We consider solitary wave excitations above the ground state of $F=1$ spin-orbit-coupled Bose-Einstein condensates (SOBECs). The low-energy properties of SOBECs in any of the three branches of the single-particle dispersion relation can be described by suitable scalar nonlinear Schrödinger (NLS) equations which we obtain using multiple-scale expansions. This enables us to examine a variety of different configurations, such as dark solitary waves associated with higher energy branches, as well as dark and bright structures in the lowest branch. The lowest branch can also exhibit a “superstripe” phase that supports solitary waves. In all cases, we provide explicit expressions for the NLS coefficients, and confirm their validity with full numerical simulations of the SOBEC system, including a harmonic confining potential.

Optical tweezers use light from a tightly focused laser beam to manipulate the motion of tiny particles. When the laser light is strongly focused its electromagnetic field is characterized by a longitudinal component which is of a magnitude comparable to the transverse ones and which has been ignored in the theoretical analysis of the tweezing forces. In our paper we revise the calculations of the various components of the radiation pressure force, within the limits of the Rayleigh regime or dipole approximation, in the case where a tiny particle interacts, in free space, with a circularly polarized optical vortex beam, by taking into account this ignored field term. We show that this term is responsible for considerable modifications in the magnitude of the various components, and, moreover, also for the appearance of terms involving the coupling of the spin angular momentum and the orbital angular momentum of the photons of the vortex beam. We compare our findings with the ones taken ignoring the longitudinal field component and the wave-vector magnitude reduction.

Parity-time $\left(\mathcal{PT}\right)$ symmetry and chirality are both actively investigated in photonics due to the original behaviors they provide. We combine $\mathcal{PT}$ symmetry and chirality in a single photonic structure by inserting a chiral material in the narrow gap between $\mathcal{PT}$-symmetric coupled waveguides. We analyze the various effects of chiral coupling between the modes, especially in the vicinity of an exceptional point. By tuning the waveguide gap we tailor the chiral coupling between non-Hermitian modes with different polarizations, which would otherwise not interact. As a result, a rich variety of qualitatively differing dispersions is achieved, from typical anticrossings to symmetry-broken and associated symmetry-recovery zones, as well as a hybrid trimodal anticrossing. Furthermore, the slot effect in the gap leads to a very strong chiral coupling, reaching bulk sensitivity values near an exceptional point, which could be useful for sensing purposes. We employ a modified two-dimensional finite-element approach to include chirality in the simulations. Additionally, we propose a compact coupled-mode theory that elucidates the physics at play and provides opportunities for the study of more complex devices.

Quantum steering plays a key role for quantum technologies, such as one-way quantum computation and quantum communication. Here, we consider a vector cavity optomechanical scheme to create quantum steering between two orthogonal polarized lights and a mechanical vibration, where cavity modes are driven by short laser pulses. By controlling the polarization behavior of a light field, we not only manipulate the steering of the vibration via polarized light, but also reversibly switch between ordinary tripartite steering and collective steering. In addition, we find that bipartite steering is more sensitive to thermal noise than tripartite steering. As a result, collective steering can be achieved within a larger parameter range. Controllable quantum steering with such a vectorial device may provide effective resources to build up steered networks.

The ground state of the quantum Rabi model (QRM) exhibits rich nonclassical states including the squeezed state, cat state, and entangled state in different parameter regimes. In this paper, we first use the polaron picture with high precision and fidelity to figure out the process of the superradiant phase transition (SPT) in view of the nonclassical states. The squeezing effect is considered to be a result of the higher-order excitations and important in order to understand the fundamental quantum physics of the SPT in the QRM. According to the polaron wave functions, the squeezed state extends across the entire range of parameters. Based on the squeezing and superradiance, a classification of the coupling strength for the nonclassical states containing the ground state is presented; the system sequentially goes through the vacuum states, the squeezed vacuum states, the squeezed cat states, and the cat states as the coupling strength increases. The process of the SPT is also demonstrated by the photon number distribution in Fock space, and then the novel behavior of parity chains for the SPT is revealed. Additionally, the nonclassical states can be intuitively characterized by the Wigner functions with analytical expressions in the polaron picture, and the Wigner negativity is evaluated to measure the nonclassicality. We find that a small but nonzero weighted antipolaron is necessary to form novel semicat states. Our paper provides guidance for preparing nonclassical states in experiments based on the light-matter interaction.

