We report the emergence of a previously unexplored species of solitary waves, viz., time-localized dark modes in integrable and nonintegrable variants of the massive Thirring model and in the three-wave resonant-interaction system, which are models broadly used in plasma physics, nonlinear optics, and hydrodynamics. They are also interesting as basic models for the propagation of nonlinear waves in media without intrinsic dispersion. An essential finding is that the condition for the existence of time-localized dark modes in these systems, which develop density dips in the course of their evolution, coincides with the condition for the occurrence of the zero-wave-number-gain (ZWG) modulational instability (MI). Systematic simulations reveal that, whenever the ZWG MI is present, such dark modes are generically excited from a chaotic background as patches embedded in complex patterns.

The superradiant phase transition (SPT) has been widely studied in cavity quantum electrodynamics (CQED). However, this SPT is still subject to ongoing debates due to the no-go theorem induced by the so-called ${\mathbf{A}}^2$ term (AT). We propose a hybrid quantum system, consisting of a single-mode cavity simultaneously coupled to both a two-level system and yttrium-iron-garnet sphere supporting magnons with Kerr nonlinearity, to restore the SPT against the AT. The Kerr magnons here can effectively introduce an additional AT tunable and strong enough to counteract the intrinsic AT, via adiabatically eliminating the degrees of freedom of the magnons. We show that the Kerr-magnon-induced SPT can exist in both cases of ignoring and including the intrinsic AT. Without the intrinsic AT, the critical coupling strength can be dramatically reduced by introducing the Kerr magnons, which greatly relaxes the experimental conditions for observing the SPT. With the intrinsic AT, the forbidden SPT can be recovered with the Kerr magnons in a reversed way. Our work paves a potential way to manipulate the SPT against the AT in hybrid systems combining CQED and nonlinear magnonics.

We propose a simple simulation of nonlocal quantum correlations among $N$ qubits using a local hidden variable source with a positive probability distribution, given that each of the $N$ observers has access to a local negative bit. Notably, unlike the Toner-Bacon protocol, no exchange of classical bits between the observers is required. Moreover, our simulation can be extended to include Popescu-Rohrlich box correlations.

An important challenge for current and near-term quantum devices is finding useful tasks that can be preformed on them. We first show how to efficiently encode a bounded $n\times n$ matrix $A$ into a linear optical circuit with $2n$ modes. We then apply this encoding to the case where $A$ is a matrix containing information about a graph $G$. We show that a photonic quantum processor consisting of single-photon sources, a linear optical circuit encoding $A$, and single-photon detectors can solve a range of graph problems including finding the number of perfect matchings of bipartite graphs, computing permanental polynomials, determining whether two graphs are isomorphic, and the $k$-densest subgraph problem. We also propose preprocessing methods to boost the probabilities of observing the relevant detection events and thus improve performance. Finally we present both numerical simulations and implementations on Quandela's Ascella photonic quantum processor to validate our findings.

The Fourier expansion of the loss function in variational quantum algorithms (VQAs) contains a wealth of information yet is generally hard to access. We focus on the class of variational circuits where constant gates are Clifford gates and parametrized gates are generated by Pauli operators, which covers most practical cases while allowing much control due to the properties of stabilizer circuits. We give a classical algorithm that, for an $N$-qubit circuit and a single Pauli observable, computes coefficients of all trigonometric monomials up to a degree $m$ in time bounded by $O\left(N{2}^m\right)$. Using the general structure and implementation of the algorithm, we reveal several aspects of Fourier expansions in Clifford plus Pauli VQAs such as (i) reformulating the problem of computing the Fourier series as an instance of the multivariate Boolean quadratic system, (ii) showing that the approximation given by a truncated Fourier expansion can be quantified by the $L^2$-norm and evaluated dynamically, (iii) the tendency of Fourier series to be rather sparse and Fourier coefficients to cluster together, and (iv) the possibility to compute the full Fourier series for circuits of nontrivial sizes, featuring tens to hundreds of qubits and parametric gates.

