By performing a joint theoretical and experimental investigation on the high-order above-threshold ionization spectrum, the dominant role of the third-return-recollision trajectories in the region near the cutoff due to the ionic Coulomb field is identified. This invalidates the key assumption adopted in the conventional laser-induced electron-diffraction (LIED) approach that the first-return-recollision trajectories dominate the spectrum according to strong-field approximation. Our results show that the incident (return) electron beams produced by the first and third returns possess distinct characteristics of beam energy, beam diameter, and temporal evolution law due to the influence of the Coulomb field, and, therefore, the extracted results in the LIED will be altered if the significance of the third-return-recollision trajectories is properly considered in the analysis. Such a Coulomb field effect should be taken into account in all kinds of laser-induced imaging schemes based on recollision.

Multipartite quantum states may exhibit different types of quantum entanglement in that they cannot be converted into each other by local quantum operations only, and fully understanding mathematical structures of different types of multipartite entanglement is a very challenging task. In this paper, from the viewpoint of Hardy's nonlocality, we compare W and GHZ (Greenberger-Horne-Zeilinger) states and show a couple of crucial different behaviors between them. Particularly, by developing a geometric model for Hardy's nonlocality problem of W states, we derive an upper bound for its maximal violation probability, which turns out to be strictly smaller than the corresponding probability of GHZ states. This gives us a new comparison between these two quantum states, and the result is also consistent with our intuition that GHZ states are more entangled. Furthermore, we generalize our approach to obtain an asymptotic characterization for general $N$-qubit W states, revealing that when $N$ goes up, the speed that the maximum violation probabilities decay is exponentially slower than that of general $N$-qubit GHZ states. We provide some numerical simulations to verify our theoretical results.

Recent experiments with identically tuned nested Mach-Zehnder interferometers which attempted to observe the location of particles inside these interferometers are analyzed. In spite of claims to the contrary, it is argued that all experiments support the same surprising picture according to which the location of the particles inside the interferometers is not described by continuous trajectories.

As has been shown elsewhere, a reasonable model of the loss of entanglement or correlation that occurs in quantum computations is one which assumes that they can effectively be predicted by a framework that presupposes the presence of irreversibilities internal to the system. It is based on the steepest-entropy-ascent principle and is used here to reproduce the behavior of a controlled-phase gate in good agreement with experimental data. The results show that the loss of entanglement predicted is related to the irreversibilities in a nontrivial way, providing a possible alternative approach that warrants exploration to that conventionally used to predict the loss of entanglement. The results provide a means for understanding this loss in quantum protocols from a nonequilibrium thermodynamic standpoint. This framework permits the development of strategies for extending either the maximum fidelity of the computation or the entanglement time.

In recent years, there has been a significant progress in the development of digital quantum processors. The state-of-the-art quantum devices are imperfect, and fully algorithmic fault-tolerant quantum computing is a matter of future. Until technology develops to the state with practical error correction, computational approaches other than the standard digital one can be used to avoid execution of the most noisy quantum operations. We demonstrate how a hybrid digital-analog approach allows simulating dynamics of a transverse-field Ising model without standard two-qubit gates, which are currently one of the most problematic building blocks of quantum circuits. We use qubit-qubit crosstalks (couplings) of IBM superconducting quantum processors to simulate Trotterized dynamics of spin clusters and then we compare the obtained results with the results of conventional digital computation based on two-qubit gates from the universal set. The comparison shows that digital-analog approach significantly outperforms standard digital approach for this simulation problem, despite of the fact that crosstalks in IBM quantum processors are small. We argue that the efficiency of digital-analog quantum computing can be improved with the help of more specialized processors, so that they can be used to efficiently implement other quantum algorithms. This indicates the prospect of a digital-to-analog strategy for near-term noisy intermediate-scale quantum computers.

Photoelectron satellite spectra and angular distribution of argon atoms were measured in the energy range of 34–43 eV, using a well-developed reaction microscope mounted on a newly built high-order harmonic generation (HHG) extreme ultraviolet (EUV) source. Various satellites were resolved and the angular distribution asymmetry parameters $\beta$ were determined for both the main lines and the satellites. It is found that our measured $\beta$ for the $3p$ and $3s$ main lines are in an excellent agreement with previous results. The $\beta$ parameters measured for most of the satellites are first reported in this energy range, providing benchmark data for testing different theoretical models.

