Counterpropagating (CP) solitons generated in high-Q microcavities not only offer useful dual-comb sources, but also provide a new platform to study soliton interactions. Here, we demonstrate and theoretically explain a manifestation of soliton trapping that occurs between CP solitons in a silica microcavity introducing a Kerr soliton dimer. In conventional soliton trapping, the group velocities of two solitons can be synchronized by a Kerr-effect-mediated interaction. The solitons can then copropagate with a fixed temporal delay. However, as shown here, when counterpumping a microcavity using slightly detuned pump frequencies and in the presence of backscattering, the group velocities of clockwise and counterclockwise solitons undergo periodic modulation instead of being locked to a constant velocity. Upon emission from the microcavity, the solitons feature a relative oscillatory motion around a locked average relative displacement with an amplitude that can be larger than the soliton pulse width. This relative motion introduces a sideband fine structure into the optical spectrum of the CP solitons. Our observation provides insights on coherently pumped soliton dimers in microcavities.

We show that the threshold condition for two pump photons to convert into a pair of the sideband ones in Kerr microresonators with high-quality factors breaks the pump laser parameter space into a sequence of narrow in frequency and broad in power Arnold tongues. Instability tongues become a dominant feature in resonators with the finesse dispersion parameter close to and above one. As pump power is increased, the tongues expand and cross by forming a line of cusps, i.e., the threshold of complexity, where more sideband pairs become unstable. We elaborate theory for the tongues and threshold of complexity, and report the synchronisation and frequency-domain symmetry breaking effects inside the tongues.

We study the simulation of the topological phases in three subsequent dimensions with quantum walks. We focus mainly on the completion of a table for the protocols of the quantum walk that could simulate different families of the topological phases in one, two, and three dimensions. We also highlight the possible boundary states that can be observed for each protocol in different dimensions and extract the conditions for their emergences. To further enrich the simulation of the topological phenomena, we include step-dependent coins in the evolution operators of the quantum walks. This leads to step dependence of the simulated topological phenomena and their properties which introduces dynamicity as a feature of simulated topological phases and boundary states. This dynamicity provides the step number of the quantum walk as a means to control and engineer the numbers of topological phases and boundary states, their numbers, types, and even occurrences.

Recently various ways to quantify coherence of a quantum state have been proposed. In this paper, several reliable quantum coherence measures based on quantum Fisher information (QFI) are presented. For pure states, this coherence measure is equivalent to that based on skew information, while for mixed states, we first define two natural coherence measures via QFI and find that the two definitions are exactly identical. By virtual of the special structure of qubit, we show that the mutual unbiased bases' (MUBs) qubit coherence is constrained only by the length of the Bloch vector. Then we provide upper bounds of the coherence measure with respect to any given basis and lower bounds in the case of a set of MUBs for both pure states and mixed states of arbitrary dimension. Finally, we investigate the connection between our coherence measures and quantum metrology, thereby providing an operational meaning for these coherence measures.

Fault-tolerant implementation of quantum gates is one of preconditions for realizing quantum computation. The platform of Rydberg atoms is one of the most promising candidates for achieving quantum computation. We propose to implement a controlled-$Z$ gate on Rydberg atoms where an amplitude-modulated field is employed to induce Rydberg antiblockade. Gate robustness against the fluctuations in the Rydberg-Rydberg interaction can be largely enhanced by adjusting amplitude-modulated field. Furthermore, we introduce a Landau-Zener-Stückelberg transition on the target atom so as to improve the gate resilience to the deviation in the gate time and the drift in the pulse amplitude. With feasible experimental parameters, one can achieve the gate with low fidelity errors caused by atomic decay, interatomic dipole-dipole force, and Doppler effects. Finally, we generalize the gate scheme into multiqubit cases, where resilient multiqubit phase gates can be obtained in one step with an unchanged gate time as the number of qubits increases.

We present a study of higher-order Raman ($\mathrm{\Lambda }, V$, and $S$) transitions between Rydberg states involving the absorption and emission of at least three photons in the interaction of two-color corotating circularly polarized laser pulses with the hydrogen atom. Our analysis is based on results of numerical solutions of the corresponding time-dependent Schrödinger equation. The results for the interaction with ($\omega ,2\omega$) fields show that the simultaneous interaction with both fields results in an excited state distribution over a broad range of magnetic quantum numbers. Indications of the impact of the $\mathrm{\Lambda }, V$, and $S$ transitions are found via the analysis of final distributions resulting from the preparation of the atom in specific Rydberg states. Furthermore, we show that similar mechanisms for the redistribution of population between Rydberg states are present in two-color fields with larger differences in the central frequencies, i.e., ($\omega ,3\omega$) and ($\omega ,4\omega$) fields.

