We introduce and demonstrate a new approach to measure the electron-hole dynamics and coherence induced by strong-field ionization using hole-assisted high-harmonic spectroscopy. The coherent driving of the infrared and XUV pulses correlates the dynamics of the core-hole and the valence-hole by coupling multiple continua, which leads to the otherwise forbidden absorption and emission of high harmonics. An analytical model is developed based on the strong-field approximation by taking into account the essential multielectron configurations. The emission spectra from the core-valence transition and the core-hole recombination are found to modulate strongly as functions of the time delay between the two pulses, suggesting that the coherent electron wave packets in multiple continua can be utilized to temporally resolve the core-valence transition in attoseconds.

Imaging the charge distributions and structures of molecules and clusters will promote the understanding of the dynamics of the quantum system. Here, we report a method by using an Ar atom as a tip to probe the charge distributions of benzene (Bz) cations in gas phase. Remarkably, the measured charge distributions of Bz$^{+}$ ($\delta_{_{\scriptstyle \rm H}}=0.204$, $\delta_{_{\scriptstyle \rm C}}=-0.037$) and Bz$ ^{2+}$ ($\delta_{_{\scriptstyle \rm H}}=0.248$, $\delta_{_{\scriptstyle \rm C}}=0.0853$) agree well with the calculated Mulliken distributions, and the structures of Bz$_{2}$ is reconstructed by using the measured charge distributions. The structures of two Bz$_{2}$ isomers (T-shaped and PD isomers) can be resolved from the measured inter-molecular potential $V(R)$ between two Bz ions, and the structures of Bz dimer agree well with the theoretical predictions.

Dual-species single-atom array in optical tweezers has several advantages over the single-species atom array as a platform for quantum computing and quantum simulation. Thus, creating the defect-free dual-species single-atom array with atom numbers over hundreds is essential. As recent experiments demonstrated, one of the main difficulties lies in designing an efficient algorithm to rearrange the stochastically loaded dual-species atoms arrays into arbitrary demanded configurations. We propose a heuristic connectivity optimization algorithm to provide the near-fewest number of atom moves. Our algorithm introduces the concept of using articulation points in an undirected graph to optimize connectivity as a critical consideration for arranging the atom moving paths. Tested in array size of hundreds atoms and various configurations, our algorithm shows a high success rate ($>97\%$), low extra atom moves ratio, good scalability, and flexibility. Furthermore, we propose a complementary step to solve the problem of atom loss during the rearrangement.

As x-ray probe pulses approach the subfemtosecond range, conventional x-ray photoelectron spectroscopy (XPS) is expected to experience a reduction in spectral resolution due to the effects of the pulse broadening. However, in the case of resonant x-ray photoemission, also known as resonant Auger scattering (RAS), the spectroscopic technique maintains spectral resolution when an x-ray pulse is precisely tuned to a core-excited state. We present theoretical simulations of XPS and RAS spectra on a showcased CO molecule using ultrashort x-ray pulses, revealing significantly enhanced resolution in the RAS spectra compared to XPS, even in the sub-femtosecond regime. These findings provide a novel perspective on potential utilization of attosecond x-ray pulses, capitalizing on the well-established advantages of detecting electron signals for tracking electronic and molecular dynamics.

Ultracold atoms trapped in optical superlattices provide a simple platform for realizing the seminal Aubry–André–Harper (AAH) model. However, this model ignores the periodic modulations on the nearest-neighbor hoppings. We establish a generalized AAH model by which an optical superlattice system can be approximately described when $V_1\gg V_2$, with periodic modulations on both on-site energies and nearest-neighbor hoppings. This model supports much richer topological properties absent in the standard AAH model. Specifically, by calculating the Chern numbers and topological edge states, we show that the generalized AAH model possesses multifarious topological phases and topological phase transitions, unlike the standard AAH model supporting only a single topological phase. Our findings can uncover more opportunities for using optical superlattices to study topological and localization physics.