One of the most remarkable features of quantum physics is that attributes of quantum objects, such as the wavelike and particlelike behaviors of single photons, can be complementary in the sense that they are equally real but cannot be observed simultaneously. Quantum measurements, serving as windows providing views into the abstract edifice of quantum theory, are basic tools for manifesting the intrinsic behaviors of quantum objects. However, a quantitative formulation of complementarity that highlights its manifestations in general measurement scenarios remains elusive. Here we develop a general framework for demonstrating quantum complementarity in the form of information exclusion relations (IERs), which incorporates the wave-particle duality relations as particular examples. Moreover, we explore the applications of our theory in entanglement detection and elucidate that our IERs lead to an extended form of entropic uncertainty relations, providing insights into the connection between quantum complementarity and the preparation uncertainty.

Edge burst is a phenomenon in non-Hermitian quantum dynamics discovered by a recent numerical study [Phys. Rev. Lett. 128, 120401 (2022)]. It finds that a large proportion of particle loss occurs at the system boundary in a class of non-Hermitian quantum walk. In this paper, we investigate the evolution of real-space wave functions for this lattice system. We find the wave function of the edge site is distinct from the bulk sites. Using time-dependent perturbation theory, we derive the analytical expression of the real-space wave functions and find that the different evolution behaviors between the edge and bulk sites are due to their different nearest-neighbor site configurations. We also find the edge wave function primarily results from the transition of the two nearest-neighbor nondecay sites. Besides, the numerical diagonalization shows the edge wave function is mainly propagated by a group of eigenmodes with a relatively large imaginary part. Our work provides an analytical method for studying non-Hermitian quantum dynamical problems.

This paper addresses the challenge of modeling relaxation dynamics in quantum many-body systems, specifically focusing on electrons in graphene nanoflakes. While quantum many-body techniques effectively describe dynamics up to a few particles, these approaches become computationally intractable for large systems. Larger systems may be tackled with a single-particle approach that, however, struggles to incorporate relaxation effects. Existing relaxation models encounter issues such as an inability to capture system complexity and violation of the Pauli principle. In this paper, we propose a single-particle model that accounts for various relaxation effects at the crossroads of quantum optics and solid-state photonics, that overcomes the limitations of previous models. Our approach is rooted in the quantum-optical Lindblad model, where relaxation rates are deactivated once the target levels saturate due to the Pauli principle. This approach is referred to as the saturated-Lindblad model. To validate the predictions of the saturated-Lindblad model, we confront them against phenomenological and many-body physics models in low-dimensional systems, including atomic chains and graphene nanoflakes. Remarkably, the saturated-Lindblad model exhibits excellent agreement with few-body calculations, distinguishing itself from other existing approaches. Moreover, by assigning different relaxation rates to different transitions, we successfully reproduce cascade deexcitation dynamics and predict emission spectra. The saturated-Lindblad model offers the ability to describe dynamics in systems of practical sizes, encompassing a wide range of structures that can be effectively captured within the single-particle description.

Many quantum paradoxes based on a realistic view of weak values were discussed in the last decades. They lead to astonishing conclusions such as the measurement of a spin component of a spin-1/2 particle resulting in $100\hslash$, the separation of a photon from its polarization, and the possibility of having three particles in two boxes without any two particles being in the same box, among others. Here, we show that the realistic view of the weak values present in these (and other) works is equivalent to a realistic (and highly controversial) view of quantum measurements, where a measurement reveals the underlying reality of the measured quantity. We discuss that all quantum paradoxes based on weak values simply disappear if we deny these realistic views of quantum measurements and weak values. Our work thus aims to demonstrate the strong assumptions and the corresponding problems present in the interpretation of these quantum paradoxes.

The core of Heisenberg's heuristic argument for the uncertainty principle, involving the famous $\gamma$-ray microscope Gedankenexperiment, hinges upon the existence of measurements that irreversibly alter the state of the system on which they are acting, causing an irreducible disturbance on subsequent measurements. The argument was put forward to justify measurement incompatibility in quantum theory, namely, the existence of measurements that cannot be performed jointly—a feature that is now understood to be different from irreversibility of measurement disturbance, though related to it. In this article, on the one hand, we provide a compelling argument showing that measurement incompatibility is indeed a sufficient condition for irreversibility of measurement disturbance, while, on the other hand, we exhibit a toy theory, termed the minimal classical theory (MCT), that is a counterexample for the converse implication. This theory is classical, hence it does not have complementarity nor preparation uncertainty relations, and it is both Kochen-Specker and generalized noncontextual. However, MCT satisfies not only irreversibility of measurement disturbance, but also the properties of no-information without disturbance and no-broadcasting, implying that these cannot be understood per se as signatures of nonclassicality.

