The study of no-signaling polytopes is addressed under an alternative approach, focusing on the skeleton graph of the polytopes rather than on the Bell inequalities solely. The paper starts with the study of the Bell scenario where one of the parties (Bob) has three output options (2223 scenario). We found that in this polytope the Popescu-Rohrlich (PR) vertices are interconnected, making up two networks around the polytope of local correlations, which is a new feature in relation to the 2222 polytope. We also found that in this scenario each CHSH inequality is violated by five PR vertices: the PR vertex associated to that inequality and its four adjacent PR vertices. Later, we study the facet structure of the corresponding polytope of local correlations. What we obtain is a one-to-one correspondence between some two-dimensional faces of the no-signaling polytope defined by specific loops in the adjacency networks of PR vertices, the lifting Bell functions, and the nontrivial facets of the local polytope. Finally, by studying the skeleton graphs of the polytopes for the general $222v_B$ scenarios, where $v_B$ is the number of possible Bob's outcomes, a striking high degeneracy is unveiled. We show that the set of local vertices exhibits a much higher degeneracy than the PR vertices, which in turn are more degenerate than a regular vertex.

We explore the extent of the induced entanglement of the mechanical modes that can be attributed to the transfer of coherence from two-mode cavity radiation in a doubly resonant optomechanical cavity. It is expected that this scheme can support a generation of mechanical oscillations with a robust degree of entanglement combined with significant controllability. The entanglement is found to be sensitive to the specific choices of the frequencies of the bichromatic drive laser. It also turns out that the degree of entanglement would be enhanced with increasing rates of injection of the atoms but with decreasing initial lengths of a doubly resonant cavity and atomic decay rates. In addition, the entanglement is found to behave qualitatively in the same way for the measures of entanglement we have applied. Since the scheme we considered can possibly be implemented with current technology and allows the quantum features of cavity radiation to be accessible for application, we anticipate that it can be utilized in the realization of continuous-variable quantum information processing.

We report on an ab initio model to describe single- and double-ionization processes in antiproton collisions with atoms and molecules. Our model is based on a fully correlated close-coupling approach and a Dyson orbital analysis. Furthermore, we employ the concept of a correlation integral in order to quantify and gain insights into the effects of electronic correlation on the ionization processes. Our model is applied to the prototype antiproton-helium and antiproton–molecular hydrogen collision systems in the keV energy range.

This work investigates the capability of the multiconfiguration Dirac-Hartree-Fock (MCDHF) method in predicting the Cu $K{\alpha }_{3,4}$ spectrum. Ab initio energy eigenvalues, relative intensities, and radiative widths are calculated for the Cu $2p$ and $2s$ satellite transitions. By fitting to the most accurate experimental Cu $K{\alpha }_{3,4}$ spectra available, we show that our $2p$ satellite energy eigenvalues agree with experiment to within 0.35 eV and that our $2p$ shake probability agrees with the $2p$ fitted intensity to within 0.05%. Our fits suggest a $I\left(2s\right)$:$I\left(2p\right)$ satellite intensity ratio (as a percentage of the total $K\alpha$ spectrum) of 0.03(1):0.76(1) Theoretical predictions of this ratio can be examined using shake probabilities. We calculate the probability of shake-off due to the sudden creation of a $1s$ hole in Cu, yielding an ab initio $I\left(2s\right)$:$I\left(2p\right)$ shake probability ratio of 0.194:0.742. Using MCDHF, the rates at which hole states, created through the shake processes, depopulate via Auger transitions are determined. These results explain the apparent discrepancy between experimental satellite intensities and shake probabilities, and characterize the Cu $K{\alpha }_{3,4}$ spectrum with a satellite intensity ratio of 0.04(1):0.76, consistent with the experiment.

We investigate the level structure of heavy hydrogenlike ions in laser beams. In heavy ions, the electrons are tightly bound by the Coulomb potential of the nucleus, which prohibits ionization even by strong lasers. However, interaction with the light field leads to dynamic shifts of the electronic energy levels. We apply a fully relativistic description of the electronic states by means of the Dirac equation. Interaction with the monofrequent laser field is treated by second-order time-dependent perturbation theory. Our formalism goes beyond the Stark long-wavelength dipole approximation and takes into account nondipole effects of retardation and interaction with the magnetic-field components of the laser beam. The resulting level shifts are relevant for experiments at present and near-future laser facilities.

