We study how the spatial profile of a deterministic coherent gate perturbation affects finite-size quasienergy statistics and operator dynamics in $\text{U}\left(1\right)$-symmetric XXZ Floquet circuits. Using a shared homogeneous reference, we compare a single modified bond with an alternating perturbation applied across the whole chain. After separating symmetry sectors and checking several known integrable forms, we find that the alternating circuits move toward circular orthogonal ensemble level statistics and change operator-space entanglement and out-of-time-order correlations at smaller local amplitudes than the single defects. For intermediate numbers of modified bonds, the response is nonmonotonic and also depends on where the bonds are placed, so bond density alone is not enough. A perturbative analysis identifies the additive weight of many modified bonds, while coherent spatial interference reshapes the coupling between nearby eigenstates.

With a unitary coin operator that is a random matrix drawn from a uniform distribution with respect to the Haar measure, we construct a variant of a discrete quantum random walk using a $d$-regular undirected connected simple graph. With each step of the walk, the coin operator random matrix is drawn independently from the distribution. The expectation value over the distribution of random unitaries for the associated quantum channel gives a depolarization channel for the coin subspace and resembles the associated classical random walk where the direction a walker steps depends upon the outcome of a fair coin or balanced $d$-sided dice. Remarkably, despite the fact that the averaged channel depolarizes the coin subspace, measurements in the vertex subspace can be designed that would reveal information about the initial quantum state, even after many iterations of the channel. We illustrate with examples of quantum walks on Cayley graphs of Abelian groups, such as the cycle and hypercube graphs. For Cayley graphs of Abelian groups, the averaged channel is dephasing in the Fourier basis. These quantum walks are examples of bipartite strongly interacting systems, where one subsystem is strongly perturbed, yet information about the initial state in the other subsystem is potentially measurable forever. Due to its decoherence, a quantum random walk with a Haar-random coin would not be useful in search algorithms but could aid in understanding quantum systems with strongly perturbed subsystems.

Adiabatic quantum algorithms must evolve slowly enough to suppress nonadiabatic transitions while remaining fast enough to be practical. In open systems, this trade-off is reshaped by decoherence. For Hamiltonians subject to dephasing Lindbladians, Avron et al. [Phys. Rev. A 82, 040304(R) (2010)] showed that a unique timetable exists that maximizes the fidelity with a target state. This optimal schedule is characterized by a constant tunneling rate along the adiabatic path. In this work we revisit their analysis and apply it to the adiabatic Grover search framework, obtaining closed-form expressions for the optimal evolution schedule, the minimum runtime, and the resulting achievable fidelity. Moreover, by invoking an energy-time uncertainty argument, we identify a critical dephasing threshold beyond which further noise-assisted acceleration is prohibited, showing that the apparent advantage of strong dephasing is a Zeno-type stabilization mechanism rather than a conventional algorithmic speedup over the unitary Grover search.

We present a theoretical investigation of the electronic structure and properties of radium monochalcogenides, with chalcogens O, S, and Se, as well as the ionic species ${\mathrm{RaO}}^{\pm }$. Our approach combines two different, partially relativistic, quantum-chemistry methods. Electronic properties are obtained using the exact two-component Hamiltonian-based coupled-cluster approach with single, double, and perturbative triple excitations $[\mathrm{CCSD}\left(\mathrm{T}\right)+\mathrm{X}2\mathrm{C}]$, while potential-energy curves are computed using an internally contracted multireference configuration-interaction method, including relativistic effects through small-core pseudopotentials and Pauli–Breit operator diagonalization ($\mathrm{MRCI}+\mathrm{Q}+\mathrm{ECP}+\mathrm{SO}$). The dimers exhibit very large permanent dipole moments and sizable dipolar polarizabilities, while the Franck-Condon factors between the lowest electronic states are highly nondiagonal. These features are discussed in terms of the divalent character of the chemical bonding in the neutral species.

