Characterizing high-dimensional entangled states is of crucial importance in quantum information science and technology. Recent theoretical progress has been made to extend Hardy's paradox into a general scenario with multisetting and multidimensional systems, which enables the maximum probability of nonlocal events to surpass the bound that was limited in Hardy's original test. Hitherto, no experimental verification has been conducted to verify Hardy's paradox, as most of the previous experimental efforts were restricted to two-dimensional systems. Here, based on two-photon high-dimensional orbital angular momentum (OAM) entanglement, we report an experiment to demonstrate Hardy's paradox for multiple settings and multiple outcomes. We demonstrate the paradox for two-setting higher-dimensional OAM subspaces up to $d=7$, which reveals that the nonlocal events increase with dimension. Furthermore, we showcase the nonlocality with an experimentally recorded probability of 36.77% for a five-setting three-dimensional OAM subspace via entanglement concentration, thus showing a sharper contradiction between quantum mechanics and classical theory.

We study the generation of frequency nondegenerate correlated photon pairs and heralded single photons via spontaneous four-wave mixing (SFWM) in a series of identical micro- or nanofibers (MNFs). Joint spectral intensity of the biphoton field generated at wavelengths of about 880 and 1310 nm has been measured under excitation by 100-ps laser pulses demonstrating good agreement with the theoretical prediction. The measured zero-time second-order autocorrelation function was about 0.2 when the emission rate of the heralded photons was of 4 Hz. The MNF-based source perfectly matches standard single-mode fibers, which makes it compatible with the existing fiber communication networks. In addition, SFWM observation in a series of identical MNFs allows for increasing the generation rate of single photons via spatial multiplexing.

We study the dynamics of the center of mass of a Brownian particle levitated in a Paul trap. We mostly focus on the overdamped regime in the context of levitodynamics, comparing theory with numerical simulations and experimental data from our Paul trap. We provide an exact analytical solution to the stochastic equation of motion, expressions for the standard deviation of the motion, and thermalization times by using the WKB method under two different limits. Finally, we prove that the power spectral density of the motion can be approximated by that of an Ornstein-Uhlenbeck process and use the found expression to calibrate the motion of a trapped particle.

A bright beam of metastable helium atoms is useful in various studies. Laser spectroscopy of helium in an atomic beam can provide a determination of the fine-structure constant and the nuclear charge radius, towards a test of quantum electrodynamics (QED). In these measurements, it is necessary to control not only the internal quantum state but also the translation motion of the helium atom. Here we present a setup producing an intense and speed-adjustable continuous beam of helium atoms. By using a combination of laser cooling, focusing, and deflection, helium atoms are prepared at the single quantum state of $2^3{S}_1$ ($m=0$), with a very narrow distribution of the longitudinal speed ($\delta v_z<\pm 4.5$ m/s). At the same time, we obtain a flux of $1.8\times {10}^{13}$ (atoms/s)/sr with a fractional fluctuation below 0.02%. The atomic-beam quality was investigated by laser spectroscopy, indicating a considerable improvement over those used in previous helium precision-spectroscopy measurements.

Polarimetric dimension and nonregularity are newly introduced concepts that characterize three-dimensional (3D) polarization states of light. We analyze the spectral polarimetric dimension and the degree of nonregularity associated with two kinds of tightly focused beams: a radially fully polarized Gaussian Schell-model (GSM) beam and a partially polarized beam composed of an incoherent superposition of two orthogonally polarized (coherent) plane-wave modes. We show that for both beams the focal field can exhibit genuine 3D and nonregular character, with even perfect nonregularity encountered for the tightly focused two-mode beam. These features originate from the partial spatial coherence and partial polarization of the incident beams, and in the limit of full coherence and polarization the three-dimensionality and nonregularity of the focal field vanish. We also find that the GSM beam can generate a nanoscale region around the focus where the field is essentially 3D unpolarized. The results demonstrate the rich polarimetric structure of focal fields and may find uses in optical particle manipulation and sensing.

Within the framework of QED theory at finite temperature, the thermal radiative corrections to spontaneous one-photon transition rates in hydrogen atom are investigated. The radiative one-loop self-energy corrections are described in the thermal case. Closed analytical expressions are derived and their numerical calcul- ations for the spontaneous decay rate of the $2p$ state are carried out. The dominance of thermal radiative corrections to a spontaneous ${\mathrm{Ly}}_{\alpha }$ decay rate over an ordinary induced transition rate up to temperatures $T<6000$ K is demonstrated.

