We measure the energy-differential cross sections for collisional excitation of the soft x-ray electric-dipole $K\alpha$ ($x+y+w$) emission from He-like oxygen (O vii), using an electron beam ion trap. Values near their excitation thresholds were extracted from the observed emissivity by rapidly cycling the energy of the exciting electron beam. This allows us to subtract time-dependent contributions of the forbidden $z$-line emission to the multiplet. We develop a time-dependent collisional-radiative model to further demonstrate the method and predict all spectral features. We then compare the extracted $x+y+w$ cross sections with calculations based on distorted-wave and $R$-matrix methods from the literature and our own predictions using the flexible atomic code. All $R$-matrix results are validated by our measurements of direct and resonant excitation, supporting the use of such state-of-the-art codes for astrophysical and plasma physics diagnostics.

Within monochromatic optical fields, we demonstrate rotating-wave structured light with a stationary wave vortex carrying intrinsic transverse orbital angular momentum orthogonal to the propagation direction of light. Remarkably, we find that such rotating-wave structured light reveals highly nontrivial features of transverse spin and orbital angular momentum densities. The normalized total angular momentum density is conserved universally, suggesting a mutual conversion of the intrinsic transverse spin and orbital angular momenta in free space. Despite such mutual conversion at the local level, integral intrinsic orbital angular momentum can be well defined with the topological charge of the vortex per photon. Moreover, the orientation of transverse spin density is governed by the direction of the Poynting momentum density, manifesting a spin-momentum-locking effect in free space.

We study force-gradient sensing with a coherently driven mechanical resonator and phase-sensitive detection of motion through the two-tone backaction evading measurement of cavity optomechanics. The response of the optomechanical system, solved by numerical integration of the classical equations of motion, shows an extended region which is monotonic to changes in force gradient. We use Floquet theory to model the fluctuations, which rise only slightly above that of the usual backaction evading measurement in the presence of the mechanical drive. The monotonic response and minimal backaction are advantageous for applications such as atomic force microscopy.

We investigate the resonance fluorescence features of a small ensemble of closely packed and moderately laser-pumped two-level emitters at resonance. The mean distance between any two-level radiators is smaller than the corresponding emission wavelength, such that the dipole-dipole interactions are not negligible. We have found that under the secular approximation, the collective resonance fluorescence spectrum consists of $2N+1$ spectral lines, where $N$ is the number of emitters from the sample. The $2N$ spectral sidebands, symmetrically located around the generalized Rabi frequency with respect to the central line at the laser frequency, are distinguishable if the dipole-dipole coupling strength is larger than the collective spontaneous decay rate. This way, one can estimate the radiators number within the ensemble via measuring of the spontaneously scattered collective resonance fluorescence spectrum. Contrarily, if the dipole-dipole coupling is of the order of or smaller than the cooperative spontaneous decay rate, but still non-negligible, the spectrum turns into a Mollow-like fluorescence spectrum, where the two sidebands' spectral lines broaden proportional to the dipole-dipole coupling strength.

From the perspective of majorization theory, we study how to enhance the entanglement of a two-mode squeezed vacuum (TMSV) state by using local filtration operations. We present several schemes achieving entanglement enhancement with photon addition and subtraction, and then consider filtration as a general probabilistic procedure consisting in acting with local (nonunitary) operators on each mode. From this, we identify a sufficient set of two conditions for these filtration operators to successfully enhance the entanglement of a TMSV state, namely, the operators must be Fock orthogonal (i.e., preserving the orthogonality of Fock states) and Fock amplifying (i.e., giving larger amplitudes to larger Fock states). Our results notably prove that ideal photon addition, subtraction, and any concatenation thereof always enhance the entanglement of a TMSV state in the sense of majorization theory. We further investigate the case of realistic photon addition (subtraction) and are able to upper bound the distance between a realistic photon-added (-subtracted) TMSV state and a nearby state that is provably more entangled than the TMSV, thus extending entanglement enhancement to practical schemes via the use of a notion of approximate majorization. Finally, we consider the state resulting from $k$-photon addition (on each of the two modes) on a TMSV state. We prove analytically that the state corresponding to $k=1$ majorizes any state corresponding to $2\le k\le 8$ and we conjecture the validity of the statement for all $k\ge 9$.

