The wave-packet convergent close-coupling approach has been used to model electron-capture and ionization processes occurring in collisions of ${\mathrm{O}}^{7+}$ and ${\mathrm{O}}^{8+}$ ions with atomic hydrogen. We use a model potential approach to represent the interactions with the hydrogenlike oxygen ion. We outline a method to obtain model potential parameters which provide significantly more accurate ground- and excited-state energies for the projectiles formed through electron capture than what is currently available in the literature. For both collision systems, our results for the total electron-capture cross section agree very well with one set of experimental measurements, but disagree with the other set. For the total ionization cross section, our results disagree with the classical trajectory Monte Carlo calculations. This difference is likely due to the choice of distribution used to describe the initial electron cloud in the classical calculations. We also calculate state-selective electron-capture cross sections, important for fusion plasma diagnostics and astrophysical research, resolved in terms of the final-state principal and orbital angular momentum quantum numbers. For the ${\mathrm{O}}^{8+}$ projectile, slight differences are observed in some of the partial cross sections when compared with other theoretical calculations, but the present results are generally in good agreement with the existing literature. At energies relevant to fusion plasma modeling, there is no state-selective data available in the literature for the ${\mathrm{O}}^{7+}+\mathrm{H}\left(1s\right)$ collision system. The presented results fill this gap.

We theoretically investigate the Coulomb focusing effect in an orthogonally polarized two-color (OTC) laser field by numerically solving the time-dependent Schrödinger equation. Based on the photoelectron momentum distribution along the light-propagation direction, the strength of the Coulomb focusing effect across all relative phases in the OTC field is quantified using a unified standard, revealing a phase dependence. Employing a semiclassical model, we conduct a comprehensive study of the underlying mechanism and find that both the unique electron looplike dynamic behavior and the electron birth time distribution play crucial roles. Their respective influences are manifested in two different aspects, i.e., altering the effective interaction time with the ionic potential and changing the proportion of rescattering electrons. We further investigate the phase-modulated Coulomb focusing effect for different atoms in the OTC field. Distinct modulation trends are observed, which arise from the differences in the proportion of rescattering electrons.

In conducting an experiment to measure a neutron electric dipole moment (nEDM), it is often necessary to determine the behavior of an ensemble of spins under time-dependent and randomly fluctuating magnetic fields. This is particularly relevant for experiments which utilize dressing magnetic fields to modify the effective Larmor frequency of the neutrons under study. In this work, we investigate a new technique to calculate the frequency shifts arising from magnetic field inhomogeneities and motional magnetic fields, particularly in the case where the spins in question are subjected to a time-periodic magnetic field. The method is based on Floquet theory, a general framework for analyzing periodic linear differential equations, and Redfield theory, which governs the time evolution of the density matrix in the presence of weak couplings to an environment. We benchmark this method against the analytical results derived both with and without dressing field, as well as against the results of a conventional Runge-Kutta integrator, and find agreement in all cases. We further study the performance of the method, and find order-of-magnitude improvements in runtime over the conventional integrator.

We propose a theoretical description of a mechanically squeezed Kerr oscillator with a single ion in a tapered trap. We show that the motion coupling between the axial and radial modes caused by the trap geometry leads to Kerr nonlinearity of the radial mode with magnitude controlled by the trap frequencies. This allows the realization of non-Gaussian quantum gates, which play a significant role in the universal set of continuous variable quantum gates. Furthermore, we show that, because of the nonlinearity of the ion trap, applying an off-resonant time-varying electric field along the trap axis causes a motion squeezing of the radial mode. Finally, we discuss the motion mode frequency spectrum of an ion crystal in a tapered trap. We show that the frequency gap between the motion modes increases with trap nonlinearity, which benefits the realization of faster quantum gates.

The interplay of topology and disorder in quantum dynamics has recently attracted significant attention across diverse platforms, including solid-state devices, ultracold atoms, and photonic systems. Here, we report on a topological Anderson transition caused by quasiperiodic modulation of the stronger intracell couplings in photonic Su-Schrieffer-Heeger lattices. As the quasiperiodic strength is varied, the system exhibits a reentrant transition from a trivial phase to a topological phase and back to a trivial phase, accompanied by the closing and reopening of the band gap around zero energy. Unlike the traditional detection of photonic topological edge modes, we measure the mean chiral displacement from the transport of light in the bulk of the lattices. In our photonic lattices with a fixed length, the propagation dynamics is retrieved by varying the wavelength of light, which tunes the interwaveguide couplings.