Nondipole effects are ubiquitous and crucial in light-matter interaction. However, they are too weak to be directly observed. In strong-field physics, motion of electrons is mainly confined in transverse plane of light fields, which suppresses the significance of nondipole effects. Here, we present a theoretical study on enhancing and controlling the nondipole effect by using the synthesized two perpendicularly propagating laser fields. We calculate the three-dimensional photoelectron momentum distributions of strong-field tunneling ionization of hydrogen atoms using the classical trajectory Monte Carlo model and show that the nondipole effects are noticeably enhanced in such laser fields due to their remarkable influences on the sub-cycle photoelectron dynamics. In particular, we reveal that the magnitudes of the magnetic and electric components of nondipole effects can be separately controlled by modulating the ellipticity and amplitude of driving laser fields. This novel scenario holds promising applications for future studies with ultrafast structured light fields.

Using a three-dimensional classical ensemble model, we investigate the dependence of relative frequency and relative initial phase for nonsequential double ionization (NSDI) of atoms driven by orthogonal two-color (OTC) fields. Our findings reveal that the NSDI probability is clearly dependent on the relative initial phase of OTC fields at different relative frequencies. The inversion analysis results indicate that adjusting the relative frequency of OTC fields helps control returning probability and flight time of the first electron. Furthermore, manipulating the relative frequency at the same relative initial phases can vary the revisit time of the recolliding electron, leading that the emission direction of Ar$^{2+}$ ions is explicitly dependent on the relative frequency.

Quantum excitation is usually regarded as a transient process occurring instantaneously, leaving the underlying physics shrouded in mystery. Recent research shows that Rydberg-state excitation with ultrashort laser pulses can be investigated and manipulated with state-of-the-art few-cycle pulses. We theoretically find that the efficiency of Rydberg state excitation can be enhanced with a short laser pulse and modulated by varying the laser intensities. We also uncover new facets of the excitation dynamics, including the launching of an electron wave packet through strong-field ionization, the re-entry of the electron into the atomic potential and the crucial step where the electron makes a U-turn, resulting in twin captures into Rydberg orbitals. By tuning the laser intensity, we show that the excitation of the Rydberg state can be coherently controlled on a sub-optical-cycle timescale. Our work paves the way toward ultrafast control and coherent manipulation of Rydberg states, thus benefiting Rydberg-state-based quantum technology.

We demonstrate the extreme ultraviolet free induction decay emission that can be significantly enhanced by employing isolated attosecond pulses. The near infrared pulses are applied to excite the neon atoms into Rydberg states coherently, and isolated attosecond pulses are used to manipulate populations of the Rydberg states and the subsequent free induction decay process. The time resolved experimental measurement of dependence of the resonance emission yield would help to understand the buildup dynamics of population of excited states. The enhancement assisted by attosecond pulses can serve as a mechanism to develop high-flux extreme ultraviolet light sources.

We report on experimental measurements of the transition dipole moments (TDMs) between the intermediate state $5{s}5{p}\, ^3\!{P}_1$ and the triplet Rydberg series $5{s}n{s}\, ^3\!{S}_1$ in an ultracold strontium gas. Here $n$ is the principal quantum number ranging from 19 to 40. The transition $5{s}5{p}\, ^3\!{P}_1$–$5{s}n{s}\, ^3\!{S}_1$ is coupled via an ultraviolet (UV) beam, inducing Autler–Townes splitting of both states. Such a splitting of the intermediate state is spectroscopically measured by using absorption imaging on a narrow transition $5{s^2}\, ^1{S}_0$–$5{s}5{p}\, ^3\!{P}_1$ in an ultracold gas of strontium atoms. The power and size of the UV beam are carefully determined, with which the TDMs are extracted from the measured Autler–Townes splitting. The experimentally obtained TDMs are compared to the calculations based on a parametric core potential, on a Coulomb potential with quantum defect, and on the open-source library Alkali Ryderg calculator, finding good agreement with the former two models and significant deviation with the latter.