Excitation energy transfer is a photophysical process upon which many chemical and biological phenomena are built. From natural small systems to synthetic multichromophoric macromolecules, energy transfer deals with the process of migration of electronic excitation energy from an excited donor to an acceptor. Although this phenomenon has been extensively studied in the past, the rapid evolution of quantum-enabled technologies has led to the question of whether nonclassical sources of light, such as entangled photon pairs, may provide us with better control (or enhancement) of energy transfer at the nanoscale. In this work we provide a comprehensive study of the joint excitation of two noninteracting two-level atoms by time-frequency-correlated photon pairs (whose central frequencies are not resonant with the individual particles) generated by means of spontaneous parametric down-conversion (SPDC). We demonstrate that while strong frequency anticorrelation between photons guarantees a large two-photon excitation (TPE) probability, photons bearing a sine cardinal spectral shape exhibit an approximately 3.8 times larger TPE signal than photons with a Gaussian spectrum. More importantly, we find that suppression of time-ordered excitation pathways does not substantially modify the TPE probability for two-photon states with a Gaussian spectral shape, whereas photons with a sine cardinal spectrum exhibit the strongest TPE signals when two-photon excitation pathways are not suppressed. Our results not only help elucidate the role of time-frequency correlations in resonance energy transfer with SPDC photons, but also provide valuable information regarding the optimal source to be used in the experimental implementation of said process.

This paper presents an implementation of the noniterative eigenchannel $R$-matrix method for the Dirac equation. It includes a brief introduction, implementation details, and results for the photoionization cross section of the beryllium atom. Beryllium is a convenient test due to small but significant relativistic effects. Our calculation aligns with other $R$-matrix calculations and experiments. It observes the change in the Fano line shape of the $\left(2pnd\right){}^{1}P$ series and reveals a coupling between triplet and singlet series observable in the ground-state photoionization cross section.

The worldline method is a powerful numerical path integral framework for computing Casimir and Casimir-Polder energies. An important challenge arises when one desires derivatives of path integral quantities—standard finite-difference techniques, for example, yield results of poor accuracy. In this work we present methods for computing derivatives of worldline-type path integrals of scalar fields to calculate forces, energy curvatures, and torques. In Casimir-Polder-type path integrals, which require derivatives with respect to the source point of the paths, the derivatives can be computed by a simple reweighting of the path integral. However, a partial-averaging technique is necessary to render the differentiated path-integral computationally efficient. We also discuss the computation of Casimir forces, curvatures, and torques between macroscopic bodies. Here a different method is used, involving summing over the derivatives of all the intersections with a body; again, a different partial-averaging method makes the path-integral efficient. To demonstrate the efficiency of the techniques, we give the results of numerical implementations of these worldline methods in atom-plane and plane-plane geometries. Being quite general, the methods here should apply to path integrals outside the worldline context (e.g., financial mathematics).

We identify that coherent interconversion can stabilize bosonic atomic-molecular mixtures by establishing a comprehensive and analytical phase diagram, which can accommodate up to two tricritical points (TPs) with one being intrinsic and the other being conditional. By varying the interaction parameters, the topology of the phase diagram gets modified while changing the number of TPs. We master the quantum tricriticality and concrete signals of the phase transition for this system. Notably, we show that the sound velocity vanishes as the atomic-molecular superfluid phase touches on the border of the phase separation lines.. Correlation functions such as pairing amplitudes and density (spin density) structure factors exhibit characteristic behaviors when the system crosses the phase transitions. Our work paves the way for future advancement toward understanding the salient physics of atom-molecular mixtures.