Bound states in the continuum (BICs) represent a unique class of physical phenomena characterized by resonant or leaky modes that remain perfectly confined, despite residing within the continuum. The high-quality $Q$-factor guided-mode resonances associated with BICs have garnered considerable attention in photonics research. In this study, we achieve electromagnetically induced transparency (EIT) with perfect chirality by coupling a high-$Q$ chiral quasi-BIC with a broad achiral resonance. We demonstrate the utilization of the topological properties of BICs to realize a high-$Q$ transparent band with maximum chirality in an all-dielectric planar metasurface. Additionally, we observe a spin-dependent slow-light effect, where the group delay difference between the two circularly polarized states exceeds 150 ps. These results reveal a straightforward mechanism for engineering ultranarrow spin-dependent EIT, which holds broad applicability across the electromagnetic spectrum, particularly in filtering and sensing.

Investigations into optical phenomena associated with nonlinear magnetoelectric effects are attracting growing attention within the scientific community. Technologies constantly demand new materials capable of exhibiting precise and controllable responses to external electromagnetic fields. In this context, the optics of such materials is of remarkable importance. Here, working in a lossless and nondispersive regime, electromagnetic wave propagation in materials presenting linear and nonlinear optical coefficients is investigated. We expand the discussion of the roles of nonlinear coefficients by examining special cases in which the contribution of the magnetoelectric optical coefficients ${\alpha }_{ij}$, ${\beta }_{ijk}$, and ${\gamma }_{ijk}$ to birefringence and nonreciprocal phenomena is elucidated. Notably, expressions that directly connect the magnetoelectric coefficients to the refractive indices of the medium are fully derived. These expressions enable the direct measurement of all components of each nonlinear magnetoelectric coefficient, providing an advancement over previous works. This development bridges theoretical models with experimental applications, offering possibilities for the optical characterization of magnetoelectric effects.

We present a semianalytic formalism for calculating the squeezing and antisqueezing spectrum in a channel waveguide side-coupled to a lossy ring resonator. Our approach first uses the semianalytic evolution of the density matrix inside the ring up to the time where the squeezing is maximized. Then noting that a conservative approximate result for the squeezing can be obtained by ignoring the effect of the pump at later times, we calculate the free evolution of the field operators in the channel waveguide at all later times. We then calculate the quadrature squeezing spectrum in the waveguide, assuming that the measurement starts at the time when the squeezing in the ring is maximized. Using these results, we determine the optimum values for the pump pulse duration and amplitude and the ring-channel coupling for the pump and signal. We find that squeezing above 10 dB can be easily achieved in the channel for antisqueezing levels of less than 22 dB.

Unitary integrable models typically relax to a stationary generalized Gibbs ensemble (GGE), but in experimental realizations dissipation often breaks integrability. In this work, we use the recently introduced time-dependent GGE (t-GGE) approach to describe the open dynamics of a gas of bosons subject to atom losses and gains. We employ tensor network methods to provide numerical evidence of the exactness of the t-GGE in the limit of adiabatic dissipation, and of its accuracy in the regime of weak but finite dissipation. That accuracy is tested for two-point functions via the rapidity distribution, and for more complicated correlations through a non-Gaussianity measure. We combine this description with generalized hydrodynamics and we show that it correctly captures transport at large scales. Our results demonstrate that the t-GGE approach is robust in both homogeneous and inhomogeneous settings.