Experiments with bosonic atoms in optical superlattices allow for the interesting possibility to study the adiabatic quantized pumping of bosonic atoms in the presence of interactions. We investigate this exotic phenomenon for bound bosonic pairs in the paradigmatic Su-Schrieffer-Heeger model where the ground state exhibits topological properties due to dimerized hoppings. We show that at unit filling there exist crossovers and phase transitions to bond-order phases of paired bosons known as the pair-bond-order phase for attractive interactions. The pair-bond-order phase is found to exhibit effective topological properties such as the presence of polarized paired edge states. This is further analyzed by studying the emergence and breakdown of the Thouless charge pumping of bound bosonic pairs by a parametric extension to the famous Rice-Mele model. Finally, we discuss how the pumping of paired bosons or different regimes of breakdown of charge pumping can be probed by state-of-the-art experiments, for example, with repulsively bound bosons.

We consider a generic Hamiltonian that is suitable for describing a uniform BCS superfluid on a lattice with a two-point basis and study its collective excitations at zero temperature. For this purpose, we first derive a Gaussian effective action for the pairing fluctuations and then extract the low-energy dispersion relations for the in-phase Goldstone and out-of-phase Leggett modes along with the corresponding amplitude (i.e., the so-called Higgs) ones. We find that while the Goldstone mode is gapless at zero momentum and propagating in general, the Leggett mode becomes undamped only with sufficiently strong interactions. Furthermore, we show that, in addition to the conventional contribution that is controlled by the energy of the Bloch bands, the velocity of the Goldstone mode has a geometric contribution that is governed by the quantum metric tensor of the Bloch states. Our results suggest that the latter contribution dominates the velocity when the former becomes negligible for a narrowband or a flatband.

A general analytic form of the full $6\times 6$ dyadic Green's function of a spherically symmetric open optical system is presented, with an explicit solution provided for a homogeneous sphere in vacuum. Different spectral representations of the Green's function are derived using the Mittag-Leffler theorem, and their convergence to the exact solution is analyzed, allowing us to select optimal representations. Based on them, more efficient versions of the resonant-state expansion (RSE) are formulated, with a particular focus on the static mode contribution, including versions of the RSE with a complete elimination of static modes. These general versions of the RSE, applicable to nonspherical optical systems, are verified and illustrated on exactly solvable examples of a dielectric sphere in vacuum with perturbations of its size and refractive index, demonstrating the same level of convergence to the exact solution for both transverse electric and transverse magnetic polarizations.

In this paper, we investigate the single-photon scattering and bound states in a one-dimensional coupled-resonator waveguide which couples to a single artificial giant atom with two or more coupling points. When the atom couples to the waveguide via two resonators, the single-photon reflection rate is characterized by either Breit-Wigner or Fano lineshapes. When the atom couples to the waveguide via multiple resonators, we show numerically how the destructive interference effect leads to a complete single-photon reflection. We also find a phase-transition phenomena for the multiresonator coupling case, which reveals that the upper bound state only exists when the atom-waveguide coupling strength is above a critical value.

We investigate a four-level tripod atomic sample trapped, respectively, in a one-dimensional order and disorder (atom number fluctuation) optical lattice to achieve the tunable and robust three-color reflections in the regimes of zero absorption. Based on electromagnetically induced transparency (EIT) and the periodic modulation of the atom number density, there are three reflection regions of high probe reflectivity in different frequency ranges, respectively. Such three-color reflections arise from the double-EIT atomic structure in the optical lattice and can also be observed by the suppression of the density of photonic states. One of them appears in the large detuning range, and the other two locate in two EIT windows, respectively, which are relatively narrow and can be dynamically manipulated flexibly. It is worth noting that such reflection scheme has strong robustness, i.e., it is immune to the atom number fluctuation in the disorder structure. Our results have potential applications in all-optical networks with respect to fabricating efficient devices (such as reflectors and diodes) for manipulating multicolor photon flows.