We conducted measurement and calculation to resolve the long-standing large discrepancy in the metastable state lifetime for the $^{88}$Sr atom between theoretical and experimental results. The present lifetime $\tau = 830_{-240}^{+600}$ s, measured using the magneto-optical trap as a photon amplifier to detect the weak decay events, is approximately 60% larger than the previous experimental value $\tau = 520_{-140}^{+310}$ s. By considering the electron correlation effects in the framework of the multiconfiguration Dirac–Hartree–Fock theory, we obtained a theoretical lifetime of 1079(54) s, which lies in the range of measurements with error bars. Furthermore, we considered the higher-order electron correlation and Breit interaction to control the uncertainty of the theoretical calculation. The significant improvement in the agreement between calculations and measurements is attributed to the updated blackbody radiation-induced decay rate.

Effective Hamiltonians in periodically driven systems have received widespread attention for realization of novel quantum phases, non-equilibrium phase transition, and Majorana mode. Recently, the study of effective Hamiltonian using various methods has gained great interest. We consider a vector differential equation of motion to derive the effective Hamiltonian for any periodically driven two-level system, and the dynamics of the spin vector are an evolution under the Bloch sphere. Here, we investigate the properties of this equation and show that a sudden change of the effective Hamiltonian is expected. Furthermore, we present several exact relations, whose expressions are independent of the different starting points. Moreover, we deduce the effective Hamiltonian from the high-frequency limit, which approximately equals the results in previous studies. Our results show that the vector differential equation of motion is not affected by a convergence problem, and thus, can be used to numerically investigate the effective models in any periodic modulating system. Finally, we anticipate that the proposed method can be applied to experimental platforms that require time-periodic modulation, such as ultracold atoms and optical lattices.

Nonresonant multiphoton ionization by femtosecond laser pulses can be applied to any molecule virtually, thereby greatly enhancing the scope of Stark decelerated molecules. For comparison, we detect decelerated and trapped ammonia molecules using two different schemes: (i) nonresonant multiphoton ionization using intense femtosecond (fs) pulses in the near infrared, and (ii) resonance-enhanced multiphoton ionization using nanosecond (ns) pulses from a tunable UV laser. The observed number of ions per shot for both schemes is similar. The fs laser detection scheme suffers from an increased background, which can be effectively eliminated by subsequent mass and velocity selection. To determine the detection volume of the ns laser detection scheme, we present measurements in which the decelerated ammonia molecules are bunched to a packet with a longitudinal spread well below $\sim$100 µm. It is concluded that the detection volume for the ns laser detection scheme is 1.5–2 times larger than that of the fs laser detection scheme.

Electronic charge of molecules can move on time scales when the nuclei stand practically still, from few hundreds of attoseconds to few femtoseconds. This ultrafast process is called “charge migration”. A typical consequence is rapid change of electronic dipole, which points to the center of charge. Corresponding linear (one-dimensional, 1D) and planar (2D) dipolar motions have already been well documented. Here we construct the first case of charge migration which causes chiral 3D dipolar motion, specifically along a helix about oriented iodo-acetylene (HCCI). Quantum dynamics simulations show that this can be induced by well-designed laser pulses.

Tractor beams, able to produce optical pulling forces (OPFs) on particles, are attracting increasing attention. Here, non-paraxial Bessel tractor beams are generated using polarization-insensitive metasurfaces. OPFs are found to exert on dielectric particles with specific radii at the axes of the beams. The strengths of the OPFs depend on the radii of the particles, which provides the possibility of sorting particles with different sizes. For the OPFs, the radius ranges of particles vary with the polarization states or topological charges of the incident beams. The change of polarizations can provide a switch between the pulling and pushing forces, which offers a new way to realize dynamic manipulation of particles. The change of topological charges leads to disjoint radii ranges for the OPFs exerting on particles, which provides the possibility of selective optical separation. Moreover, we study the behaviors of particles in the tractor beams. The simulation results reveal that linearly or circularly polarized tractor beams can pull particles a sufficient distance towards the light source, which verifies the feasibility of separating particles.