We establish the complete positivity of a quantum dynamical map associated with the quantum optical master equation of the partial secular Bloch-Redfield type, which describes the generation of noise-induced Fano coherences in three-level $\mathrm{\Lambda }$ and V systems driven by incoherent blackbody radiation. To this end, we calculate the equation's Kossakowski matrix by evaluating the action of the Liouvillian on the orthonormal $\mathrm{SU}(N$) basis matrices and then computing a sum of matrix-product traces [M. J. W. Hall et al., Phys. Rev. A 89, 042120 (2014)]. We also show that the Kossakowski matrix can be calculated using an alternative approach based on the Moore-Penrose pseudo-inverse of a rectangular tensor composed of the structure constants of $\mathrm{SU}(N$). Using these approaches, we find that the eigenvalues of the Kossakowski matrix are identical for the $\mathrm{\Lambda }$ and V systems, implying hitherto undetected similarities in their dissipative dynamics, further elucidated by examining the diagonal Lindblad operators constructed from Kossakowski matrix spectra. The approaches explored here enable the extraction of the Kossakowski matrix directly from the Liouvillian in the system eigenstate basis and do not require one to provide an operator form of the quantum master equation. They are expected to be particularly advantageous in situations where quantum master equations are defined only by their Liouvillians, facilitating valuable insights into dissipative processes in open multilevel quantum systems of prime importance in precision measurement, photosynthetic light harvesting, and quantum thermodynamics.

Quantum machine learning (QML), as an extension of classical machine learning that harnesses quantum mechanics, facilitates efficient learning from data encoded in quantum states. Training a quantum neural network (QNN) typically demands a substantial labeled training set for supervised learning. Human annotators, often experts, provide labels for samples through additional experiments, adding to the training cost. To mitigate this expense, there is a quest for methods that maintain model performance over fully labeled data sets while requiring fewer labeled samples in practice, thereby extending few-shot learning to the quantum realm. Quantum active learning (QAL) estimates the uncertainty of quantum data to select the most informative samples from a pool for labeling. Consequently, a QML model is supposed to accumulate maximal knowledge as the training set comprises labeled samples selected via sampling strategies. Notably, the QML models trained within the QAL framework are not restricted to specific types, allowing performance enhancement from the model architecture's perspective towards few-shot learning. Recognizing symmetry as a fundamental concept in physics ubiquitous across various domains, we leverage the symmetry inherent in quantum states induced by the embedding of classical data for model design. We employ an equivariant QNN capable of generalizing from fewer data with geometric priors. We benchmark the performance of QAL on two classification problems, observing both positive and negative results. QAL effectively trains the model, achieving performance comparable to that on fully labeled data sets by labeling less than 7% of the samples in the pool with unbiased sampling behavior. Furthermore, we elucidate the negative result of QAL being overtaken by random sampling baseline through miscellaneous numerical experiments. Our work lays the groundwork for real-world applications of QML by addressing realistic experimental costs.

Hybrid systems consisting of superconducting circuits and mechanical resonators are a promising platform for quantum technologies. However, realizing high-performance preparation and manipulation of phonon states remains an outstanding challenge due to the complex energy levels and interactions of hybrid systems. Here, we propose a noise-resistant phonon manipulation scheme. Using engineered superconducting-resonator dispersive interactions, we tailor a control sequence combined with dynamical decoupling pulses, which forms the basis for a universal set of phonon gate operations. Using this gate design, we demonstrate the preparation of Schrödinger cat states of phonons, the creation of complex phonon superposition states, and the encoding and transfer of quantum information into phonon states. In addition, we show that the fidelity of the state preparation can reach 95% under realistic parameters and that these designs significantly reduce the error effects compared to existing schemes. These results provide a complete toolbox for superconducting-mechanics hybrid systems.

Robust quantum control can achieve noise resilience of quantum systems and quantum technological devices. While the need for noise resilience grows with the number of fluctuating quantities, and thus typically with the number of qubits, most numerically exact optimal control techniques are limited to systems of few interacting qubits. This paper exploits quantum control that avoids explicit reference to quantum states in exponentially large Hilbert space. Exemplary control protocols for spin chains are discussed in terms of noise-resilient preparation of highly entangled states.

We investigate the higher repulsive branches of one-dimensional (1D) bosonic and fermionic quantum gases beyond the super-Tonks-Girardeau regime, utilizing the Bethe-Ansatz method and exact diagonalization of small trapped clusters. In contrast to the well-studied lowest branches that are characterized by spin-charge separation, we demonstrate the emergence of strong spin-charge correlation in all higher branches with hard-core interactions. This manifests in distinct quasimomentum distributions and energy spectra for bosons and spin-1/2 fermions, despite their fermionization. Furthermore, trapped fermions in higher branches exhibit novel spin textures, intricately linked to charge excitations, necessitating a coupled multichain description beyond single effective spin-chain models. Our findings unveil a rich interplay between spin and charge degrees of freedom in highly excited 1D systems, opening avenues for exploring novel quantum phenomena beyond the conventional paradigm of low-lying states.