The Stark-interference technique is commonly used to amplify the feeble parity-violating signal in atomic experiments. As a result, interpretation of these experiments in terms of electroweak observables requires knowledge of the Stark-induced $E1$ transition amplitudes or, equivalently, transition polarizabilities. While the literature assumes that these transition polarizabilities do not depend on the nuclear spin, here we prove the contrary. The nuclear-spin dependence arises due to the hyperfine mixing of atomic states and requires a third-order perturbation theory (one hyperfine interaction and two electric-dipole interactions) treatment. We demonstrate that the so far neglected tensor contribution appears in the transition polarizability and present numerical results for the nuclear-spin-dependent corrections to the $6S_{1/2}\to 7S_{1/2}$ transition polarizability in ${}^{133}\mathrm{Cs}$. We investigate the effect of these corrections to transition polarizabilities on the extraction of the ${}^{133}\mathrm{Cs}$ anapole moment from the Boulder experiment [Science 275, 1759 (1997)]. We also consider their effect on the extraction of the ratio between the scalar and vector transition polarizabilities from the measurements [Phys. Rev. A 55, 1007 (1997)]. While the corrections are minor at the current level of experimental accuracy, our analysis provides a framework for future experiments.

We study localization properties and mobility edges of a generalized spinful Aubry-André-Harper (AAH) model, which is the dimensional reduction of the two-dimensional Hofstadter model with a non-Abelian SU(2) gauge potential. Depending on whether the quasiperiod is comparable with the lattice size, the model has different localization properties. In the noncomparable case, the generalized AAH model still retains duality properties. Tuning the non-Abelian gauge can make the system undergo an unconventional reentrant localization phase transition as the strength of quasiperiodic potential increases. Furthermore, mobility edges exist in the mixed phase where the localized states sit at the center of spectra. Nevertheless, the non-Abelian gauge potential results in more mobility edges than that the Abelian gauge potential does, when the model is in the semiclassical limit where the quasiperiod is comparable with the lattice size. Moreover, exact expressions of the mobility edges and localization phase diagrams are analytically obtained by a semiclassical method.

We study the zero-energy collision of three identical spin-polarized fermions with short-range interactions in one dimension. We derive the asymptotic expansions of the three-body wave function when the three fermions are far apart or one pair and the third fermion are far apart, and the three-body scattering hypervolume $D_F$ appears in the coefficients of such expansions. If the two-body interaction is attractive and supports two-body bound states, $D_F$ acquires a negative imaginary part related to the amplitudes of the outgoing waves describing the departure of the resultant bound pair and the remaining free fermion. For weak interaction potentials, we derive an approximate formula of the hypervolume by using the Born expansion. For the square-barrier and the square-well potentials and the Gaussian potential, we solve the three-body Schrödinger equation to compute $D_F$ numerically. We also calculate the shifts of energy and of pressure of spin-polarized one-dimensional Fermi gases due to a nonzero $D_F$ and the three-body recombination rate in one dimension.

Open systems with non-Hermitian degeneracies called exceptional points show a significantly enhanced response to perturbations in terms of large energy splittings induced by a small perturbation. This reaction can be quantified by the spectral response strength of the exceptional point. We extend the underlying theory to the general case where the dimension of the Hilbert space is larger than the order of the exceptional point. This generalization allows us to demonstrate an intriguing phenomenon: The spectral response strength of an exceptional point increases considerably and may even diverge to infinity under a parameter variation that eventually increases the order of the exceptional point. This dramatic behavior is in general not accompanied by a divergence of the energy eigenvalues and is shown to be related to the well-known divergence of Petermann factors near exceptional points. Finally, an accurate and robust numerical scheme for the computation of the spectral response strength based on the general theory and residue calculus is presented.

A beam of single-photon vortex has been detected with near-field diffraction. Our single-photon beam is generated with a weak coherent pulse method and subsequently imprinted with the orbital angular momentum of the vortex using a zeroth-order vortex half-wave retarder. The detection is employed with a two-grating Talbot near-field effect. Instead of using expensive single-photon image detection, we use a mask grating and an avalanche photodiode for the detection. This technique can identify the order and charge of the vortex from different count rates. Our technique can be utilized in the field of quantum information technology such as quantum cryptography using orthogonal states of an optical vortex beam.