Strong-field photoelectron holography encodes ultrafast electron dynamics through momentum-space interference. However, the orbit-resolved origin of spiderlike spin fringes and the mechanism underlying their target dependence remain unclear. Here we resolve both issues by analyzing photoelectron spin textures generated during tunneling ionization. We use the Coulomb quantum-orbit strong-field approximation, benchmarked against time-dependent Schrödinger equation simulations for ${\mathrm{He}}^{+}$ and Xe, to separate orbital-channel and quantum-orbit contributions. Spiderlike fringes arise from interference between $p$-orbital ionization channels with different magnetic quantum numbers within an individual orbit class and therefore do not require interorbit interference. The observable polarization along these fringes, however, depends on the balance among orbit-class contributions. The decomposition associates the opposite first-leg polarizations of ${\mathrm{He}}^{+}$ and Xe with different relative weights of laser-deflected and forward-scattered trajectories, consistent with target-dependent Coulomb focusing. Photoelectron spin textures thus complement momentum distributions as probes of Coulomb-driven strong-field dynamics.

We theoretically investigate second-order sideband generation (SSG) and tunable slow-fast light propagation via strong nonlinear coupling among photons, phonons, and excitons in two coupled microcavities, consisting of an active (gain) cavity and a passive (loss) cavity that contains a quantum well and an optical parametric amplifier (OPA). By solving the Heisenberg-Langevin equations perturbatively, we obtain analytical expressions for the optical transmission and SSG efficiency. We found that the OPA significantly enhances both the transmission and the sideband generation efficiency. The enhancement becomes stronger in the active-passive configuration than in the passive-passive case and is further reinforced in the exciton-photon strong-coupling regime. Numerical analysis of higher-order sidebands further reveals substantial amplification induced by the OPA, strong exciton-photon coupling, and intercavity tunneling. In addition, by tuning system parameters, a controllable transition between slow and fast light is achieved in the output field, which could be utilized for an all-optical switching mechanism. These findings highlight the potential of coupled exciton-optomechanical cavities for tunable nonlinear frequency conversion and light-propagation control, with possible applications in optical switching, frequency comb generation, and quantum information technologies.

The cavity-mediated coupling between magnons in an yttrium-iron-garnet (YIG) sphere and a superconducting qubit has recently been demonstrated as a new platform for preparing macroscopic quantum states. Here, based on this system, we propose to entangle two magnon modes in two YIG spheres by driving the qubit with a two-tone field and by appropriately choosing the frequencies and strengths of the two driving fields. We show that strong entanglement can be achieved with fully feasible parameters. We further provide a detection scheme for experimentally verifying the entanglement. Our results indicate that macroscopic entanglement between two magnon modes in 2-mm-sized YIG spheres, involving more than ${10}^{18}$ spins, can be realized using currently available parameters, which finds promising applications in fundamental studies, such as macroscopic quantum mechanics and the test of unconventional decoherence theories.

The correlation structure of multitime quantum processes—succinctly described by quantum combs—is an important resource for many quantum information protocols and control tasks. Inspired by approaches for quantum states, we introduce quantifiers of the practical utility of quantum processes that satisfy monotonicity properties, thus overcoming shortcomings in previous state-motivated approaches. Applying these quantifiers to the problem of noise reduction of a quantum process under open-loop control, they are shown to represent the largest amount of temporal mutual information that a process can possibly exhibit. In addition, we study their resource composition behavior and connect them to the recently introduced notion of generalized comb divergences. Finally, in light of these new quantifiers, we reinterpret the numerical findings of Berk et al. [npj Quantum Inf. 9, 104 (2023)] on the relationship of dynamical decoupling and non-Markovian memory, which were based on insufficient resource quantifiers, and show that their main conclusion—the interpretation of dynamical decoupling as a resource distillation—still holds.

Accurate modeling of driven light-matter interactions is essential for quantum technologies, where natural and synthetic atoms are used to store and process quantum information, mediate interactions between bosonic modes, and enable nonlinear operations. In systems subject to multitone drives, however, the theoretical description becomes challenging and existing models cannot quantitatively reproduce the experimental data. Here, we derive an effective Hamiltonian that retains slowly rotating terms, providing a general framework for accurately describing driven dynamics across platforms. As a concrete application, we validate the theory in circuit QED, where it quantitatively reproduces experimentally measured ac Stark shifts and captures key interactions such as two-mode squeezing and beam splitting. Our results establish a broadly applicable tool to engineer driven interactions in quantum information processing platforms.

