We numerically integrate the Schrödinger equation for a model helium atom irradiated by an intense few-cycle laser pulse and find the emitted XUV spectra. They demonstrate resonant Fano peaks at the frequencies of the transitions from the doubly excited autoionizing states (AISs) to the ground state. Studying XUV intensity and phase, we show that the resonant peak should be described with an essentially complex asymmetry parameter. The Stark shift of the AIS and the decay of the AIS due to photoionization by the laser field result in the asymmetry parameter describing a spiral path in the complex plane when the laser pulse duration increases. This behavior, in particular, corresponds to the decrease of the resonant contribution with the increase of the laser pulse duration, in agreement with published experimental data. Moreover, we find a remarkable similarity of this behavior of the asymmetry parameter in the complex plane to that of the classical double pendulum. We conclude that transient perturbation of the natural frequency and friction in the upper pendulum can be treated as a classical analogy of the transient frequency shift and extra depopulation of the excited state in the quantum system demonstrating the Fano resonance.

Above-threshold detachment of electrons from negative ions by a strong low-frequency elliptically polarized laser field is considered using the strong-field approximation. The detachment probability amplitude is expressed via integral over times of highly oscillatory functions. Particular attention is devoted to application of the asymptotic methods to solve these integrals. For the direct detachment only the integral over the detachment time appears, while for the high-order above-threshold detachment the double integral over the detachment and rescattering times should be solved. Depending on the ellipticity of the laser field, a critical photoelectron energy exists for which the standard saddle-point method fails. The problem can be solved by properly deforming the integration contour in the complex time plane and, for energies higher than this critical energy, taking into account only one of the two saddle-point solutions. However, this procedure still leaves a spike in the photoelectron spectrum near this critical energy. This problem is cured applying the uniform approximation. A formula for the transition amplitude in the uniform approximation is derived, and it is shown how this formula should be modified for the energies higher than the critical one. For high-order above-threshold detachment many more saddle-point solutions contribute. They are classified into pairs. For the saddle-point method each pair produces a spike in the spectrum which spoils the total spectrum. When the contribution of each pair is treated using the uniform approximation with a careful choice of the phase factors after the anti-Stokes transition the agreement with the exact numerical results becomes excellent.

Hybrid plasmonic-photonic domains showcase remarkable non-Hermitian characteristics, offering unique benefits through their ability to exhibit gain and loss simultaneously. Here, in-depth analysis has been conducted on a multilayered structure that is enhanced by the presence of plasmonic antennae. These antennae are strategically placed to optimize the interaction with light, potentially improving the performance of the structure in various applications. The hybrid waveguide resonance mode, generated by combining the surface-plasmon polariton with waveguide modes and activated by the antenna modes, characterizes the multilayer. The antennae aid in hybridizing waveguide and surface-plasmon modes; in turn, the multilayer structure enhances near-field intensity and allows for tuning far-field responses through multiple resonances. The interaction between antennae is characterized by dark and bright modes, which can create bound states in the continuum because of destructive interference in radiation losses. The nonconservative nature of the open system enables exploration of non-Hermitian properties, complex topology, and exceptional points. Analytical, numerical, and experimental results enable the study of the system's behavior, and display that exceptional points significantly boost emission levels when measurements are taken in their vicinity. In conclusion, systems with hybrid architecture and including plasmonic antennae can provide enhanced sensitivity and demonstrate their potential value across a diverse array of applications. These include advanced sensing technologies and innovative communication systems, where the complete density matrix can be simplified, and non-Hermitian Hamiltonian frames can be utilized to explore entanglement dynamics and analyze systems much larger than feasible.

The squeezed state of a magnon has garnered widespread attention due to its distinct properties and potential applications. Although preliminary experimental verification has been achieved, numerous technical and theoretical challenges persist. Here, we investigate the nonlinear coupling within the magnon-cavity-qubit system and propose an innovative method for achieving magnon squeezing. By employing virtual photons as a mediator for magnon-qubit interactions and optimizing system parameters, we induce nonlinear coupling. This approach not only enhances the interaction strength between magnons and qubits but also opens up an alternative pathway for achieving magnon squeezing. Our findings significantly enrich the fields of quantum physics and magnetism by providing a method for magnon squeezing, thereby laying a solid foundation for future advancements and applications in quantum technology.

In this paper, the 16 Stokes parameters of a two-photon system are organized into a $4\times 4$ Stokes matrix. This formalism is then applied to analyze the polarization properties of annihilation photons produced by electron-positron annihilation, with a particular focus on their evolution during Compton scattering interactions. The study demonstrates that when one of the subsystems undergoes Compton scattering, quantum entanglement preserves the polarization symmetry between the scattered and noninteraction photons, while in the absence of entanglement, this symmetry is lost. Moreover, the method simplifies the calculation of the joint differential cross section for multiple scattering scenarios and provides a means to characterize the polarization properties of the individual subsystems. These findings highlight the impact of quantum correlations on scattering behavior, and they offer a framework for further investigation into the polarization dynamics of entangled photon pairs.