We examine giant resonant enhancement of high-order harmonics generation (HHG) in the Mn atom and its singly charged ${\mathrm{Mn}}^{+}$ ion. Our theoretical model combines single active electron tunneling and propagation with correlation-enhanced recombination. Previously, this approach demonstrated its useability for the Xe atom [Phys. Rev. A 100, 013404 (2019)]. Spin-polarized Hartree-Fock and random-phase approximation with exchange calculations are carried out to evaluate the correlation enhancement factor in Mn and ${\mathrm{Mn}}^{+}$, which is then compared with other reported calculations and the experiment.

Based on the numerical solution of the full-dimensional time-dependent Schrödinger equation, we study the above-threshold ionization of atomic hydrogen by subpicosecond laser pulses at wavelengths ranging from 300 nm to 800 nm, in which regime substructures of the photoelectron energy spectra resulting from quantum interference are entangled with those induced by the Freeman resonance. By analyzing the time-dependent populations and ac Stark energy shifts of the relevant atomic levels, and observing the pulse intensity dependence and the pulse length evolution of the spectra, we clearly identify the origins of the substructures exhibited in the spectra.

We provide a comprehensive study of atomic Raman and Bragg diffraction when coupling to a pair of counterpropagating light gratings (double diffraction) or to a single one (single diffraction) and discuss the transition from one case to the other in a retroreflective geometry as the Doppler detuning changes. In contrast to single diffraction, double Raman loses its advantage of high diffraction efficiency for short pulses and has to be performed in a Bragg-type regime. Moreover, the structure of double diffraction leads to further limitations for broad momentum distributions on the efficiency of mirror pulses, making the use of (ultra)cold ensembles essential for high diffraction efficiency.

The recent experimental realization of spin-orbit (SO) coupling for ultracold bosons and fermions opens an exciting avenue for engineering quantum matter that may be challenging to realize in solid-state materials such as SO-coupled pseudospin-1 fermions. While one-dimensional SO coupling for spin-1 bosons has been experimentally realized, the generation of two-dimensional (2D) SO coupling and its topological properties are largely unexplored. Here we propose an experimental scheme for realizing a 2D Rashba-type SO coupling in a square lattice for pseudospin-1 Fermi gases. Because of the extended spin degree of freedom, many interesting topological phases could exist without relying on lattice point group symmetries that are crucial in solid-state materials. These exotic phases include triply degenerate points, quadratic band touching, a large Chern number ($C=5$) superfluid with five Majorana modes, triple-Weyl fermions, and so on. Our scheme can be generalized to larger spins and provides a distinctive route for engineering topological quantum matter by utilizing large spin degrees of freedom, instead of specific lattice symmetries.

Atoms with different internal states can exhibit different responses to an artificial magnetic field. An atomic gas mixture of two different components can therefore be interpreted as a mixture of two atomic gases carrying different synthetic charges. We consider the superfluid state of such unequally charged Fermi gases coupled to a magnetic field via the orbital effect and trapped in a torus geometry. The orbital coupling to the magnetic field favors an inhomogeneous superfluid state with optimum finite-center-of-mass-momentum pairing. The resulting population-balanced orbital Fulde-Ferrell (FF) state is robust against the magnetic field and does not undergo pair breaking, unlike the conventional BCS and Fulde-Ferrell-Larkin-Ovchinnikov-type pairing states under the Zeeman effect. We contrast the homogeneous and inhomogeneous cases, emphasizing the advantages of the unequally charged systems, and present their momentum distributions. We conclude that an unequally charged atomic Fermi gas system orbitally coupled to an artificial magnetic field provides an ideal candidate for experimental realization of the FF state.

Light-pulse atom interferometers are powerful quantum sensors, however, their accuracy for example in tests of the weak equivalence principle is limited by various spurious influences like stray magnetic fields or blackbody radiation. Improving the accuracy therefore requires a detailed assessment of the size of such deleterious effects. Here we present a systematic operator expansion to obtain phase shift and contrast analytically in powers of a perturbation potential. The result can either be employed for robust straightforward order-of-magnitude estimates or for rigorous calculations. Together with general conditions for the validity of the approach, we provide a particularly useful formula for the phase including wave-packet effects.