From the solution of the time-dependent Schrödinger equation (TDSE), it has been shown that in a two-color laser pulse the relative phase that optimizes terahertz (THz) radiation also optimizes the asymmetry of the angular distribution of photoelectrons. Here, we show that a second-order strong-field approximation can accurately calculate the asymmetry of photoelectrons and thus can locate the phase delay that optimizes THz generation, doing so four orders of magnitude faster than TDSE calculations. We further trace that this is possible because THz emission originates from a free-free radiative transition between the rescattered electron and the target ion, similar to other electron-ion collisions in the laser field, e.g., high-harmonic generation and nonsequential double ionization. Our results pave the way to locate the optimal phase delay for any typical laser pulses for maximal THz generation in the laboratory.

Optical phase gradient metasurfaces (PGMs) have provided unprecedented opportunities for arbitrarily controlling wave propagation via the generalized Snell's law (GSL). However, the whole picture of wave diffraction therein has not been clearly presented, particularly for the incident angles beyond the critical angle. Although a parity-dependent diffraction effect was found in acoustic metagratings, little is known about whether this effect holds true in typical optical PGMs. Here we demonstrate the universality of the parity-dependent diffraction effect by employing some optical PGMs with popular designs, such as all-dielectric and plasmonic meta-atoms. It is first shown that the parity in optical PGMs plays a significant role in determining the diffraction physics, producing a robust reversal effect of outgoing waves from the reflection to the transmission side. As an alternative degree of freedom in PGMs, the parity-dependent diffraction effect, together with the GSL, provides a complete theory to manipulate wave fields, further advancing various explorations in unique wave phenomena and promising applications.

We theoretically calculate the density dynamic structure factor of one-dimensional Fermi superfluid with Raman-type spin-orbit coupling, and analyze its main dynamical character during phase transition between Bardeen-Cooper-Schrieffer superfluid and topological superfluid. Our theoretical results display four kinds of single-particle excitations induced by the two-branch structure of the single-particle spectrum, and the cross single-particle excitation is much easier to see in the spin dynamic structure factor at a small transferred momentum. Also we find a new rotonlike collective mode emerges at a fixed transferred momentum $q\simeq 2k_F$, and it only appears once the system enters the topological superfluid state. The occurrence of this rotonlike excitation is related to the switch of the global minimum in the single-particle spectrum from $k=0$ to $k\simeq 2k_F$.

We investigate the system of a heavy impurity immersed in a degenerate Fermi gas, where the impurity's internal degree of freedom (pseudospin) is manipulated by a series of radio-frequency (RF) pulses at several different times. Applying the functional determinant approach, we carry out an essentially exact calculation of the Ramsey-interference-type responses to the RF pulses. These responses can be regarded as multidimensional (MD) spectroscopy of the system in the time domain. Similar to their one-dimensional counterpart studied by Knap et al. [M. Knap et al., Phys. Rev. X 2, 041020 (2012)], these responses are universal functions. A Fourier transformation of the time intervals gives the MD spectroscopy in the frequency domain, providing insightful information on the many-body correlation and relaxation via the cross peaks, e.g., the off-diagonal peaks in a two-dimensional spectrum. These features are inaccessible for the conventional, one-dimensional absorption spectrum. Our scheme provides a different method to investigate many-body nonequilibrium physics beyond the linear response regime with accessible tools in cold atoms.

The many-body entanglement between two finite (size-$d$) disjoint vacuum regions of noninteracting lattice scalar field theory in one spatial dimension, i.e., a ${(d_A\times d_B)}_{\mathrm{mixed}}$ Gaussian continuous variable system, is locally transformed into a tensor-product core of ${(1_A\times 1_B)}_{\mathrm{mixed}}$ entangled pairs. Accessible entanglement within these core pairs exhibits an exponential hierarchy and as such identifies the structure of dominant region modes from which vacuum entanglement could be extracted into a spatially separated pair of quantum detectors. Beyond the core, the remaining modes of the halo are determined to be $AB$ separable in isolation, as well as separable from the core. However, state preparation protocols that distribute entanglement in the form of ${(1_A\times 1_B)}_{\mathrm{mixed}}$ core pairs are found to require additional entanglement in the halo that is obscured by classical correlations. This inaccessible (bound) halo entanglement is found to mirror the accessible entanglement, but with a step behavior as the continuum is approached. It remains possible that alternate initialization protocols that do not utilize the exponential hierarchy of core-pair entanglement may require less inaccessible entanglement. Entanglement consolidation is expected to persist in higher dimensions and may aid classical and quantum simulations of asymptotically free gauge field theories, such as quantum chromodynamics.