Quantum Darwinism offers an explanation for the emergence of classical objective features (those we are used to at macroscopic scales) from quantum properties at the microscopic level. The interaction of a quantum system with its surroundings redundantly proliferates information to many parts of the environment, turning it accessible and objective to different observers. However, given that one cannot probe the quantum system directly, only its environment, how to determine whether an unknown quantum property can be deemed objective? Here we propose a probabilistic framework to analyze this question and show that objectivity implies a Bell-like inequality. Among several other results, we show quantum violations of this inequality, a device-independent proof of the nonobjectivity of quantum correlations. We also implement a photonic experiment where the temporal degree of freedom of photons is the quantum system of interest, while their polarization acts as the environment. Employing a fully black-box approach, we achieve the violation of a Bell-like inequality, thus certifying the nonobjectivity of the underlying quantum dynamics in a fully device-independent framework.

We study the Leggett-Garg inequality (LGI) of a two-level system (TLS) undergoing coherent dynamics described by a non-Hermitian Hamiltonian and Lindblad equation with no quantum jumps. The nonlinear Bloch equation for the TLS density matrix predicts violations of LGI above the TLS Lüders bound of $3/2$, approaching the extremal case of LGI parameter $K_3=3$ in the $\mathcal{PT}$-symmetric region and the $\mathcal{PT}$-broken region. We show that these findings are reproduced by using postselection to remove instantaneous quantum jumps from a three-level system described by the Lindblad equation with a single spontaneous emission dissipator. We trace the $K_3$ excesses beyond the standard quantum limit of $3/2$ to a nonuniform speed of evolution on the Bloch sphere. Finally, we consider the effects of competing Lindblad dissipators on the postselected non-Hermitian dynamics and the viability of observing $K_3$ exceeding the Lüder bound and approaching its algebraic maximum of three in current experimental setups.

We investigate the thermodynamic uncertainty relation (TUR), i.e., a tradeoff between entropy production rate and relative power fluctuations, for nondegenerate three-level and degenerate four-level maser heat engines. In the nondegenerate case, we consider two slightly different configurations of the three-level maser heat engine and contrast their degree of violation of the standard TUR. We associate their different TUR-violating properties to the phenomenon of spontaneous emission, which gives rise to an asymmetry between them. Furthermore, in the high-temperature limit, we show that the standard TUR relation is always violated for both configurations. For the degenerate four-level engine, we study the effects of noise-induced coherence on the TUR. We show that, depending on the parametric regime of operation, noise-induced coherence can either suppress or amplify the relative power fluctuations.

A common situation in quantum many-body physics is that the underlying theories are known but too complicated to solve efficiently. In such cases one usually builds simpler effective theories as low-energy or large-scale alternatives to the original theories. Here the central tasks are finding the optimal effective theories and proving their equivalence to the original theories. Recently quantum computing has shown the potential of solving quantum many-body systems by exploiting its inherent parallelism. It is thus an interesting topic to discuss the emergence of effective theories and design efficient tools for finding them based on the results from quantum computing. As the first step towards this direction, in this paper, we propose two approaches that apply quantum computing to find the optimal effective theory of a quantum many-body system given its full Hamiltonian. The first algorithm searches the space of effective Hamiltonians by quantum phase estimation and amplitude amplification. The second algorithm is based on a variational approach that is promising for near-future applications.

Characterizing large noisy multiparty quantum states using genuine multipartite entanglement is a challenging task. In this paper, we calculate lower bounds of genuine multipartite entanglement localized over a chosen multiparty subsystem of multiqubit stabilizer states in the noiseless and noisy scenarios. In the absence of noise, adopting a graph-based technique, we perform the calculation for arbitrary graph states as representatives of the stabilizer states and show that the graph operations required for the calculation have a polynomial scaling with the system size. As demonstrations, we compute the localized genuine multipartite entanglement over subsystems of large graphs having linear, ladder, and square structures. We also extend the calculation for graph states subjected to single-qubit Markovian or non-Markovian Pauli noise on all qubits and demonstrate, for a specific lower bound of the localizable genuine multipartite entanglement corresponding to a specific Pauli measurement setup, the existence of a critical noise strength beyond which all of the postmeasured states are biseparable. The calculation is also useful for arbitrary large stabilizer states under noise due to the local unitary connection between stabilizer states and graph states. We demonstrate this by considering a toric code defined on a square lattice and computing a lower bound of localizable genuine multipartite entanglement over a nontrivial loop of the code. Similar to the graph states, we show the existence of the critical noise strength in this case also and discuss its interesting features.