In this work we present a comprehensive analysis of a post-selective attack on quantum key distribution protocols employing phase-encoded linearly independent coherent states (or similar alternatives). The attack relies on multimode projection onto a Fock subspace and enables probabilistic extraction of information by an eavesdropper. We derive analytical expressions for the information accessible to the adversary, and show that it depends only on three protocol parameters: the mean photon number of the signal states, the phase separation in the information basis, and the expected optical loss of the quantum channel. Several optical realizations of phase-encoded quantum key distribution protocols are analyzed to illustrate the applicability of the results. Possible countermeasures against the proposed attack are also discussed.

One of the primary challenges of photonic quantum information processing is the on-demand preparation of multiple single-photon-level quantum states from probabilistic photon-pair sources. Motivated by recent developments in frequency-bin-encoded photonic quantum information processing, here we consider active time multiplexing to generate $n$-photon states, where $n$ single photons with $n$ distinct frequencies occupy the same spatiotemporal mode. We devise an approach that uses optical quantum memories to manipulate the temporal mode of heralded single photons and an array of fiber Bragg grating reflectors to jointly manipulate the frequency and temporal modes of the photons, overlapping $n$ photons in $n$ separate frequency bins into a single spatiotemporal mode. We calculate multiphoton state generation rates that, accounting for loss, are realistically achievable with commercially available hardware. Using only a single free-space switchable delay loop for an optical quantum memory, this scheme could feasibly produce eight-photon states at an average rate of 1 kHz.

We report high-resolution spectroscopy of ionic-core transitions in alkaline-earth-metal Rydberg atoms. We perform high-resolution measurements of isotope shifts and the hyperfine splitting of dipole transitions in the ionic core. A key element of this work is the reduction of the linewidth by more than two orders of magnitude, enabled by the dynamical control of the Rydberg electron's orbit, which significantly enhances the spectral resolution. Furthermore, to unambiguously identify the frequency shift, we directly compare the core ion's spectrum with a signal from a single trapped ion serving as a frequency reference, which represents a completely isolated system.

We extend the correlated B-spline basis function (C-BSBF) method to high-precision calculations of the ionization energies of helium Rydberg $n{}^{1,3}P$ states ($n=24–35$). Using a unified basis set, we evaluate nonrelativistic energies, relativistic corrections of order $m{\alpha }^4$ (including finite-nuclear-mass recoil), QED contributions of order $m{\alpha }^5$, and partial $m{\alpha }^6$ terms (singlet-triplet mixing, one- and two-loop radiative corrections, and the logarithmic contribution). The remaining higher-order contributions are estimated via $1/n^3$ scaling. The resulting ionization energies achieve kilohertz-level accuracy and are in excellent agreement with independent Hylleraas calculations, thereby providing cross validation between two distinct theoretical approaches. From these data, the quantum-defect parameters are determined and used to extrapolate the ionization energies up to $n=102$. Combining our Rydberg ionization energies with high-precision experimental $2S\to nP$ transition frequencies yields the ionization energies for the metastable $2^{1}S$ and $2^{3}S$ states as $960332040.532{\left(10\right)}_{\mathrm{stat}}{\left(20\right)}_{\mathrm{sys}}$ and $1152842742.7271{\left(52\right)}_{\mathrm{stat}}{\left(25\right)}_{\mathrm{sys}}\mathrm{MHz}$, respectively. The C-BSBF result for the $2{}^{1}S$ state is consistent with the experimental ionization energy obtained from Rydberg-series extrapolation, while for the $2{}^{3}S$ state the difference is 0.019(10) MHz.

The single-electron charge-transfer process in ${\mathrm{H}}^{+}-{\mathrm{H}}_2$ collisions is investigated using a fully correlated two-active-electron semiclassical atomic-orbital close-coupling method in the energy range of 0.1–100 keV/u, and the orientation and vibrational effects of the molecular target are taken into account under the sudden approximation. Total and state-selective charge-exchange cross sections for this system are provided, and excellent agreement is found between our calculations and the available experimental data for total charge-transfer cross sections in the energy range of 0.2–100 keV/u. The vibrational effect from the molecular target make a big contribution to the total charge–transfer cross sections for energies below 2 keV/u, and it leads to an increase of 8%–185% for the total charge-exchange cross sections in the 0.1–1 keV/u range. Furthermore, absolute angular differential cross sections at different energies are reported, and their oscillatory structures are inferred to originate from Fraunhofer-type diffraction.