The atom-atom-anion three-body recombination (TBR) and collision induced dissociation (CID) processes of the $^{3}$He–$^{3}$He–T$^-$ system at ultracold temperatures are investigated by solving the Schrödinger equation in the adiabatic hyperspherical representation. The variations of the TBR and CID rates with the collision energies in the ultracold temperatures are obtained. It is found that the $J^{\varPi}=1^-$ symmetry dominates the TBR and CID processes in most of the considered collision energy range. The rate of TBR (CID) into (from) the $l=1$ anion is larger than those for the $l=0$ and $l=2$ anions, with the $l$ representing the rotational quantum number of $^{3}$HeT$^-$. This can be understood via the nonadiabatic couplings among the different channels.

Optical gain and loss of microcavity greatly affect the quality of lasing, how to improve optical gain and decrease optical loss is of great significance for the preparation of laser. In this study, four types standard microdisks with different thicknesses of 2.2 µm, 1.9 µm, 1.7 µm, and 1.45 µm were fabricated by micromachining technology process to modulate optical gain and loss of microdisk lasing. The whispering gallery mode lasing in the ultraviolet range of GaN microdisk devices was investigated for these devices in order to clarify the effect of microdisk thickness on device characteristics. The quality factor $Q$ and lasing mode number for different thicknesses are calculated from the stimulated spectra. The lifetimes of the exciton combination properties of the devices were observed using time-resolved PL spectroscopy. The lasing modes are modulated, and the lifetime decreases, while the $Q$ factor of the devices first increases and then decreases with decreasing thickness. All these results are induced by optical gain and loss competition.

We study charge transfer of a multi-electron collision system Li$^{2+}$ + Ar using the time-dependent density functional theory non-adiabatically coupled to the molecular dynamics. By implementing the particle number projection method, the single- and double-charge transfer cross sections are extracted at MeV energies, which are in good agreement with the experimental data available. The analysis of charge transfer probabilities shows that for energies higher than 1.0 MeV, the single-charge transfer occurs for a broader range of impact parameters, while the double-charge transfer is dominated by close collisions. To gain the population of captured electrons on the projectile, we compute the orbital projection probabilities. It is found that the electrons of the Ar atom will most possibly transfer to the $2p$ orbitals of the Li$^{2+}$, and only a small portion of captured electrons distribute on the $s$ orbitals. This work verifies the capability of the present methodology in dealing with charge transfer in dressed ion collisions at MeV energies.

Trapped atoms on photonic structures inspire many novel quantum devices for quantum information processing and quantum sensing. Here, we demonstrate a hybrid photonic-atom chip platform based on a GaN-on-sapphire chip and the transport of an ensemble of atoms from free space towards the chip with an optical conveyor belts. Due to our platform's complete optical accessibility and careful control of atomic motion near the chip with a conveyor belt, successful atomic transport towards the chip is made possible. The maximum transport efficiency of atoms is about $50\%$ with a transport distance of $500\,\mathrm{µ m}$. Our results open up a new route toward the efficient loading of cold atoms into the evanescent-field trap formed by the photonic integrated circuits, which promises strong and controllable interactions between single atoms and single photons.

Nonlinear terahertz (THz) radiation from gas media usually relies on the asymmetric laser-induced current produced by ultra-intense two-color laser fields with a specific phase delay. Here a new scheme is proposed and theoretically investigated, in which the radiation is generated by spatially inhomogeneous fields induced by relatively low-intensity monochromatic lasers and an array of single triangular metallic nanostructures. Our simulations are based on the classical photocurrent model and the time-dependent Schrödinger equations separately. It is found that the collective motion of the ionized electrons can be efficiently controlled by the inhomogeneous field, resulting in strong residual currents. The intensity of the THz radiation could be enhanced by about two orders of magnitude by increasing the spatial inhomogeneity of the field.