In the accompanying paper [Y. Mitsuhashi, et al., Phys. Rev. Lett. 134, 180404 (2025)], we established the method of characterizing the maximal order of asymptotic unitary designs generated by symmetric local random circuits, and explicitly specified the order in the cases of ${\mathbb{Z}}_2, \mathrm{U}\left(1\right)$, and $\mathrm{SU}\left(2\right)$ symmetries. Here, we provide full details on the derivation of the main theorems for general symmetry and for concrete symmetries. Furthermore, we consider a general framework where we have access to a finite set of connected compact unitary subgroups, which includes symmetric local unitary gate sets.

A pure multipartite quantum state is called absolutely maximally entangled if all reductions of no more than half of the parties are maximally mixed. However, an $n$-qubit absolutely maximally entangled state exists only when $n$ equals 2, 3, 5, and 6. A natural question arises when it does not exist: Which $n$-qubit pure state has the largest number of maximally mixed $\lfloor n/2\rfloor$-party reductions? We denote this number by $\mathrm{Qex}\left(n\right)$. It was shown that $\mathrm{Qex}\left(4\right)=4$ by Higuchi and Sudbery [Phys. Lett. A 273, 213 (2000)] and $\mathrm{Qex}\left(7\right)=32$ by Huber et al. [Phys. Rev. Lett. 118, 200502 (2017)]. In this paper, we give a general upper bound of $\mathrm{Qex}\left(n\right)$ by linking the well-known Turán problem in graph theory and provide lower bounds using constructive and probabilistic methods. In particular, we show that $\mathrm{Qex}\left(8\right)=56$, which is the third known value for this problem.

The dynamics and processes involved in particle-molecule scattering, including nuclear dynamics, are described and analyzed using various quantum information quantities throughout the different stages of the scattering. The main process studied and characterized with the information quantities is the interatomic Coulombic electronic capture, an inelastic process that can lead to dissociation of the target molecule. The analysis focuses on a one-dimensional transversely confined NeHe molecule model used to simulate the scattering between an electron $e^{-}\left(\text{particle}\right)$ and a ${\text{NeHe}}^{+}$ ion (molecule). The time-independent Schrödinger equation is solved using the finite element method with a self-developed Julia package FEMTISE to compute potential energy curves and the parameters of the interactions between particles. The time-dependent Schrödinger equation is solved using the multiconfiguration time-dependent Hartree algorithm. The time-dependent electronic and nuclear probability densities are calculated for different electron incoming energies, evidencing elastic and inelastic processes that can be correlated to changes in von Neumann entropy, von Neumann mutual information, and Shannon mutual information. The expectation value of the position of the particles, as well as their standard deviations, are analyzed along the whole dynamics and related to the entanglement during the collision and after the process is over, thus highlighting the dynamics of entanglement generation. It is shown that the correlations generated in the collision are partially retained only when the inelastic process is active.

Graph states are the key resources for measurement- and fusion-based quantum computing with photons, yet their creation is experimentally challenging. We optimize a hybrid graph-state generation scheme using a single quantum emitter and linear optics Bell-state measurements called fusions. We first generate a restricted class of states from a single quantum emitter and then apply fusions to create a target graph state, where we use a dynamic programming approach to find the construction that requires the lowest possible number of fusions. Our analysis yields a lookup table for constructing $\approx 2.8\times {10}^7$ nonisomorphic graph states with the minimum number of fusions. The lookup table covers all graph states with up to eight qubits and several other ones with up to 14 qubits. We present construction protocols of selected graph states and provide the lookup table. For large graph states that are not in the lookup table, we derive bounds for the required number of fusions using graph-theoretic properties. Finally, we use the lookup table to search for the best graph codes for loss-tolerant encodings, given a fixed number of fusions for their construction.

Two-way microwave-optical quantum transduction is essential to connecting distant superconducting qubits via optical fiber and to enable quantum networking at a large scale. In Blésin, Tian, Bhave, and Kippenberg's article [Phys. Rev. A 104, 052601 (2021)], they lay out a two-way quantum transducer converting between microwave photons and telecom-band photons by way of an intermediary gigahertz-band phonon mode utilizing piezoelectric and optomechanical interactions respectively (and are the first to work out the quantum piezoelectric coupling). In this work, we examine both the piezoelectric, and optomechanical interactions from first principles, and together with the evanescent coupling between optical modes, discuss what parameters matter most in optimizing this kind of quantum transducer. For its additional utility, we have also compiled a table of relevant properties of optical materials that may be used as elements in transducers.

