We analyze the impact of spin-orbit and Rabi couplings on the dynamical stability of spin-orbit-coupled spin-1 Bose-Einstein condensates for ferromagnetic (FM) and antiferromagnetic (AFM) interactions. Determining the collective excitation spectrum through Bogoliubov–de Gennes theory, we characterize the dynamical stability regime via modulational instability. For AFM interactions, the eigenspectrum reveals the presence of both stable and unstable avoided crossings (UACs), with the first-excited branch undergoing a double unstable avoided crossing. In contrast, with ferromagnetic interactions, only a single UAC, which occurs between the low-lying and first-excited branches, is observed. Furthermore, the eigenvectors demonstrate the transition from densitylike to spinlike behavior, as the collective excitation shows the transition from the stable to the unstable mode for both the FM and AFM interactions. In the multiband instability state, eigenvectors display spin-density mixed mode, while they show spin-flip nature in the avoided crossing regime. Our analysis suggests that spin-orbit coupling enhances the instability gain, while Rabi coupling plays the opposite role. Finally, we corroborate our analytical findings of stable and unstable regimes through numerical simulations of the dynamical evolution of the condensates by introducing the perturbations upon quenching the trap strength. The dynamical phases show the formation of complex domains with AFM interaction, which may be attributed to the double unstable avoided crossings in such a system.

We demonstrate the effect of pulse shaping in momentum-selective atomic Bragg diffraction. We compare temporal square pulses, which produce sidelobes in momentum space, with other shapes that can produce more nearly square momentum distributions. We produce pulses that simultaneously address two sets of velocity classes and demonstrate that we can control the differential phase imprinted on them in a way that is insensitive to laser phase fluctuations. Our work marks a significant step forward in testing Bell inequalities using massive particles entangled in momentum.

Since the first experimental realization of Bose-Einstein condensates in a spin-orbit-coupled Zeeman lattice, a wide range of applications have been found in these systems. Here, we systematically study the ground-state phase diagram of the systems. The spin-orbit-coupled Zeeman lattice generates multiple energy minima in the lowest Bloch band. These minima may locate at Brillouin-zone edges and center, or inside the Brillouin zone. Interacting atoms condensing into these minima constitute ground-state quantum phases. Among these phases, the band-edge condensation states have no analog in either spin-orbit-coupled homogeneous systems or lattice systems without the spin-orbit coupling. They exist in a very broad parameter regime of the spin-orbit-coupled Zeeman lattice system. We address superfluidity of the band-edge states by analyzing their elementary excitations and superfluid fraction.

Pulsation dynamics of laser cavity solitons is studied numerically in broad-area semiconductor lasers with an intracavity saturable absorber with and without carrier transverse diffusion. We show that the existence range of the self-pulsing laser cavity solitons and their oscillation dynamics are affected by the carrier transverse diffusion such that larger diffusion coefficients lead to self-pulsation dynamics of smaller amplitude and shorter period. Moreover, we studied the interaction of two self-pulsing structures and found regimes of in-phase, antiphase, and partially antisynchronized oscillations depending on the control parameter values and the initial separation distance. The effect of carrier transverse diffusion on the interaction dynamics is also investigated, and it is found that the synchronization time increases with the diffusion coefficient. Synchronization-driven spatial adjustment of self-pulsing laser cavity solitons is also reported, where attractive or repulsive forces are observed to form between the dynamical localized structures.

In some anisotropic bulk media (for example, biaxial weakly absorbing crystals) there are special directions along which the plane-wave field distribution has a singular profile of the form $\propto \left(\mathbf{n}\mathbf{r}\right)exp\left(iq\mathbf{n}\mathbf{r}\right)$. They are also known as Voigt waves. Similar singular profiles also arise in the theory of surface electromagnetic waves in anisotropic media. In this work we consider surface polaritons at the interface of two generally different uniaxial media. The optic axes of both media are parallel to the interface. One of the specific solutions, called bisingular, greatly simplifies the dispersion equation for surface polaritons. In this case the analytical solution in closed form is found, and existence conditions are determined. It is shown that a bisingular surface polariton exists only for certain angles between the optic axes, which are found from two cubic equations. All the parameters of the bisingular surface polariton depend only on permittivities and this angle. If one medium is weakly anisotropic or both media are almost the same, then two angles exist. In the general case of two arbitrary media there can be from two to six such angles.