High-resolution photoelectron energy spectra of osmium anions are obtained using the slow-electron velocity-map imaging method. The energy levels of excited states $^{4}\!F_{7/2}$, $^{4}\!F_{5/2}$ and $^{4}\!F_{3/2}$ of Os$^{-}$ are determined to be 148.730(13), 155.69(15), and 176.76(13) THz [or 4961.09(41), 5193.4(49), and 5896.1(42) cm$^{-1}$], respectively. The lifetime of the opposite-parity excited state $^{6}\!D_{9/2}^{\rm o}$ is determined to be 201(10) µs using a cold ion trap, about 15 times shorter than the previous result 3(1) ms. Our high-level multi-configuration Dirac–Hartree–Fock calculations yield a theoretical lifetime 527 µs. Our work shows that the laser cooling rate of Os$^{-}$ is as fast as that of Th$^{-}$. The advantages of Os$^{-}$ are its near-IR range cooling transition and simple electronic structure, which make Os$^{-}$ a promising candidate for laser cooling of negative ions. We propose a general approach to produce cold atoms and molecules based on the sympathetic cooling of negative ions in combination with a threshold photodetachment.

We develop a quantum optical description of radiation from a two-level system (TLS) in strong laser fields, which provides a clear insight into the final states of the TLS and the harmonics field. It is shown that there are two emission channels: the Rayleigh-like channel and the Raman-like channel, which correspond to the TLS ending up in the ground state and excited state after the emission, respectively. The numerical result shows that the harmonics are mainly produced by the Rayleigh-like channel. In addition, according to the coherence of emission among the emitters, the radiation is divided into coherent parts that result from the semi-classical dipole oscillation and incoherent parts that result from the quantum fluctuations of the dipole moment. In the weak field limits, the Rayleigh-like channel corresponds to the coherent parts, and the Raman-like channel corresponds to the incoherent parts. However, in strong laser fields, both channels contribute to coherent and incoherent radiation, and how much they contribute depends on the final excitation. By manipulating the laser field, we can make the Rayleigh-like channel produce either coherent or incoherent radiation.

The Mott insulator and superfluid phase transition is one of the most prominent phenomena in ultracold atoms. We report the observation of a novel 2D quantum phase transition between the Mott insulator and $\pi$ superfluid in a shaking optical lattice. In the deep optical lattice regime, the lowest $S$ band can be tuned to Mott phase, while the higher $P_{x,y}$ bands are itinerant for having larger bandwidth. Through a shaking technique coupling the $s$-orbital to $p_{x,y}$-orbital states, we experimentally observe the transition between the states of the $S$ and $P_{x,y}$ bands, leading to a quantum phase transition from two-dimensional $s$-orbital Mott phase to the $p_{x,y}$-orbital superfluid which condensed at $(\pi,\pi)$ momentum. Using the band-mapping method, we also observe the changes of atomic population in different energy bands during the transition, and the experimental results are well consistent with theoretical expectations.

We show that it is feasible to generate sub-40-attosecond pulses with near-infrared few-cycle pulses centered at 800 nm. With proper gating technique, super-broadband continuum spectrum extending from 50 eV to above 200 eV can be obtained, and the intrinsic atto-chirp can be satisfactorily compensated with C filter, producing isolated attosecond pulses with duration of 33 as. According to the wavelength scaling law of high-order harmonic generation, the proposed scheme is of great significance to develop high-flux ultrashort attosecond sources.