Pair creation by a $\gamma$ ray in a high-intensity electromagnetic field (the nonlinear Breit-Wheeler process) is sensitive to the $\gamma$-ray polarization. Here we study the stability required in order to measure this polarization dependence in a head-on collision between a high-power linearly polarized laser and a relativistic electron beam, where the $\gamma$ rays produced via nonlinear Compton scattering are highly polarized. We find that the laser strength parameter $a_0$ has to be known to within a few percent and that the alignment of the laser and electron beam cannot fluctuate by more than half the spot size in order to attribute the reduction in positron yield to polarization effects. These collision parameters are difficult to achieve, but may be realized in future precision experiments that focus on beam stability.

We present the precise frequency measurement of the $2{}^{3}S_1\text{–}8{}^{3}D_1$ two-photon transition in ${}^4\mathrm{He}$ at 544 nm. The 544-nm light is generated by second-harmonic generation with a power-amplified 1088 nm external cavity diode laser (ECDL). To determine the absolute frequency of the transition, the ECDL is stabilized to an optical frequency comb. The absolute frequency of $2{}^{3}S_1\text{–}8{}^{3}D_1$ transition is experimentally determined to be 1 101 410 219.422(54) MHz with a fractional uncertainty of $4.9\times {10}^{-11}$. By combining our measured transition frequency with the theoretically calculated ionization energy of the $8{}^{3}D_1$ state [Yerokhin et al., Phys. Rev. A 102, 012807 (2020) and G. Drake, Springer Handbook of Atomic, Molecular, and Optical Physics (Springer Nature, New York, 2023)] we determine the ionization energy of the $2{}^{3}S_1$ state to be 1 152 842 742.652(54) MHz. The $2{}^{3}S_1$ Lamb shift is then deduced to be 4057.403(54) MHz according to the theoretical calculation  [Patkóš et al., Phys. Rev. A 103, 042809 (2021)]. We compare our experimental result with previous measurements and find excellent agreement [Dorrer et al., Phys. Rev. Lett. 78, 3658 (1997); Luo et al., Phys. Rev. A 94, 062507 (2016); Zheng et al., Phys. Rev. Lett. 119, 263002 (2017); Zheng et al., Phys. Rev. A 99, 032506 (2019); Huang et al., Phys. Rev. A 97, 032516 (2018); Pastor et al., Phys. Rev. Lett. 92, 023001 (2004); Phys. Rev. Lett. 97, 139903(E) (2006); Rengelink et al., Nat. Phys. 14, 1132 (2018); Clausen et al., Phys. Rev. Lett. 127, 093001 (2021); Clausen et al., Phys. Rev. A 111, 012817 (2025)]. A discrepancies of approximately 0.4 MHz remains between the theoretical predictions and the experimental results for the $2{}^{3}S_1$ state.

In this work, we extend the study of the interplay between scaling symmetries and statistics to one-dimensional fluids by studying the Calogero model in a harmonic trap modulated through time. The latter harbors an interpretation in terms of free particles imbued with exclusion statistics and is an example of a scale invariant fluid in one-dimension displaying $\text{SO}(2,1)$ dynamical symmetry preserved by harmonic traps. Taking advantage of the dynamical symmetry, two experimentally relevant drive protocols spanning both quasistatic and nonadiabatic regimes are investigated and universal signatures of the interactions and exclusion statistics are uncovered in the ground-state echo amplitude and closely related ground-state fidelity. In particular, under both periodic modulation and slow drive through the gapless point of the trap frequency, enhanced interactions and exclusion are shown to favor the proliferation of defects and to hinder their annihilation, which leads to a universal decrease of ground-state fidelities and echo amplitudes. We also show that increasing exclusion sparks a sharp suppression of the likelihood of intermediate echoes beyond those imposed by the commensurability of a periodic drive and the natural frequency of the trap.

We develop a linear-response theory to provide a unified description of two recent spectroscopy protocols for probing one-dimensional supersolid states realized in cold-atom systems. Both protocols involve applying a periodic optical potential to excite the supersolid and determine its excitation frequencies and density response characteristics. This information can be used to estimate the superfluid fraction. We validate our linear-response theory against nonlinear mean-field simulations of the dynamics for ring, box, and harmonically trapped cases. A key focus is the behavior at the band edge—the regime occurring when the optical potential used to excite the system has a wavelength that is twice the value of the supersolid lattice constant. Here symmetry can be used to selectively excite a mode from one of the two low-energy gapless excitation bands. We also consider the application of the spectroscopy protocols to determine the superfluid fraction, showing the relationship to hydrodynamic theory and a Josephson-junction array model. Our analysis and results suggest that the phase protocol we introduce has potential advantages for experimental application over the density protocol.