Many calculations in strong-field quantum field theory are carried out by using a simple field geometry, often neglecting the spatial field envelope. In this paper, we simulate the electron-diffraction quantum dynamics of the Kapitza-Dirac effect in a Gaussian beam standing light wave. The two-dimensional simulation is computed in a relativistic framework, by solving the Dirac equation with the fast Fourier-transform split operator method. Except the numerical propagation method, our results are obtained without applying approximations and demonstrate that a spin flip in the Kapitza-Dirac effect is possible. We further discuss properties such as the validity of a plane-wave approach for the theoretical description, the influence of the longitudinal polarization component due to laser beam focusing, and higher-order diffraction peaks in Kapitza-Dirac scattering.

We propose a feasible scheme to implement a one-dimensional non-Hermitian Su-Schrieffer-Heeger model with long-range hopping using the electrical circuit system. We investigate the admittance spectrum and node voltage density distribution of the current system. The non-Bloch $\mathcal{PT}$ symmetry and its breaking can be effectively demonstrated using the saddle point theory and the ratio of complex eigenenergy. It is important to note that the phase boundary of non-Bloch $\mathcal{PT}$ symmetry is solely determined by the long-range hopping strengths. Moreover, we observe that the system exhibits zero-admittance topological end and gap modes with varying strength relations between intercell and long-range hopping. We further represent the winding number phase diagram, which reveals the distinct topological phase transition from $w=\pm 1$ to $w=0$. Our work provides a method to simulate nonreciprocal topolectrical circuits and contributes to understanding the interplay between topology and non-Hermiticity.

The quantum version of Kramers turnover theory is generalized beyond the parabolic barrier approximation. The result is a uniform instanton-based quantum Kramers turnover theory that does not display any divergence at what is known as the crossover temperature. The theory is analyzed using a model of a particle trapped in a cubic potential. As the temperature is lowered, the maximum in the Kramers turnover curve moves to lower friction values. When the temperature is sufficiently low, the quantum rate at low friction becomes almost independent of the friction strength.

By exploiting the complexity intrinsic to quantum dynamics, quantum technologies promise a host of computational advantages. One such advantage lies in the field of stochastic modeling, where it has been shown that quantum stochastic simulators can operate with a lower memory overhead than their best classical counterparts. This advantage is particularly pronounced for continuous-time stochastic processes; however, the corresponding quantum stochastic simulators heretofore prescribed operate only on a quasi-continuous-time basis and suffer an ever-increasing circuit complexity with increasing temporal resolution. Here, by establishing a correspondence with quantum trajectories (a method for modeling open quantum systems), we show how truly continuous-time quantum stochastic simulators can be embedded in such open quantum systems, bridging this gap and obviating previous constraints. We further show how such an embedding can be made for discrete-time stochastic processes, which manifest as jump-only trajectories, and discuss how viewing the correspondence in the reverse direction provides a means of studying structural complexity in quantum systems themselves.

We consider violations of a Clauser-Horne-Shimony-Holt-type steering inequality for quantum states of systems of indistinguishable particles restricted by a particle-number superselection rule. We check for violations in noninteracting Bose-Einstein condensate states, NOON states, and relative phase eigenstates, by using two copies of the states for bypassing the superselection rule. The superselection rule prevents the states from maximally violating the steering inequality, but the steering inequality violations are higher than Bell inequality violations for the same states. This in particular implies, in certain cases, that visibilities of the steering inequality violations are higher than the same for Bell inequality violations, for admixtures with white noise. We also found that an increase in the number of particles in the noninteracting condensate states results in a decrease of the violation amount of the steering inequality.

An exactly solvable Kitaev model in a two-dimensional square lattice exhibits a topological quantum phase transition which is different from the symmetry-breaking transition at zero temperature. When the ground state of a nonlinearly perturbed Kitaev model with different strengths of perturbation taken as the initial state is quenched to a pure Kitaev model, we demonstrate that various features of the dynamical state, such as the Loschmidt echo and time-averaged multipartite entanglement, can determine whether the initial state belongs to the topological phase or not. Moreover, the derivatives of the dynamical quantifiers can faithfully identify the topological quantum phase transition, which is present at equilibrium. When the individual qubits of the lattice interact with the local thermal bath repeatedly, we observe that block entanglement in dissipative dynamics can, nevertheless, distinguish the equilibrium phases from which the system starts evolution.