We present a method that the atomic transition frequency measurement relies on the accurate wavemeter, optical frequency comb and stable Fabry–Pérot cavity to precise determination of stable even isotope shift on single Yb$^{+}$ ion ($A=168$, 170, 172, 174, 176). The $6s$ ${}^{2}\!S_{1/2} \leftrightarrow 6p\,{}^{2}\!P_{1/2}$   and $5d\,{}^{2}\!D_{3/2} \leftrightarrow 6s\,{}^{3}[3/2]_{1/2}$   resonance dipole transition frequencies are preliminarily measured by using a wavemeter which is calibrated by the 729 nm clock laser of ${}^{40}$Ca$^{+}$. Meanwhile, those frequencies are double checked by using optical frequency comb for correction of deviation. Ultimately, by changing frequency locking points at an ultralow expansion cavity more slightly and monitoring the corresponding atomic fluorescence changing with 17%, we finally improve the resonant frequency uncertainty to $\pm 6$ MHz, which is one order of improvement in precision higher than previously published measurements on the same transitions. A King-plot analysis with sensitivity to coupling between electrons and neutrons is carried out to determine the field and mass shift constants. Our measurement combined with existing or future isotope shift measurements can be used to determine basic properties of atomic nuclei, and to test new forces beyond the Standard Model.

We report experimental and theoretical investigations of wavelength dependence of Rydberg state excitation (RSE) process of Ar subject to intense laser fields. By simultaneously measuring ionization and RSE yields of Ar atoms subject to strong laser fields at a series of wavelengths, we obtain the wavelength scaling law of the ratio of Ar$^{*}$ over Ar$^{+}$ with respect to the laser intensity, and this result can be well reproduced by a nonadiabatic model, but not by the classical-trajectory Monte Carlo model. Our results indicate that the nonadiabatic corrections of the photoelectron tunneling exit and tunneling probability play a significant role at shorter wavelengths. Analysis shows that the wavelength dependence phenomenon is due to the interplay of the nonadiabatic effect, wave-packet diffusion and Coulomb focusing effect of the liberated electron.

By numerically solving time-dependent Schrödinger equations, we investigate the generation of electron currents, ultrafast magnetic fields and photoelectron momentum distributions (PMD) when circularly polarized laser pulses interact with a Ne dimer in the charge migration (CM) process. By adjusting the laser wavelength, we consider two cases: (i) coherent resonance excitation ($\lambda=76$ nm) and (ii) direct ionization ($\lambda=38$ nm). The results show that the current and magnetic field generated by the Ne dimer under resonance excitation are stronger than under direct ionization. This phenomenon is due to the quantum interference between the initial state $2p\sigma_{\rm g}$ and the excited state $3s\sigma_{\rm g}$ under resonance excitation, so the CM efficiency of the dimer can be improved and the strength of the PMD under different ionization conditions is opposite to the strength of the electron current and induced magnetic field. In addition, we also find that both $2p\pi_{\rm g}$ and $2p\pi_{\rm u}$ have coherent resonance excitation with $3s\sigma_{\rm g}$ state and generate periodic oscillating currents for the Ne dimer. The study of the dynamics of the Ne dimer under different ionization conditions lays a foundation for research of ultrafast magnetism in complex molecular systems.

We demonstrate the flexible tunability of excitation transport in Rydberg atoms, under the interplay of controlled dissipation and interaction-induced synthetic flux. Considering a minimum four-site setup, i.e., a triangular configuration with an additional output site, we study the transport of a single excitation, injected into a vertex of the triangle, through the structure. While the long-range dipole-dipole interactions between the Rydberg atoms lead to geometry-dependent Peierls phases in the hopping amplitudes of excitations, we further introduce on-site dissipation to a vertex of the triangle. As a result, both the chirality and destination of the transport can be manipulated through the flux and dissipation. In particular, we illustrate a parameter regime where our Rydberg-ring structure may serve as a switch for transporting the injected excitation through to the output site. The underlying mechanism is then analyzed by studying the chiral trajectory of the excitation and the time-dependent dissipation. The switchable excitation transport reported here offers a flexible tool for quantum control in Rydberg atoms, and holds interesting potentials for applications in quantum simulation and quantum information.