The probability of photon emission of a charged particle traversing a strong field becomes modified if vacuum polarization is considered. This feature is important for fundamental quantum electrodynamics processes present in extreme astrophysical environments and can be studied in a collision of a charged particle with a strong laser field. We show that for today's available 700 GeV (6.5 TeV) protons and the field provided by the next generation of lasers, the emission spectra peak is enhanced due to the vacuum polarization effect by 30% (suppressed by 65%) in comparison to the traditionally considered Compton process. This striking phenomenon offers an alternative path to the laboratory-based manifestation of vacuum polarization.

We analyze nondipole effects arising in ionization and high-order harmonic generation for a two-dimensional hydrogen atom irradiated with either low- or high-frequency laser pulses. In the low-frequency case, the electron wave packet dynamics is dominated by rescattering processes within the laser pulse. Here both odd- and even-order harmonics are generated in the direction of the laser field propagation and polarization, respectively. For high-frequency pulses, such rescattering processes can be neglected. We demonstrate that a significant portion of photoelectrons is detected opposite to the laser pulse propagation direction as a consequence of their postpulse wave packet spreading and interaction with the parent ion. This is accompanied by rich interference structures formed in the momentum distributions of photoelectrons. Our results follow from the numerical solution of the time-dependent Schrödinger equation, which is based on the Suzuki-Trotter scheme with the split-step Fourier approach. The method relies on a Hamiltonian decomposition, where except for the components depending exclusively on the momentum or on the position operators, there are also terms depending on both momentum and position operators in particular configurations. We demonstrate that, as long as the latter does not depend on noncommuting coordinates of the momentum and position operators, nondipole effects in laser-matter interactions can be studied without applying extra approximations and unitary operations.

The isolation and control of disparate degrees of freedom underpins quantum simulators. We advance the programmability of cold atom quantum simulators with a realization of the dynamic coupling of spatial and spin degrees of freedom. We experimentally demonstrate that violent spatial evolutions tune long-lived coherent spin dynamics and develop a model of quantum spin mixing incorporating the spatial evolution via time-dependent spin-spin interactions. Our results may open new paths towards the simulation of quantum spin models with tunable interactions via tailored spatial dynamics.

Dynamical characterization of topological phases under quantum quench dynamics has been demonstrated as a powerful and efficient tool. Previous studies have focused on systems of which the Hamiltonian consists of matrices that commute with each other and satisfy Clifford algebra. In this work we consider the characterization of topological phases with Hamiltonians that are beyond the minimal model. Specifically, the quantum quench dynamics of two types of layered systems is studied, which consist of matrices of Hamiltonians that do not all satisfy Clifford algebra. We find that the terms which anticommute with others can hold common band-inversion surfaces, which controls the topology of all the bands, but for other terms, there is no universal behavior and they need to be treated case by case.

This work investigates the effect of chirality-induced spin selectivity (CISS) on quantum coherence in the radical-pair (RP) mechanism of avian magnetoreception. Additionally, we examine the correlation of global and local coherence measures with the yield of the signaling state in the RP model. We find that both relative entropy of global coherence and local coherence in the radical pair increase with CISS. However, only global coherence shows a strong correlation with the signaling state yield and thus indicates a plausible utilitarian role for the avian compass. We also analyze the interplay of dipolar and exchange interaction with the CISS and their effect on the coherence of the radical-pair spin. Further, we analyze the effect of environmental decoherence along with CISS. We conclude that a high CISS results in a high correlation of global coherence with signaling state yield. We propose that CISS might play an important role in developing quantum technologies by sustaining coherence in radical-pair-like quantum systems.

In this work, we study the thermal quantum coherence and fidelity in a two-level system with spin-orbit coupling. The proposed model involves a single electron in a double quantum dot with Rashba spin-orbit coupling in the presence of an external magnetic field. In our scenario, the thermal entanglement of the single electron is driven by the charge and spin qubits, the latter controlled by Rashba coupling. Analytical expressions are obtained for thermal concurrence and correlated coherence using the density matrix formalism. The main goal of this work is to provide a good understanding of the effects of temperature and several parameters in quantum coherence. In addition, our findings show that we can use the Rashba coupling to tune the thermal entanglement, quantum coherence, and the thermal fidelity behavior of the system. Moreover, we focus on the role played by thermal entanglement and correlated coherence responsible for quantum correlations. We observe that the correlated coherence is more robust than the thermal entanglement in all cases, so quantum algorithms based only on correlated coherence may be stronger than those based on entanglement.