We demonstrate time-domain observation of the core-hole decay of xenon atoms based on photoelectron interferometry, using phase-coherent extreme ultraviolet pulse pairs generated from a synchrotron light source. By utilizing a tandem undulator configuration, we induced interference between innershell photoelectron wave packets and measured the resulting Ramsey fringes via photoelectron spectroscopy. The contrast of these fringes exhibits a clear decay as a function of the femtosecond-scale time delay between the two pulses, directly reflecting the temporal evolution of the core hole. This synchrotron-based interferometry holds the potential to probe the ultrafast electron dynamics with element selectivity and site selectivity.

Polarization skyrmions are usually characterized through a unit Stokes vector field, which defines a map from the transverse plane to the Poincaré (Bloch) sphere and allows one to assign a skyrmion number under appropriate boundary conditions. This description assumes complete polarization. In many optical situations, however, the local polarization state is partially polarized and is therefore described by a spectral polarization matrix rather than by a single Jones vector. We formulate a density-matrix-based effective extension of the skyrmion-number construction for statistically stationary, quasimonochromatic paraxial fields in terms of the normalized spectral polarization matrix. Its eigenvectors define two orthogonal polarization eigenstates, corresponding to antipodal points on the sphere, while the eigenvalue imbalance is fixed by the local degree of polarization (DoP) $P\left(\mathbf{r}\right)$. Averaging the opposite skyrmion-density contributions of these two eigenpolarizations leads to an effective skyrmion number in which the usual pure-state density is weighted by $P\left(\mathbf{r}\right)$. The resulting quantity reduces to the conventional skyrmion number for fully polarized fields and assigns zero weight to locally unpolarized regions. In general, it is not an integer topological invariant; rather, it is an integrated effective measure, constructed from local Stokes parameters, which can be interpreted geometrically as a DoP-weighted signed solid-angle integral of the polarization eigentexture on the Poincaré sphere. We illustrate the construction with a regular analytic family of polarization eigentextures with the winding order $m$ and fixed radial DoP profiles, showing how the effective number depends jointly on the winding order and on the spatial overlap between the DoP and the skyrmion density.

Selective pumping along an unstable periodic orbit in a fully chaotic billiard laser induces strong intensity localization, making the orbit appear effectively stable. This effect arises not from a single linear resonance, but from self-organized phase locking of multiple linear modes via nonlinear gain dynamics. We define this as a “nonlinear scar mode.” Since fully chaotic billiards host diverse unstable orbits, this mechanism enables tailored lasing features, such as directional emission and low thresholds. Our findings establish nonlinear scar modes as a design principle for two-dimensional microcavity lasers.

We study theoretically the synchronization of two arrays of coupled lasers with complex mode dispersion, separated by a spectrally detuned barrier in one- and two-dimensional geometries. We demonstrate that when lasing at the Dirac point, the synchronization persists for an order of magnitude higher barriers than in the arrays with a usual parabolic dispersion or a purely dissipative coupling. We interpret this effect as the Klein tunneling of the laser coherence through the barrier. Our numerical findings are supported by an analysis of the delocalization of the linearized eigenmodes of the arrays, which enhances the synchronization.

Owing to the radiation-force-induced nonlinearity, cavity optomechanical systems (COMSs) exhibit a rich variety of dynamical phenomena, including backaction-induced oscillations, chaos, mechanical amplitude locking, and anomalous stabilization, under different driving conditions and system parameters. Here we identify a dynamical behavior characterized by a staircaselike evolution of the mechanical resonator energy in a COMS with an intermediate intrinsic mechanical frequency driven by a two-tone field. This behavior emerges when the frequency difference between the two drive tones matches the intrinsic mechanical frequency. We systematically investigate its dependence on key system parameters, including the mechanical frequency and quality factor, as well as on driving conditions such as unequal drive powers and detuning of the drive-tone frequency difference from the mechanical resonance. We further reveal several distinctive features unique to COMSs with intermediate mechanical frequencies, including incomplete stability of the transient states, an emergent bifurcation induced by drive-power imbalance, and strikingly different system responses to positive and negative drive-tone detunings. These phenomena are absent in COMSs with either very high or very low mechanical frequencies. Our results fill a gap in the understanding of the dynamics of two-tone-driven COMSs. From an application perspective, the rapid growth of mechanical energy associated with the staircase dynamics provides a promising route toward phonon lasing, while the strong sensitivity to drive-tone detuning suggests potential applications in high-precision sensing.