Spin is one fundamental property of microscopic particles. A lot of theoretical work has postulated the possible coupling between spin and gravitation, which could result in the violation of the equivalence principle. In our recent joint mass-and-energy test of the weak equivalence principle with a 10-m ${}^{85}\mathrm{Rb}\text{−}{}^{87}\mathrm{Rb}$ dual-species atom interferometer, the Eötvös parameters of four ${}^{85}\mathrm{Rb}\text{−}{}^{87}\mathrm{Rb}$ combinations with specific atomic spin states were measured to the ${10}^{-10}$ level [L. Zhou et al., Phys. Rev. A 104, 022822 (2021)]. Here these experimental results are used to constrain the postulated spin-gravity coupling effects. The bounds on the spin-independent and spin-dependent anomalous passive gravitational mass tensors in Lämmerzahl's model are set to the ${10}^{-10}$ level, which improves existing bounds by three orders of magnitude. The constraints to the spin-independent electron- and proton-gravity coupling parameters in the gravitational standard-model extension are set to the ${10}^{-6}\mathrm{GeV}$ level.

The skyrmion, which is characterized by a topological integer, is a structure that is topologically stable against local disturbances. The huge potential of skyrmions for use in magnetic storage systems has drawn considerable research interest among physicists. Recently, optical skyrmions were discovered to have some excellent properties. However, all of these optical skyrmions have been observed in surface plasmons that consist of evanescent waves. This type of optical skyrmion is difficult to manipulate and also difficult to apply in practice. Here we describe several skyrmionic optical structures with different skyrmion numbers in a free-space linear optical system. Because of the convenience of operation using free-space optics, with the exception of the original applications of skyrmions, skyrmionic optical structures can also be applied widely, ranging from manipulation of tiny objects to propagation with information over long distances.

In recent years, there has been tremendous focus on identifying whether effective descriptions of open quantum systems such as master equations can accurately describe the dynamics of open quantum systems. One particular question is whether they provide the correct steady state in the long time limit. The transient regime is also of interest. Description of evolution by various master equations—some of them being not completely positive—is benchmarked against exact solutions [see, e.g., R. Hartmann and W. T. Strunz, Phys. Rev. A 101, 012103 (2020)]. An important property of true evolution is its non-Markovian features, which are not captured by the simplest completely positive master equations. In this paper we consider a non-Markovian yet completely positive evolution (known as refined weak coupling or cumulant equation) for the spin-boson model with an overdamped Drude-Lorentz spectral density and arbitrary coupling. We benchmark it against a numerically exact solution, as well as against other master equations, for different coupling strengths and temperatures. We find the cumulant to be a better description in the weak-coupling regime where it is supposed to be valid. For the examples considered it shows superiority at moderate and strong couplings in the low-temperature regime for all examples considered. In the high-temperature regime, however, its advantage vanishes. This indicates that the cumulant equation is a good candidate for simulations at weak to moderate coupling and low temperature. Our calculations are greatly facilitated due to our concise formulation of the cumulant equation by means of representation of the density matrix in the SU(N) basis.

Witnessing quantum correlation in real, practical experimental setups is a key focus for the development of near-future quantum technologies. While there are several experimental protocols for witnessing entanglement, detecting quantum discord remains far more elusive. A recently proposed quantum discord witness offers an experimentally accessible setup which allows the quantum discord to be witnessed through a nonlinear combination of correlation functions. Interestingly, the experimental setup can be mapped to Bayesian game theory allowing for an extended generalization of the proposed witness. Subsequently, it is shown that there is a direct link between the expected payoff in Bayesian game theory and the previously proposed quantum discord witness by uniting these two concepts through the established CHSH game.

The optical tweezer experiment with neutral atoms is a focal topic in cold-atom physics due to its significant potential in quantum computing and simulation. Here we present the realization of a dual-species optical tweezer for both Rb and K atoms, taking a step towards creating a polar-molecule optical tweezer array. Initially, Rb and K atoms are collected using a dual magneto-optical trap and further cooled to $7\mu \mathrm{K}$ for Rb and $10\mu \mathrm{K}$ for K. By employing 850-nm tweezer beams, we demonstrate the ability to capture individual Rb or K atoms. The filling ratios of Rb and K can be finely adjusted by controlling the atomic densities of both species. Utilizing the postselection technique, we can create a deterministic array of two-species atoms, paving the way for future polar-molecule array formation.