In this paper we calculate the ground-state energy of benzene under spatial deformations by using the variational quantum eigensolver. The primary goal of the study is to estimate the feasibility of using quantum computing Ansätze on near-term devices to solve problems with a large number of orbitals in regions where classical methods are known to fail. Furthermore, by combining our advanced simulation platform with real quantum computers, we provide an analysis of how the noise, inherent to quantum computers, affects the results. At the center of our study are the hardware efficient and quantum unitary coupled-cluster (QUCC) Ansätze. First, we find that the hardware efficient Ansatz has the potential to outperform mean-field methods for extreme deformations of benzene. However, key problems remain at equilibrium, preventing real chemical application. Moreover, the hardware efficient Ansatz yields results that strongly depend on the initial guess of parameters (in both noisy and noiseless cases) and optimization issues have a higher impact on their convergence than noise. This is confirmed by comparison with real quantum computing demonstrations. On the other hand, the QUCC Ansatz alternative exhibits deeper circuits. Therefore, noise effects increase and are so extreme that the method never outperform mean-field theories. Our dual simulator, (8–16)-qubit QPU computations of QUCC Ansatz appears to be much more sensitive to hardware noise than shot noise, which further indicates where the noise-reduction efforts should be directed. Finally, the study shows that the QUCC method better captures the physics of the system as the QUCC method can be utilized together with the Hückel approximation. We discussed how going beyond this approximation sharply increases the optimization complexity of such a difficult problem.

Temporal Kerr cavity solitons are pulses of light that can persist in coherently driven, dispersive resonators with Kerr-type nonlinearity. Many studies have shown that such solitons can react to parameter inhomogeneities (e.g., variations in the complex amplitude of the driving field) by experiencing a temporal drift. The vast majority of such studies assume that the inhomogeneity varies slowly across the soliton, leading to the prediction that the soliton drift rate is linearly proportional to the gradient of the inhomogeneity at the soliton position. However, the assumption of a slowly varying inhomogeneity may not hold true in all situations, e.g., when using bichromatic driving or in the presence of third-order dispersion that gives rise to an extended dispersive wave tail. Here we report on theoretical and numerical results pertaining to the behavior of dissipative temporal Kerr cavity solitons under conditions where parameter inhomogeneities vary nonlinearly across the width of the soliton. In this case, the soliton velocity is dictated by the full overlap between its so-called adjoint neutral mode and the parameter perturbation, which we show can yield dynamics that are manifestly at odds with the common wisdom of motion dependent solely upon the gradient of the inhomogeneity. We also investigate how the presence of third-order dispersion and the associated dispersive wave tail changes the motion induced by parameter inhomogeneities. We find that the dispersive wave tail as such does not contribute to the soliton motion; instead, higher-order dispersion yields counterintuitive influences. Our results provide new insights into the behavior of temporal cavity solitons in the presence of parameter inhomogeneities and can impact systems employing pulsed or bichromatic pumping and/or resonators with non-negligible higher-order dispersion.

We propose a scheme to enhance entanglement and asymmetric steering between two mechanical oscillators in an optomechanical system with coherent feedback control. In the system, an optical cavity interacts with two mechanical oscillators and an auxiliary cavity whose output field is fed back into the input port of the optical cavity via a feedback loop. Due to the coherence between the auxiliary cavity and feedback, we derive the effective decay rate and a nonzero frequency shift of the optical cavity. Consequently, the induced beam-splitter-type and parametric-type interactions between the two mechanical oscillators are modulated, which leads to the enhancement of entanglement and the generation of asymmetric steering even if the two mechanical oscillators possess identical decoherence properties. And the direction of asymmetric steering can be controlled by tuning the jumping phase between two cavities and optimizing the ratio of drive asymmetry. In contrast to the method of adding losses or noises to one subsystem at the cost of reducing steerability, this scheme provides an active way to achieve enhanced asymmetric steering. Furthermore, the steady-state and dynamical entanglement and asymmetric steering can be generated in the unresolved-sideband regime, which is friendly for experimental implementation.