Spin-$\frac{1}{2}$ two-leg ladder models under a magnetic field have a well-known phase diagram. In this paper, we use multipartite nonlocality (a measure of multipartite quantum correlations) to characterize the quantum correlations in the ladder models at zero temperature. Both finite-size and infinite-size ladders are considered. We investigate the global nonlocality ${\mathcal{S}}_g=\mathcal{S}\left(\right|{\mathrm{\Psi }}_g\rangle )$, which describes quantum correlations of the ground states $|{\mathrm{\Psi }}_g\rangle$ of the entire lattices, and the partial nonlocality ${\mathcal{S}}_p=\mathcal{S}\left({\widehat{\rho }}_n\right)$, which describes quantum correlations of the reduced states ${\widehat{\rho }}_n$ of some sublattices in the ladders. We find that as the magnetic field $\lambda$ increases, the global nonlocality ${\mathcal{S}}_g$ presents a single-peak curve. Moreover, the logarithmic measure $ln{\mathcal{S}}_g$ changes dramatically at the two critical fields ${\lambda }_{c_1}$ and ${\lambda }_{c_2}$ of the models and thus signals the quantum phase transitions in the models. For the partial nonlocality ${\mathcal{S}}_p$, in the regions ${\lambda }_{c_1}<\lambda <{\lambda }_{c_2}$, we observe that the ${\mathcal{S}}_p\left(\lambda \right)$ curve shows an oscillation. Numerical results reveal that the underling mechanism is the “major component transitions” in the reduced states ${\widehat{\rho }}_n$ of the sublattices. More importantly, the oscillation of the partial nonlocality ${\mathcal{S}}_p$ is modulated by the single-peak curve of the global nonlocality ${\mathcal{S}}_g$. The result provides valuable clues about the relation between partial nonlocality and global nonlocality in low-dimensional quantum models.

We use the uncertainty relation between the operators associated with the total number of particles and with the relative phase of two bosonic modes to construct entanglement and Einstein-Podolsky-Rosen steering criteria. These can be tested experimentally in a variety of systems, such as optical fields, Bose-Einstein condensates, and mechanical oscillators. While known entanglement criteria involving the phase observable typically require us to perform interference measurements by recombining the two systems, our criteria can be tested through local measurements at two spatially distinct positions to investigate the nonlocal nature of quantum correlations. We present simple examples where our criteria are violated and show their robustness to noise. Apart from being useful for state characterization, they might find application in quantum information protocols, for example, based on number-phase teleportation.

We study the macroscopic realism (macrorealism) through the two- and three-time Leggett-Garg inequalities (LGIs) in a two-interacting-qubits system. The two qubits are coupled either with two bosonic (thermal or photonic) baths or fermionic (electronic) baths. We study both how the equilibrium and nonequilibrium environments influence the LGIs. One way to characterize the nonequilibrium condition is by the temperature difference (for the bosonic bath) or the chemical potential difference (for the fermionic bath). We also study the heat or particle current and the entropy production rate generated by the nonequilibrium environments. Analytical forms of LGIs and the maximal value of LGIs based on the quantum master equation beyond the secular approximation are derived. The LGI functions and the corresponding maximal value have separated contributions, the part describing the coherent evolution and the part describing the coupling between the system and environments. The environment-coupling part can be from the equilibrium environment or the nonequilibrium environment. The nonequilibrium dynamics is quantified by the Bloch-Redfield equation which is beyond the Lindblad form. We found that the nonequilibriumness quantified by the temperature difference or the chemical potential difference can lead to the LGIs' violations or the increase of the maximal value of LGIs, restoring the quantum nature from certain equilibrium cases where LGIs are preserved. The corresponding nonequilibrium thermodynamic cost is quantified by the nonzero entropy production rate. The violation enhancement increases with the increase of the entropy production rate under certain nonequilibrium conditions. Therefore, the LGIs' violation enhancement can be realized by the thermodynamic nonequilibrium cost. Our results shed light on the nature of the macrorealism and the relationship between the nonequilibriumness and the quantum temporal correlation. Our finding of the nonequilibrium promoted LGIs' violations suggests a new strategy for the design of quantum information processing and quantum computational devices to maintain the quantum nature and quantum correlations for long.