An exciton-polariton condensate in semiconductor microcavities provides a novel kind of nonequilibrium superfluid. Recently, an experiment [P. Stepanov et al., Nat. Commun. 10, 3869 (2019)] aimed at understanding the role of the reservoir in determining the superfluidity of a polariton quantum fluid was carried out based on a polariton condensate under resonant pumping. Inspired by it, here we investigate the superfluid behavior of an exciton-polariton condensate under nonresonant pumping by studying the drag force exerted on a classical impurity moving therein. Focusing on the gapped phase of the condensate, we identify the respective contributions of the condensate mode and the reservoir mode to the drag force. In the regime of a low impurity velocity, we find a strong drag force that is mainly contributed by the diffusive reservoir modes, due to their slow diffusive constant at low momenta. When the velocity increases toward the sound velocity, the drag force is reduced as a result of a decreased effect of the reservoir mode. For high velocities, the drag force is dominantly contributed by the gapped condensate modes, showing a linear increase with the velocity similarly to that of its equilibrium counterpart. When the condensate approaches the turning point between the gapped and the gapless phases, the drag force exhibits behavior similar to that of an equilibrium superfluid. Our findings reveal how the reservoir modes affect the superfluidity of a polariton condensate under nonresonant pumping.

A comprehensive analysis of the dynamics of two Rydberg-dressed particles (bosons or fermions) tunneling from a potential well into open space is provided. We show that the dominant decay mechanism switches from sequential tunneling to pair tunneling when the interaction strength is tuned below a certain critical value. These critical values can be modified by tuning the effective range of the interaction potential. By comparing the dynamics of bosons and fermions, we show that there are significant differences between the two cases. In particular, increasing the interaction range modifies the tunneling rate in opposite ways for fermions and bosons. Furthermore, for the fermionic system much stronger attractive interactions are needed to achieve pair tunneling. The results provide insight into the dynamics of tunneling systems and, in light of recent realizations of tunneling few-body systems and Rydberg dressing of atoms, they offer promise for future experiments.

A comb spectrum generating device based on Bragg grating superstructures with gain and loss is suggested in this paper. The paper includes a comprehensive analysis of the device formulation and generation and manipulation of the comb spectrum with a number of degrees of freedom such as duty cycle, sampling period, and gain-loss parameter. For applications such as RF traversal filters and tunable multiwavelength laser sources, the reflected intensities of the comb resulting from the superstructures should have uniform intensities, and this is guaranteed by optimizing the physical length of the device and gain and loss in the unbroken $\mathcal{PT}$-symmetric regime. Alternatively, it can be accomplished by reducing the duty cycle ratio of the superstructure to extremely small values in the broken $\mathcal{PT}$-symmetric regime. Such a customization will degrade the reflectivity of the conventional grating superstructures, while it gives rise to narrow spectral lines with high reflectivity in the proposed system. Remarkably, combs with an inverted envelope are generated for larger values of gain and loss.

We investigate nonanalyticities (e.g., zeros and poles) of refractive index $n\left(\omega \right)$ and group index $n_g\left(\omega \right)$ in different optical media. We first demonstrate that while a Lorentzian dielectric has no nonanalyticity in the upper half of the complex frequency plane (CFP), its group index—which governs the pulse-center propagation—possesses zeros in the same region. Thus we classify the nonanalyticities as in the (a) first order (refractive index or reflection) and (b) second order (group index or group delay). The latter contains the derivative of the former. Then, we study a possible connection between the negative superluminal velocities and the presence of nonanalyticities in the upper half of the CFP. We show that the presence of nonanalyticities in the upper half of the CFP for (a) the first-order and (b) the second-order responses are accompanied by the appearance of negative (a) phase and (b) group velocities, respectively. We also distinguish between two kinds of superluminosity, $v>c$ and $v<0$, where we show that the second one ($v<0$) appears with the violation of KKRs.

The principal use of photonic crystals is to engineer the photonic density of states, which controls light-matter coupling. We theoretically show that strained two-dimensional photonic crystals can generate artificial electric and magnetic fields that act on light and we show that particular strain patterns give rise to highly degenerate Landau levels. Since photonic crystals are not in general described by tight-binding models, we employ a multiscale expansion of the full continuum wave equation. Using numerical simulations, we observe dispersive Landau levels which we show can be flattened by engineering a pseudoelectric field. Artificial fields yield a design principle for aperiodic nanophotonic systems.

The role of the spatiotemporal degrees of freedom in the preparation and observation of squeezed photonic states, produced by parametric down-conversion, is investigated. The analysis is done with the aid of a functional approach under the semiclassical approximation and the thin-crystal approximation. It is found that the squeezed state loses its minimum uncertainty property as the efficiency of down-conversion is increased, depending on the conditions of the homodyne measurements with which the amount of squeezing is determined.

The interaction of matter with quantum light leads to phenomena which cannot be explained by semiclassical approaches. Of particular interest are states with broad photon-number distributions which allow processes with high-order Fock states. Here, we analyze a Jaynes-Cummings–type model with three electronic levels which is excited by quantum light. As quantum light we consider coherent and squeezed states. In our simulations we include several loss mechanisms, namely, dephasing, cavity, and radiative losses which are relevant in real systems. We demonstrate that losses allow one to control the population of electronic levels and may induce coherent population trapping, as well as lead to a redistribution of the photon statistics among the quantum fields and even to a transfer of the photon statistics from one field to another. Moreover, we introduce and analyze a quantity, the quantum polarization, and demonstrate its fundamental difference compared to the classical polarization. Using the quantum polarization and the third-level population, we investigate electromagnetically induced transparency in the presence of quantum light and show its special features for the case of squeezed light. Finally, quantum correlations between fields are studied and analyzed in the presence of different types of losses.