We investigate the Schwinger pair production in combined electric fields using quantum field theoretical simulations, where static and oscillating electric fields are finitely extended and spatially separated. We find that the pair production can be dynamically assisted until the two fields are separated much farther than one Compton length. We show the signature of dynamical assistance, in the position of both static and oscillating fields, based on the position-based production rate. Moreover, we investigate the impact of spatial width on dynamical assistance to show that the production rate is suppressed when the width of any of the two fields exceeds one Compton length and potential heights are constant.

In the adiabatic perturbation theory, Berry curvature is related to the generalized force, and the quantum metric tensor is linked with energy fluctuation. While the former is tested with numerous numerical results and experimental realizations, the latter is less considered. Quantum Fisher information, the key to quantum precision measurement, is a four times quantum metric tensor. It is difficult to relate the quantum Fisher information with some physical observable. One interesting candidate is the square of the symmetric logarithmic derivative, which is usually tough to obtain, both theoretically and experimentally. The adiabatic perturbation theory enlightens us to measure the energy fluctuation to directly extract the quantum Fisher information. In this article we first adopt an alternative way to derive the link of energy fluctuation to the quantum Fisher information. Then we numerically testify to the direct extraction of the quantum Fisher information based on adiabatic perturbation in two-level systems and simulate the experimental realization in a nitrogen-vacancy center with experimentally practical parameters. Statistical models such as the transverse-field Ising model and Heisenberg spin chains are also discussed to compare with the analytical result and show the level crossing, respectively. Our discussion will provide a practical scheme to measure the quantum Fisher information and will also be a benefit to quantum precision measurement and the understanding of the quantum Fisher information.

Calculating the trace of the product of $mn$-qubit density matrices (multivariate trace) is a crucial subroutine in quantum error mitigation and information measure estimation. We propose a unified multivariate trace estimation (UMT) which conceptually unifies the previous qubit-optimal and depth-optimal approaches with tunable quantum circuit depth and number of qubits. The constructed circuits have $\lceil (m-1)/s\rceil$ or $n\lceil (m-1)/s\rceil$ depth corresponding to $(s+m)n$ or $s+mn$ qubits for $s\in \{1,\cdots ,\lfloor m/2\rfloor \}$, respectively. Such flexible circuit structures enable people to choose suitable circuits for different hardware devices. We apply UMT to virtual distillation to achieve exponential error suppression and design a family of concrete circuits to calculate the trace of the product of eight and nine $n$-qubit density matrices. A numerical example shows that the additional circuits still mitigate the noise expectation value under the global depolarizing channel.

We propose a protocol to perform long-range distribution of near-maximally entangled multiphoton states, allowing versatile applications such as quantum key distribution (QKD) and quantum metrology which can provide alternatives to state-of-the-art protocols. Our scheme uses resources available within the current integrated quantum photonic technology: squeezed vacuum states and photon-number-resolving detectors. The distributed entanglement can be certified by Bell tests which have the potential to be loophole free, and may be directly used in well-established QKD protocols. Generally, this provides measurement-device-independent levels of security, which may be upgraded to fully device-independent security if the Bell test is loophole free. In both cases, the protocol is robust to extremely high transmission losses, matching the optimal $O\left(\sqrt{\eta }\right)$ scaling of key rate with channel transmittance.

We report measurements of the hyperfine coupling constant for the $8p{}^{2}P_{1/2}$ level of atomic cesium, ${}^{133}\mathrm{Cs}$, with a relative uncertainty of $\approx 0.019$%. Our result is $A=42.933\left(8\right)$ MHz, in good agreement with recent theoretical results. We also examine the hyperfine structure of the $8p{}^{2}P_{3/2}$ level, and derive new values for the energies of the $8p{}^{2}P_{1/2}$ and $8p{}^{2}P_{3/2}$ levels of cesium.