The nonadiabatic holonomic quantum computation based on three-level systems has wide applicability experimentally due to its simpler energy level structure requirement and inherent robustness from the geometric phase. However, in previous conventional schemes, the states of the calculation subspace have always leaked to the noncomputation subspace, resulting in less robustness than anticipated. Recent efforts to address this problem are at the cost of excessively long gate time, which will lead to more decoherence-induced errors. Here, we propose a solution to the problem without the severe limitation of the much longer gate time. Specifically, we implement arbitrary holonomic gates via a three-segment Hamiltonian, where the gate time depends on the rotation angle, and the smaller the rotation angle, the shorter the gate time will be. Compared with the previous solutions, our numerical simulations indicate that the decoherence-induced gate errors of our scheme are greatly decreased and the robustness of our scheme is also better, particularly for small-angle rotation gates. Moreover, we provide a detailed physical realization of our proposal on a two-dimensional superconducting quantum circuit. Therefore, our protocol provides a promising alternative for future fault-tolerant quantum computation.

Measurements are presented for the low-energy positron annihilation spectra of the alkane molecules propane through octane. The downshift of the vibrational Feshbach resonances of the infrared-active C-H stretch vibrational modes is used to obtain updated values for the positron-molecule binding energies for these molecules. These binding energies correct previously reported values which had systematic energy shifts due to several small, but significant, errors in the analyses. Once these energy shifts are corrected, the old and new measured spectra are in good agreement. This updated analysis procedure is applied to other previously measured alkanes, providing updated binding energies for these molecules. The resulting data are shown to be in good agreement with a recent model-potential calculation and solve issues regarding the interpretation of previous measurements.

Multiparticle thermal Casimir interactions are investigated, mostly in terms of the Casimir entropy, from the point of view of multiple-scattering processes. The geometry of the scattering path is depicted in detail, and the contributions from different types of channels, namely, the transverse, longitudinal, and mixing channels, are demonstrated. The geometry of the path can strongly influence the weight of each channel in the path. Negativity and nonmonotonicity are commonly seen in multiparticle Casimir entropy, the sources of which are diverse, including the geometry of the path, the types of polarization mixing, the polarizability of each particle, etc. Thermal contributions from multiparticle scatterings can be significant in the system, while the zero-temperature multiparticle scattering effects are insignificant. Limiting behaviors from a multiparticle configuration to a continuum are briefly explored.

Obtaining many excited-state solutions of the multidimensional time-independent Schrödinger equation (TISE) has turned out to be crucial for the development of quantum chemistry, computational materials science, and atomic structure theory. In this work we report a numerical method based on neural networks to solve many excited states of multidimensional TISE. The effectiveness of our approach in eigenvalue problems of TISE has been demonstrated via numerical experiments on typical analytically solvable TISE, i.e., harmonic oscillators and hydrogen-like atoms. The results indicate that introducing variance of energy as the primary loss function is capable of solving the convergence imbalance problem when multiple loss terms coexist, thereby improving the accuracy of wave functions. We propose a method to explain the physical significance of neural network wave functions, laying a foundation for neural networks in atomic structure calculations. Furthermore, we find the following features: the convergence accuracy decreases as the absolute value of the system's energy eigenvalue becomes smaller, from which we develop a scheme to prevent the decreases of the convergence accuracy. Our work offers a viable insight for further study of multielectron Schrödinger equations for atom and molecules. These findings may enrich the understanding of intrinsic mechanisms for specific physical, chemical, and biological processes.

We have investigated the energies and spectroscopic properties such as lifetimes, Landé $g_J$ factors, and hyperfine-structure constants of the neutral atoms P through Mc belonging to group 15, singly ionized atoms ${\mathrm{S}}^{+}$ through ${\mathrm{Lv}}^{+}$ of group 16, and doubly ionized atoms ${\mathrm{Cl}}^{2+}$ through ${\mathrm{Ts}}^{2+}$ of group 17 of the periodic table. These elements have $n{p}^3$ configurations with $n=3–7$, which are highly open shell and expected to exhibit strong electron-correlation effects. We have used the four-component Dirac-Coulomb Hamiltonian along with the Gaunt term and a relativistic effective core potential while employing the relativistic multireference configuration-interaction method to perform the calculations with sufficient accuracy and compare the results with the available literature data. These comparisons suggest that our predicted values, for which experimental data are not available, are reliable enough to be useful for future applications.