Precise control of the transport of ultracold atoms in optical lattices is essential for the exploration of quantum phenomena and the development of advanced quantum technologies. Harnessing the distinctive characteristics of the system near integrability, we investigate the tunneling control of dipolar bosons confined within a triple-well circuit via dipole polarization orientation. Based on this control mechanism, we propose a quantum magnetic compass. Remarkably, this device achieves high sensitivity to small changes in magnetic-field orientation, using a nonentangled initial state, making it a promising candidate for sensing applications.

Subcycle optical pulse is of great importance for ultrafast science and technology. While a narrower pulse can offer a higher temporal resolution, so far the pulse width has not reached the limit of half an optical cycle. Here we propose to generate deep-sub-cycle (i.e., less than half a cycle, sub-half-cycle) ultrafast optical pulses that break the half-cycle limit via inverse Compton scattering of a deep-subwavelength-confined THz optical driving field by relativistic electrons. Quantitatively, based on a deep-subwavelength-confined 0.4-THz 1-ps pulsed driving field, we obtain a 0.095-cycle 3.6-fs-width mid-infrared femtosecond pulse with a peak frequency of ∼26 THz (∼38 fs in optical cycle) by using a 3-MeV 1-fs electron bunch, and a 0.091-cycle 180-as-width visible attosecond pulse with a peak frequency of ∼0.49 PHz (∼2.0 fs in optical cycle) by using a 15-MeV 50-as electron bunch, respectively. Below the optical damage threshold of the sapphire prisms used for field confinement, the single-pulse photon number can exceed ${10}^5$. Such pulses may open opportunities for studying light-matter interaction on the deep-sub-cycle level, and pave a way to unprecedented optical technology ranging from temporal super-resolution optical microscopy and spectroscopy to unconventional atom/molecule polarization and manipulation.

Quantum physics can be extended into the complex domain by considering non-Hermitian Hamiltonians that are parity-time ($\mathcal{PT}$) symmetric. These exhibit exceptional points (EPs) where the eigenspectrum changes from purely real to purely imaginary values and have useful properties enabling applications such as accelerated entanglement generation and the delay of the sudden death of entanglement in noisy systems. An interesting question is whether similar beneficial effects can be achieved away from EPs, since this would extend the available parameter space and make experiments more accessible. We investigate this by considering a $\mathcal{PT}$-symmetric optomechanical system but also consider what happens when two-mode squeezing interactions are included, taking us into the pseudo-Hermitian regime. The addition of squeezing is motivated by an attempt to extend the lifetime of the system's entanglement. While this does not prove to be the case, rich dynamics are nonetheless observed in both the pseudo-Hermitian and $\mathcal{PT}$-symmetric systems, including the sudden death and revival of entanglement under certain conditions. In both cases, we find that the sudden disappearance of entanglement can be mitigated at EPs and also show that the revival of entanglement is quite robust to thermal noise in a group of parameters away from the EPs. This investigation extends our understanding of non-Hermitian systems and opens a new perspective for the development of quantum devices in non-Hermitian systems even away from EPs.

Efficient telecom frequency conversion (TFC) in atomic systems is crucial for integrating atom-based quantum nodes into low-loss fiber-optic quantum networks. Here, we demonstrate high-efficiency TFC from 795 to 1367 nm in a cold ${}^{87}\mathrm{Rb}$ ensemble via diamond-type four-wave mixing (FWM), achieving conversion efficiencies of 66 and 80% at optical depths of 75 and 110, respectively, using a weak coherent probe field. These results surpass all previously reported values in atomic systems, enabled by a systematic investigation of the built-in V-type and cascade-type electromagnetically induced transparency spectra that guided the optimization of FWM conditions. Although this work employs coherent fields, our previous theoretical study has shown that quantum states can be preserved with high fidelity during the conversion process, highlighting the promise of diamond-type atomic FWM as a robust interface for long-distance quantum communication.