We derive STIRAP (stimulated Raman adiabatic passage) shortcuts to adiabaticity maximizing population transfer in a three-level $\mathrm{\Lambda }$ quantum system, using spin-to-spring mapping to formulate the corresponding optimal control problem on the simpler system of a classical driven dissipative harmonic oscillator. We solve the spring optimal control problem and obtain analytical expressions for the impulses, the durations of the zero control intervals, and the singular control, which are the elements composing the optimal pulse sequence. We also derive suboptimal solutions for the spring problem, one with less impulses than the optimal and others with smoother polynomial controls. We then apply the solutions derived for the spring system to the original system, and compare the population transfer efficiency with that obtained for the original system using numerical optimal control. For all dissipation rates used, the efficiency of the optimal spring control approaches that of the numerical optimal solution for longer durations, with the approach accomplished earlier for smaller decay rates. The efficiency achieved with the suboptimal spring control with less impulses is very close to that of the optimal spring control in all cases, while that obtained with polynomial controls lies below, and this is the price paid for not using impulses, which can quickly build a nonzero population in the intermediate state. The analysis of the optimal solution for the classical driven dissipative oscillator is not restricted to the system at hand but can also be applied in the transport of a coherent state trapped in a moving harmonic potential and the transport of a mesoscopic object in stochastic thermodynamics.

Detachment of electrons from negative ions by the ultrashort few-cycle laser pulses in the near-infrared spectral region is investigated using a theory based on the strong-field approximation. The differential detachment probability is computed by the numerical integration or by using the saddle-point method. The symmetry properties of the photoelectron momentum distribution are analyzed for the elliptically polarized as well as for the bi-elliptical driving field with the sine-square pulse envelope and compared with those obtained using a long pulse with a flat envelope. We show that the differential detachment probability depends, to a significant extent, on the polarization state of the driving light. For the direct electrons, i.e., the electrons which, after the detachment, do not interact with the parent atom, this is not the case if the process is induced by a long pulse with a flat envelope. In addition to the polarization state, the photoelectron momentum distribution also depends on the value of the absolute phase. We show that the qualitative and quantitative characteristics of the momentum distribution can accurately be controlled using this parameter as a control knob. This control is particularly successful in the regions where the rescattered electrons are dominant due to the fact that they stay longer in the applied laser field.

We demonstrate that the $4f^{14}6s6p\left({}^{3}P_2\right)-4f^{13}5d6s^2\left({}^{3}P_2^{*}\right)$ transition in neutral ytterbium (Yb) can serve as an additional clock transition with the highest fine-structure constant (${\alpha }_e$) varying sensitivity coefficient $[q=-46165(3000\left)\right]$. We demonstrate a scheme to attain simultaneous magic trapping conditions for this clock transition with other two proposed clock transitions $4f^{14}6s^2\left({}^{1}S_0\right)-{}^{3}P_2$ and ${}^{1}S_0-{}^{3}P_2^{*}$, which also possess very large $q$ values. These conditions can be realized by subjecting Yb atoms to a bias magnetic field with tuning the particular polarization angles of the trapping laser. Upon realization, it can serve as the most potential optical lattice clock to probe ${\alpha }_e$ variation.

We introduce a method for optically detected magnetic resonance for atomlike systems in an optical resonator. Driving the spin transitions of these systems with microwaves causes changes to the populations of the ground-state spin levels, which we detect by changes in the optical cavity frequency. The technique is useful for characterizing experiments aimed at quantum microwave-to-optical transduction because it provides a way of only probing the spin transitions of the atomlike systems that are in the optical resonator's mode. We demonstrate the technique using a cryogenic erbium-doped whispering-gallery-mode resonator inside a microwave resonator. We compare our results with more standard electron paramagnetic resonance to show that our optical modes are confined to a region of large microwave magnetic-field amplitude. Our optical modes have a $Q$ factor better than ${10}^8$ making them the highest $Q$-factor resonators studied with cryogenic rare-earth-ion dopants, allowing us to report ensemble strong coupling between the erbium dopants and an optical whispering-gallery-mode resonator.