In non-Hermitian systems, multiple eigenstates can merge into one state at so-called exceptional points (EPs). This paper presents the coalescence of the multiple EPs based on a waveguide. For the ring cavities located at the second-order EPs with desired chiral modes, the waveguide provides unidirectional couplings among the modes of the second-order EPs, which merges the second-order EPs into one high-order EP. The principle of this framework and properties of this system are presented by analyzing the effective Hamiltonian of the system, a mode-coupling diagram, and numerical simulations.

In this paper we determine a lower bound to the mean value of the quantum potential for an arbitrary state. Furthermore, we derive a generalized uncertainty relation that is stronger than the Robertson-Schrödinger inequality and hence also stronger than the Heisenberg uncertainty principle. The mean value is then associated to the nonclassical part of the covariances of the momenta operator. This imposes a minimum bound for the nonclassical correlations of momenta and gives a physical characterization of the classical and semiclassical limits of quantum systems. The results obtained primarily for pure states are then generalized for density matrices describing mixed states.

We consider different settings of the task to distinguish pure orthogonal quantum states under local operations and a limited amount of classical communication. In the first setting, the spatially separated parties are allowed to perform only local projective measurements without any classical communication during the measurements. Under such a restricted class of operations, if the states are indistinguishable, then within a second group of settings, the parties are allowed to use an additional resource for the distinguishing. The additional resource is either a pure entangled state or multiple identical copies of the given states. Comparisons between these two types of resources are also done in certain cases. Both probabilistic discrimination and perfect discrimination of the states are considered for the second group of settings. Within a third setting, the parties perform local projective measurements with a restriction on the availability of classical communication during the measurements. But in this setting the parties are allowed to use a maximally entangled state as a resource.

We devise a multiphoton interferometry scheme for sampling a quadratic function of a specific immanant for any submatrix of a unitary matrix and its row permutations. The full unitary matrix describes a passive, linear interferometer, and its submatrix is used when photons enter in and are detected at subsets of possible input and output channels. Immanants are mathematical constructs that interpolate between the permanent and determinant; contrary to determinants and permanents, which have meaningful physical applications, immanants are devoid of physical meaning classically but here are shown to be meaningful in a quantum setting. Our quadratic form of immanants is sampled by injecting vacuum and single photons into interferometer input ports such that the photon arrival times are entangled, in contrast to previous methods that control arrival times without entangling. Our method works for any number of photons, and we solve explicitly the quadratic form for the two-, three-, and four-photon cases.

Structural vulnerability of a network can be studied via two key notions in graph theory: articulation points (APs) and bridges, representing nodes and edges whose removal will disconnect the network, respectively. Fundamental properties of APs and bridges in classical random networks have been studied recently. Yet, it is unknown if those properties still hold in quantum networks. Quantum networks allow for the transmission of quantum information between physically separated quantum systems. They play a very important role in quantum computing, quantum communication, and quantum sensing. Here we offer an analytical framework to study the structural vulnerability of quantum networks in terms of APs and bridges. In particular, we analytically calculate the fraction of APs and bridges for quantum networks with arbitrary degree distribution and entangled qubits in pure states. We find that quantum networks with swap operations have lower fractions of APs and bridges than their classical counterparts. Moreover, we find that quantum networks under low-degree swap operations are substantially more robust against AP attacks than their classical counterparts. These results help us better understand the structural vulnerability of quantum networks and shed light on the design of more robust quantum networks.

In Li et al. [Phys. Rev. A 98, 020502(R) (2018)] it was claimed that the model-potential computations of the Lamb shift on the ${}^{2}P_{1/2}-{}^{2}P_{3/2}$ fine structure in fluorinelike uranium lead to a discrepancy between theory and experiment. Later, it was reported by Volotka et al. [Phys. Rev. A 100, 010502(R) (2019)] that ab initio QED calculation, including the first-order one-electron QED contributions and the related effects of two-electron screening, yields the result which restores the agreement between theory and experiment and strongly disagrees with the model-potential Lamb-shift values. In the present paper, the model Lamb-shift operator [Shabaev et al., Phys. Rev. A 88, 012513 (2013)] is used to evaluate the QED effects on the ${}^{2}P_{1/2}-{}^{2}P_{3/2}$ fine structure in F-like ions. The calculations are performed by incorporating this operator into the Dirac-Coulomb-Breit equation employing different methods. It is demonstrated that the methods, based on including the Lamb-shift operator either into the Dirac-Fock equations or into the calculations by perturbation theory, lead to the theoretical results which are in good agreement with each other and with experiment. The restriction of these results to the first order in the QED effects leads to a value which agrees with the aforementioned ab initio QED result.