We present a theoretical study on the elastic Rayleigh scattering of x-ray photons by closed-shell atoms. Special attention is paid to the transfer of linear polarization from the incident to the outgoing photons. To study this process, we apply the density-matrix formalism combined with the relativistic perturbation theory. This formalism enables us to find general relations between the Stokes parameters of the incident and scattered photons. By using these expressions, we revisit the recent proposal to use Rayleigh scattering for the analysis of the polarization purity of synchrotron radiation. We show that this analysis can be performed without any need for the theoretically calculated scattering amplitudes, if the linear polarization of the scattered light is measured simultaneously at the azimuthal angles $0^{\circ }$ and ${45}^{\circ }$ with respect to the plane of the synchrotron. To illustrate our approach, we present detailed calculations for scattering of 145 keV photons by lead atoms.

The prompt and delayed dissociations of ${\mathrm{N}}_2{\mathrm{O}}^{2+}\to {\mathrm{N}}^{+}+\mathrm{N}{\mathrm{O}}^{+}$ induced by electron capture collisions of 56-keV/u ${\mathrm{Ne}}^{4+}$ and ${\mathrm{Ne}}^{8+}$ ions are studied by measuring ion-pair fragments and charge-changed projectiles in triple coincidence. For the prompt dissociation, the kinetic energy release (KER) distributions are obtained and indicate this process is mainly attributed to the decay of the low-lying $1{}^3\mathrm{\Sigma }^{-}$ state ${\mathrm{N}}_2{\mathrm{O}}^{2+}$ into ${\mathrm{N}}^{+}\left({}^{3}P\right)+{\mathrm{NO}}^{+}\left({}^1\mathrm{\Sigma }^{+}\right)$. For the delayed dissociation, since it is of difficulty to directly retrieve three-dimensional momentum vectors of ionic fragments for all the events from ${\mathrm{N}}_2{\mathrm{O}}^{2+}$ dissociation in flight, an alternative way is proposed to only catch and analyze the events from which the ${\mathrm{N}}_2{\mathrm{O}}^{2+}$ breakup direction is perpendicular to the time-of-flight axis. The KER distributions for these events are obtained and the metastable states of ${\mathrm{N}}_2{\mathrm{O}}^{2+}$ are confirmed to be $1{}^3\mathrm{\Pi }$ and $2{}^3\mathrm{\Pi }$. The survival time spectra for the metastable ${\mathrm{N}}_2{\mathrm{O}}^{2+}$ are also extracted and the mean lifetimes for the $1{}^3\mathrm{\Pi }$ and $2{}^3\mathrm{\Pi }$ conjoin, as well as the lifetimes for the identified individual metastable state, are deduced both in ${\mathrm{Ne}}^{8+}$ and ${\mathrm{Ne}}^{4+}$ collision experiments.

We propose a scheme for investigating the Unruh effect, i.e., the thermal radiation observed by uniformly accelerating observers or detectors, with ultracold atoms coupled by an off-resonant laser. We find that periodic modulation of the Rydberg-dressing interaction between atoms plays a significant role in the manipulation of a thermal bath in a uniformly accelerating reference frame. Our calculations show that the resulting Unruh temperature strongly depends on the driving frequency of the modulated Rydberg-dressing potential and the soft-core nature of the dressing interaction, i.e., the soft-core radius and height. These results reveal the potential applications of ultracold atoms with controllable long-range interaction in a wide variety of analog gravity phenomena.

We investigate the position- and momentum-space two-body correlations in a weakly interacting, harmonically trapped atomic Bose-Einstein condensed gas at low temperatures. The two-body correlations are computed within the Bogoliubov approximation and the consequences of the finite system size are highlighted in contrast to the spatially homogeneous case. In the position space, we recover the antibunching induced by the repulsive interatomic interaction in the condensed fraction localized around the trap center and the bunching in the outer thermal cloud. In the momentum space, bunching signatures appear for either equal or opposite values of the momentum and display peculiar features as a function of the momentum and the temperature. In analogy to the optical Hanbury-Brown and Twiss effect, the amplitude of the bunching signal at closeby momenta is fixed by the chaotic nature of the matter field state and its linewidth is shown to be set by the (inverse of the) finite spatial size of the associated in-trap momentum components. In contrast, the linewidth of the bunching signal at opposite momenta is only determined by the condensate size.

We present the classical electromagnetic theory framework of reflection of a light beam carrying orbital angular momentum (OAM) by a magnetic structure with generic symmetry. Depending on the magnetization symmetry, we find a change in the OAM content of the reflected beam due to magneto-optic interaction and an asymmetric far-field intensity profile. This leads to three types of magnetic helicoidal dichroism (MHD), observed when switching the OAM of the incoming beam, the magnetization sign, or both. In cases of sufficient symmetries, such as domain walls and magnetic vortices, we establish analytical formulas that link an experimentally accessible MHD signal to the magneto-optical Kerr effect (MOKE) constants.