Using electric dipoles to describe light-matter interactions between two entities is a conventional approximation in physics, chemistry, and materials science. However, the lack of material structures makes the approximation inadequate when the size of an entity is comparable to the spatial extent of electromagnetic fields or the distance of two entities. In this study, we develop a unified theory of radiative and nonradiative resonance energy transfer based on transition current density in a theoretical framework of macroscopic quantum electrodynamics. The proposed theory allows us to describe polariton-assisted resonance energy transfer between two entities with arbitrary material structures in spatially dependent electromagnetic fields. To demonstrate the generality of the proposed theory, we rigorously prove that our theory can cover the main results of the transition density cube method and the plasmon-coupled resonance energy transfer. We believe that this study opens a promising direction for exploring light-matter interactions beyond the scope of electric dipoles and provides insights into materials physics.

The design of detection strategies for multipartite entanglement stands as a central importance on our understanding of fundamental quantum mechanics and has had a substantial impact on quantum information applications. However, accurate and robust detection approaches are severely hindered, particularly when the number of nodes grows rapidly like in a quantum network. Here we present a general and operational framework that generates alternative entanglement witness for an arbitrary targeted state. The framework enjoys a systematic and high-efficient character and allows to substantiate genuine multipartite entanglement for a variety of states that arise naturally in practical situations and to dramatically outperform currently standard methods. With excellent noise tolerance, our framework should be broadly applicable to witness genuine multipartite entanglement in various practically scenarios and to facilitate the best use of entangled resources in the emerging area of quantum network.

We demonstrate resonant detection of rf electric fields from 240 to 900 MHz (very high frequency to ultrahigh frequency) using electromagnetically induced transparency to measure orbital angular momentum $L=3\to L^{\prime }=4$ Rydberg transitions. These Rydberg states are accessible with three-photon infrared optical excitation. By resonantly detecting rf in the electrically small regime, these states enable a new class of atomic receivers. We find good agreement between measured spectra and predictions of quantum defect theory for principal quantum numbers $n=45$ to 70. Using a superhetrodyne detection setup, we measure the noise floor at $n=50$ to be $13\mu \mathrm{V}/\left(\mathrm{m}\sqrt{\mathrm{Hz}}\right)$. Additionally, we utilize data and a numerical model incorporating a five-level master equation solution to estimate the fundamental sensitivity limits of our system.

Features of the radiation spectra are investigated with the quantum electrodynamic theory for the nonlinear scattering interaction between relativistic electron and few-cycle linearly polarized laser pulse. The angle between the moving direction of the electron and that of the laser pulse affects the scattered radiation characteristics. When the electron has a head-on collision with the laser pulse having a carrier envelope phase as zero, the angular distribution of the scattered radiation spectrum is symmetrical with respect to the laser pulse propagation direction. Such a symmetry disappears when the electron collides with the laser obliquely and the direction of most of the radiation energy shifts at an acute angle towards the electron direction. The CEP has significant effects on the spectral angular distribution. The CEP influence is determined by the exertion of the electromagnetic field on the motion of the electron, whose trajectory overlap in phase space leads to the interference between scattered radiation in different intervals of the interaction process. The supercontinuum radiation could be produced under certain CEPs. An external electrostatic field applied as the environment for the laser-electron interaction is another important factor affecting the scattered radiation features by modulating the harmonic components and their angular distribution. This study may help the analysis of the spectral data from relativistic laser-solid target experiments.

We derive the path-length distribution (PLD) of light rays diffusely incident on a refractive sphere and slab in the geometric optics case. Refraction affects the width of the distribution and internal reflection introduces longer path lengths. Polarization is also taken into account and has a minor effect on the shape. For the slab, we also consider the effect of a small amount of scattering in the medium which adds to the tail of the PLD and significantly increases the mean path length. The absorption is calculated from the PLD, from which we approximate the absorption in the slab and sphere to the second order in the absorption coefficient, and also to the second order in the scattering coefficient for the slab.

We investigate the nontrivial characteristics of modulational instability (MI) in a system of Bragg gratings with saturable nonlinearity. We also introduce an equal amount of gain and loss into the existing system, which gives rise to an additional degree of freedom, due to the concept of $\mathcal{PT}$ symmetry. We obtain the nonlinear dispersion relation of the saturable model and discover that such dispersion relations for both the conventional and $\mathcal{PT}$-symmetric cases contradict the conventional Kerr and saturable systems by not displaying the typical signature of loop formation in either the upper branch or lower branch of the curve drawn against the wave number and detuning parameter. We employ a standard linear stability analysis to study the MI dynamics of the continuous waves perturbed by an infinitesimal perturbation. The main objective of this paper is twofold: We investigate the dynamics of the MI gain spectrum at the top and bottom of the photonic band gap followed by a comprehensive analysis carried out in the anomalous and normal dispersion regimes. As a result, this perturbed system driven by the saturable nonlinearity and gain or loss yields a variety of instability spectra, which include the conventional sidebands, monotonically increasing gain, the emergence of a single spectrum in either of the Stokes wave-number regions, and so on. In particular, we observe a remarkably peculiar spectrum, which is caused predominantly by the system parameter, though the perturbation wave number boosts the former. We also address the impact of all the physical parameters considered in the proposed model, which include the coupling coefficient, dispersion parameter, and saturable nonlinearity on the phenomenon of MI for different $\mathcal{PT}$-symmetric regimes ranging from the unbroken one to the broken one in greater detail.