We present a detailed study of the generation of photon pairs by spontaneous four-wave mixing in a bicavity system with a single atom inside. Cavity modes lead to larger coupling strength than that of the normally assumed thermal vacuum, and thus higher generation rates are expected. Unlike approaches that have emerged so far, this paper presents an approach based on the solution of the master equation. This method allows us to investigate the photon statistics more strictly, and we show that the choices of pumping detuning and leaking rate of the cavities significantly affect the degree of the autocorrelation, and a higher-purity heralded single photon source is possible. We also derive a concise expression for the output state and the generation rate for the extreme cases where the cavity leaking rate is much smaller or larger than the rate of spontaneous emission.

Giant atoms, which couple to the environment at multiple discrete points, exhibit various nontrivial phenomena in quantum optics due to their nonlocal couplings. In this study, we propose a one-dimensional cross-stitch ladder lattice featuring both a dispersive band and a flat band. By modulating the relative phase between the coupling points, the giant atom selectively interacts with either band. First, we analyze the scenario where the dispersive and flat bands intersect at two points, and the atomic frequency lies within the band. Unlike the small atom, which simultaneously interacts with both bands, a single giant atom with a controllable phase interacts exclusively with the dispersive or flat band. Second, in the band-gap regime, where two atoms interact through bound-state overlaps manifesting as dipole-dipole interactions, we demonstrate that giant atoms enable deterministic long-range hopping and energy exchange with higher fidelity than small atoms. These findings provide promising applications in quantum information processing, offering enhanced controllability and selectivity for quantum systems and devices.

Telecloning is a protocol introduced by Murao et al. [Phys. Rev. A 59, 156 (1999)] to distribute copies of an unknown quantum state to many receivers in a way that beats the trivial “clone-and-teleport” protocol. In the last decade, a new type of teleportation called port-based teleportation, in which the receiver can recover the state simply by looking at the correct port without having to actively perform correction operations, has been widely studied. In this paper, we consider the analog of telecloning, in which conventional teleportation is replaced by the port-based variant. To achieve this, we generalize the optimal measurement used in port-based teleportation and develop a new one that achieves port-based telecloning. Numerical results show that, in certain cases, the proposed protocol is strictly better than the trivial clone-and-teleport approach.

We investigate the ground-state structures and vortex configurations in a two-component Bose-Einstein condensate under the influence of spin-orbital-angular-momentum coupling (SOAMC) with a high spatial inhomogeneity and high characteristic orbital angular momentum. By modulating the coupling strength, we uncover two distinct quantum phases: a stripe phase at low coupling strengths and a new vortex-necklace phase at higher coupling intensities. The latter is characterized by vortices forming a ring-shaped structure that acts as a domain wall, a unique phase boundary between a central stripe phase and an outer single-momentum phase. For a better understanding of this new mixed phase of the system, we develop an analytical model to describe the domain-wall radius as a function of coupling strength, which aligns well with numerical simulations. Our findings contribute to the understanding of SOAMC-driven quantum phase transitions and domain-wall formation, offering new insights into topological phenomena in ultracold atomic systems.

In this work, we explore the dynamics of time-varying photonic media with an optical Kerr nonlinearity and an associated phase transition. The interplay between a periodically modulated permittivity and the nonlinearity induces a continuous transition of electromagnetic waves to a state with broken spatial and time translation symmetries. This transition gives rise to a lattice-like wave pattern, in many ways similar to a spatial crystallization in solids. Symmetry breaking triggers the emergence of soft, Goldstone-like modes, which propagate as deformations of the lattice structure, as well as massive Higgs-like modes—spatially uniform oscillations of the field amplitude. We extend the analysis of the nonequilibrium symmetry breaking to $2+1$-dimensional time-varying media and discuss pattern formation as well as the connection to discrete dissipative time crystals.

We compare two amplification schemes, photon addition and then subtraction ($\widehat{a}{\widehat{a}}^{†}$) and successive photon addition (${\widehat{a}}^{†}{}^2$), applied to hybrid entangled states (HESs) and superpositions of coherent states (SCSs). We show that the fidelity and gain of the amplification schemes for HESs are the same as those of coherent states. However, SCSs show quite nontrivial behavior by the amplification schemes, depending on the amplitudes of coherent states, the number of coherent-state components, and the relative phases between the components. This implies that appropriate amplification schemes for SCSs should be chosen depending on the tasks and specific forms of the states. To investigate the quality of amplified states, we calculate the quantum Fisher information, a measure of quantum phase estimation. In terms of the quantum Fisher information, the $\widehat{a}{\widehat{a}}^{†}$ scheme tends to show better performance for relatively small amplitudes ($\alpha \lesssim 0.9$), while the ${\widehat{a}}^{†}{}^2$ scheme is better in the larger amplitude regime. The performance of the two schemes becomes indistinguishable as the amplitude grows sufficiently large.