation, reproduces the Dirac hydrogenic spectrum with a doubled solution space and an indefinite inner product. Its exact Stark shifts must equal the Dirac ones; whether its indefinite-metric perturbation theory delivers them is not guaranteed. We compute the first- and second-order Stark shifts of the $n=1$–3 levels of hydrogenic ions from H to U$^{91+}$ in the Schrödinger, Dirac and FV$\frac{1}{2}$ theories, evaluating all spectral sums, continuum included, by the Dalgarno–Lewis method. The FV$\frac{1}{2}$ and Dirac shifts coincide level by level, to working precision at first order (closed-form roots) and to 45 significant figures at second order. Both ingredients are needed: without the explicit spin–field coupling the first-order roots become complex, and without the negative-norm states the $(Z\alpha)^2$ coefficient is wrong. Norm sign and parity are locked in the doubled space: a potential lifting the $2s_{1/2}$–$2p_{1/2}$ degeneracy needs its own spin–gradient term, and shifts placed by hand on the $\kappa$-labeled states give a spurious imaginary linear Stark shift. The relativistic linear Stark effect is smaller by finite factors, but a second-moment sum rule shows the non-relativistic linear Stark strength is conserved, the missing part reappearing in singular $(Z\alpha)^{-2}$ second-order terms. For levels with a fine-structure partner, an exact intra-shell/regular split of the second-order shift gives the $(Z\alpha)^0$ coefficients analytically and relates the $\pi^2$ terms of the $(Z\alpha)^2$ coefficients to the singular ones. The Dirac weak/strong-field crossover for $n=2$ of hydrogen lies near 2.8~kV cm$^{-1}$, and the perturbative window set by the fine structure grows as $Z^5$.
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