In a continuous-variable quantum key distribution (CV-QKD) protocol, which is based on heterodyne detection at the receiver, the application of a noiseless linear amplifier (NLA) on the received signal before the detection can be emulated by the postselection of the detection outcome. Such a postselection, which is also called a measurement-based NLA, requires a cutoff on the amplitude of the heterodyne-detection outcome to produce a normalizable filter function. Increasing the cutoff with respect to the received signals results in a more faithful emulation of the NLA and nearly Gaussian output statistics at the cost of discarding more data. While recent works have shown the benefits of postselection via an asymptotic security analysis, we undertake an investigation of such a postselection utilizing a composable security proof in the realistic finite-size regime, where this tradeoff is extremely relevant. We show that this form of postselection offers only a small fraction of the asymptotic improvement in the finite-size regime. This postselection can improve the secure range of a CV-QKD over lossy thermal channels if the finite block size is sufficiently large and the optimal value for the filter cutoff is typically in the non-Gaussian regime. The relatively modest improvement in the finite-size regime as compared to the asymptotic case highlights the need for new tools to prove the security of non-Gaussian cryptographic protocols. These results also represent a quantitative assessment of a measurement-based NLA with an entangled-state input in both the Gaussian and non-Gaussian regime.

We investigate the fragmentation dynamics of a hydrogen atom impacted by a positron whose energy is just above the atomic ionization potential (i.e., near threshold), using a classical trajectory Monte Carlo approach with a Heisenberg potential (CTMC-H). The cross sections of various fragmentation channels are calculated, addressing the Wannier threshold law that depicts the impact energy dependent cross section for the three-body breakup channel. In particular, we investigate the collision configurations for individual fragmentation channels and calculate the correlated spectrum of the angular distribution and energy allocation (CoSAE) of the outgoing electron and positron to uncover the mechanism of their highly imbalanced energy partition. Finally, we extend our study to positron-hydrogen collisions in the presence of an external electromagnetic field and show that the fractions of three different channels, i.e., three-body breakup, positronium (Ps) formation, and Rydberg excitation, can be considerably modulated with a laser pulse of peak intensity as weak as ${10}^{11}\mathrm{W}$/${\mathrm{cm}}^2$.

Fully quantum ab initio calculations of the collision-induced shapes of two rovibrational ${\mathrm{H}}_2$ lines perturbed by He provide an unprecedented subpercent agreement with ultra-accurate cavity-enhanced measurements. This level of consistency between theory and experiment hinges on a highly accurate potential energy surface and a realistic treatment of the velocity changing and dephasing collisions. In addition to the fundamental importance, these results show that ab initio calculations can provide reference data for spectroscopic studies of planet atmospheres at the required accuracy level and can be used to populate spectroscopic line-by-line databases.

We report on a comparative study of strong-field ionization of alkaline-earth-metal atoms by intense femtosecond laser pulses from near-infrared to midinfrared wavelengths. By collecting the ionization signals only produced within the central portion of the laser focus, the focus volume effect is largely reduced and the saturation intensities for different alkaline-earth-metal atoms are reliably determined, which permits us to directly test the strong-field-ionization theories. We demonstrate that the Perelomov-Popov-Terent'ev model accurately predicts the experimental ionization yields and saturation intensities in general for arbitrary values of the Keldysh parameter, while the Ammosov-Delone-Krainov simulations agree with the experiments for the tunneling-ionization regime and also for the regime when the Keldysh parameter is around 1. Our work presents benchmark data for strong-field ionization of alkaline-earth metals over a broad range of laser parameters and confirms the validity of Keldysh's picture for such atoms.

The theory of the thin-vapor-layer linear optical properties is presented for the case of specular reflection of atoms colliding with the walls. The effects of light absorption and the shift in the resonance frequency are taken into account by means of self-consistent calculation of the field and polarization in a gaseous medium. The obtained formulas reproduce the complex dependence of the spectral line profile on the gas layer thickness and allow one to determine numerically the exact values of the “blueshift.” It was shown that, despite the low vapor concentration, the resonance shift is on the same order of magnitude as the width of the spectral lines.