The introduction of higher-order topological insulators featuring topological corner states has been an extremely active topic for the generation of confined modes. Recent studies show that corner states are not an exclusive feature of topological insulators but can also exist as bound states in the continuum (BIC). This suggests that corner states may exist in different topological phases that exhibit a higher-order character, beyond the conventional bulk-edge correspondence. In this paper, we show that corner states can appear in a higher-order Dirac semimetal phase of a two-dimensional photonic lattice. Such a lattice displays a dispersion relationship with a fourfold degeneracy, which serves to induce a higher-order topological phase transition, leaving a Dirac point between two edge bands. Just as Dirac modes exist in regular lattice defects, the nonconventional bulk-edge-corner correspondence leads to corner states in the Dirac semimetal phase. Notably, the induced corner state exhibits an algebraic decay characteristic of long-range interactions, predicted by the massless Dirac equation. The analysis of the field profile and quality factors indicates that the state is not a BIC, but rather a corner-Dirac state. We show that if the photonic lattice does not present a higher-order phase transition the existence of the corner-Dirac state does not arise. The results obtained in this paper indicate that corner states can exist throughout the entire topological phase transition process, even in the semimetal phase, and pave the way to generate light-matter interactions with such a photonic bath.

As an important tool in science and industry, the ultrafast fiber laser has two paradigmatic pulse regimes of the single-soliton (SS) and the soliton molecule (SM), which have been individually researched thoroughly. However, the mutual transformation between two states is a fuzzy problem with significance in understanding the soliton dynamics and actively manipulating lasing states. Here, we reveal an intermediate state that spontaneously switches between SS and SM without external triggering in experiments and simulations. Two critical processes dominating the switching are the soliton splitting and SM partial annihilation, which are found to be induced by the high-order soliton effect and spontaneous symmetry breaking, respectively. Moreover, statistics of this intermediate state reveal the spectral rogue waves induced by the SM with high spectral brightness. This work sheds light on the switching dynamics between nonlinear states in the out-of-equilibrium systems, and has potential for intelligent manipulation of lasing states.

Optical solitons are known to be classically stable objects which are robust to perturbations. In this work we show that due to quantum-mechanical effects, an optical soliton that is initially in a classical soliton coherent state will shed photons into the continuum and hence decay. The standard formulation of the quantized soliton uses the linearized version of the quantum nonlinear Schrödinger equation in the background of the classical soliton, and the quantized soliton remains stable in this approximation. We show that if higher-order interaction terms are taken into account, the soliton is no longer stable, and its photon number decreases quadratically as a function of the number of soliton cycles. We compute the power spectrum for the continuum radiation and find a narrow band that is localized about the initial soliton momentum with a cutoff that is inversely proportional to the initial soliton width.

Magnonic frequency combs, a direct analog of optical frequency combs in the field of spin waves, have recently received considerable attentions due to their potential applications in high-precision magnonic frequency metrology. However, due to the weak nonlinear interaction of magnons, it is difficult to generate a wide-bandwidth magnonic frequency comb under a low power drive. Here, we present an efficient mechanism for the generation of a wide-bandwidth magnonic frequency comb via a two-tone microwave driving in a magnomechanical system. Numerical simulations show that the magnetostrictive effect can be greatly enhanced by the beat frequency signal from the two-tone microwave driving field, and a robust magnonic frequency comb can be observed at low power. Furthermore, abundant nonperturbative features appear in the magnonic spectrum, implying that the magnons as bosons may also be similar to photons in atom-molecular systems. Our scheme thus provides a pathway for the generation of a flat magnonic frequency comb under a low-power driving condition that may be beneficial for precision metrology based on magnonic platforms as well as the understanding of nonlinear magnomechanic dynamics.