A system of one electron in a Coulomb potential in an optical cavity is solved using a tensor-product light-matter basis. The problem was treated at the level of the Pauli-Fierz Hamiltonian describing both light and matter quantum mechanically. The effect of cavity size on the energy levels and high harmonic generation (HHG) spectrum is studied. We have shown that the energy levels, transition states, entanglement, and the HHG spectrum can be strongly influenced by changing the cavity size.

We present quantum Monte Carlo calculations of the properties of a two-component mass-imbalanced Fermi gas, corresponding to the ${}^6\mathrm{Li}\text{−}{}^{40}\mathrm{K}$ mixture. We compute the equation of state of the unpolarized system as a function of the scattering length with particular attention paid to the unitary limit, where the effect of the effective range of the interaction is explored. In order to investigate differences from the equal-mass case we also compute the pair-distribution function and the momentum distribution over a range of interaction strengths, which can provide information about the structure of the system. Finally, we compute the heavy-light quasiparticle spectrum for the full crossover regime. Our theoretical predictions, based on quantum Monte Carlo calculations, should inform future theoretical studies and can be tested by experiments with ultracold fermionic gases.

We derive the two-time linear response theory for out-of-equilibrium pumped systems, generic pump-probe delays, and probe frequencies. Such a theory enormously simplifies the numerical calculations, for instance, of the optical conductivity with respect to the actual procedure, which requires computing the effect of the probe pulse for each time delay with respect to the pump pulse. The theory is given for a generic observable and pumped Hamiltonian and then specialized for a system with a quadratic Hamiltonian and its transient optical properties, exploiting the dynamical projective operatorial approach (DPOA). The theory is complemented by a set of crucial numerical guidelines that help perform actual calculations in a computationally affordable way. The optical response (differential transient reflectivity and absorption) of a prototypical three-band (core, valence, and conduction) model in the extreme ultraviolet regime is analyzed in detail to illustrate the theory and its application. Using some generalizations of the density of states, we provide a systematic approach to exploring the optical properties in terms of the system band-structure features and the pump parameters. Such an analysis can be extremely helpful in understanding the actual results of experimental optical measurements. Moreover, we study the effects of interband and intraband transitions, the local dipole coupling, and single and multiphoton processes. The latter is further investigated by varying the central frequency of the pump pulse to have different regions of the first Brillouin zone in resonance with it. We also study the effect of varying the pump pulse intensity. Finally, we study and analyze the transient optical properties in the probe pulse regime of the infrared and visible.

A topological valley photonic crystal cavity supports robust multiple resonant modes and is therefore widely applied in broadband Purcell enhancement, laser, channel filter, and sensor. So far, its robustness has been conventionally attributed to the different topological properties of the valley photonic crystals that make up the interface. In this work, we clarify that the distribution of the circular polarization topological singularities is instrumental in generating robust and regularly spaced resonant modes. We demonstrate a photonic crystal ring resonator, constructed entirely by a deformed line-defect waveguide. Through simulations, we validate that the ring resonator retains the most advantages of a valley photonic crystal ring resonator. This work can instruct the development of a high-performance and robust photonic crystal cavity.

In this paper, we study the synchronization of qubits induced by the dynamical Casimir effect in an atom-cavity quantum electrodynamics system. Our investigation revolves around a pragmatic configuration of a quantum system, where two superconducting qubits are coupled to a shared coplanar waveguide resonator terminated at one end by a superconducting quantum interference device. The theoretical analyses of the system dynamics reveal sufficient conditions for ensuring synchronization which are anticipated to be accomplished by photon generation in the resonator. By numerically analyzing the time evolution of the system, we confirm that the conditions are satisfied by photon generation via the dynamical Casimir effect, resulting in qubit synchronization. Notably, we unveil a remarkable feature that is unique to synchronization induced by the dynamical Casimir effect: the differences in the initial states of qubits and the differences in the coupling strengths of qubits to an electromagnetic field affect the synchronization independently without overlap between these factors.