The precise characterization of dynamics in open quantum systems often presents significant challenges, leading to the introduction of various approximations to simplify a model. One commonly used strategy involves Markovian approximations, assuming a memoryless environment. In this study, such approximations are not used and an analytical dynamical depiction of an open quantum system is provided. The system under consideration is an oscillator that is surrounded by a bath of oscillators. The resulting dynamics are characterized by an integrodifferential equation, which can be solved numerically. Remarkably, an exactly analytical solution exists when its reservoir spectral density is Lorentzian. Furthermore, this analytical solvability remains when rectangular pulse control is applied to the system, though with different parameters due to the pulses. Moreover, distinct dynamical regions emerge, depending on certain parameter values. Notably, the steady-state average excitation number (AEN) of the system shows rapid escalation with increasing non-Markovianity, reflecting the intricacies of real-world dynamics. In cases where there is detuning between the system frequency and the environmental central frequency within a non-Markovian regime, the AEN maintains its initial value for an extended period. Interestingly, the exact analytical solution with pulse control inherently protects the quantum system from decoherence effects without using approximations, while this protection fades as the system approaches the Markovian regime. The pulse control can not only prolong the relaxation time of the oscillator, but can also be used to speed up the relaxation process, depending on the specifications of the pulse.

The power and expressivity of deep classical neural networks can be attributed to nonlinear input-output relations. Such nonlinearities are at the heart of many computational tasks, such as data classification and pattern recognition. Quantum neural networks, on the other hand, are necessarily linear as they process information via unitary operations. Here we show that effective nonlinearities can be implemented in these platforms by exploiting the relationship between information processing and many-body quantum dynamics. The crucial point is that quantum many-body systems can show emergent collective behavior in the vicinity of phase transitions, which leads to an effectively nonlinear dynamics in the thermodynamic limit. In the context of quantum neural networks, which are necessarily finite, this translates into metastability with transient nonergodic behavior. By using a quantum neural network the architecture of which is inspired by dissipative many-body quantum spin models, we show that this mechanism indeed allows one to realize nonlinear data classification, despite the underlying dynamics being local and linear. Our proof-of-principle study may pave the way for the systematic construction of quantum neural networks with emergent nonlinear properties.

Quantum optimization, for both classical and quantum functions, is one of the most well-studied applications of quantum computing, but recent trends have relied on hybrid methods that push much of the fine-tuning off onto costly classical algorithms. Feedback-based quantum algorithms, such as FALQON, avoid these fine-tuning problems but at the cost of additional circuit depth and a lack of convergence guarantees. In this work we take the local greedy information collected by Lyapunov feedback control and develop an analytic framework to use it to perturbatively update previous control layers, similar to the global optimal control achievable using Pontryagin optimal control. This perturbative methodology, which we call feedback-based optimally controlled quantum states (FOCQS), can be used to improve the results of feedback-based algorithms, like FALQON, providing better convergence but still no formal guarantees of convergence. Furthermore, this perturbative method can be used to push smooth annealing-like control protocol closer to the control optimum, even providing an iterative approach, albeit with diminishing returns. In numerical testing, we show improvements in convergence and required depth due to these methods over existing quantum feedback control methods.

Demonstrations of quantum advantage for certain sampling problems have generated considerable excitement for quantum computing and have further spurred the development of circuit-model quantum computers, which represent quantum programs as a sequence of quantum gates acting on a finite number of qubits. Amongst this excitement, analog quantum computation has become less prominent, with the expectation that circuit-model quantum computers will eventually be sufficient for emulating analog quantum computation and thus rendering analog quantum computation obsolete. In this work we explore the basic requirements for emulating a specific analog quantum computation in the circuit model: the preparation of a biased superposition of degenerate ground states of an Ising Hamiltonian using an adiabatic evolution. We show that the overhead of emulation is substantial even for this simple problem. This supports using analog quantum computation for solving time-dependent Hamiltonian dynamics in the short term and midterm, assuming analog errors can be made low enough and coherence times long enough to solve problems of practical interest.

Secure quantum remote sensing (SQRS) uses quantum states to gather information about distant objects or environments while ensuring secure data transmission against eavesdropping. It has potential applications in various fields, including environmental monitoring, military surveillance, and disaster response, where both data accuracy and transmission security are critical. Recent experiments have demonstrated the feasibility of SQRS using entanglement states. Here, we experimentally demonstrate an SQRS that can estimate a phase without requiring entanglement, offering the practical advantage that single-qubit states are easier to prepare. We successfully estimate the preset phase information at a remote site over a fiber distance of 50 km, which serves as a key step toward long-distance applications.

Atom-cavity systems offer unique advantages for building large-scale distributed quantum computers by providing strong atom-photon coupling while allowing for high-fidelity local operations of atomic qubits. However, in prevalent schemes where the photonic state is encoded in polarization, cavity birefringence introduces an energy splitting of the cavity eigenmodes and alters the polarization states, thus limiting the fidelity of remote entanglement generation. To address this challenge, we propose a scheme that encodes the photonic qubit in the frequency degree of freedom. The scheme relies on resonant coupling of multiple transverse cavity modes to different atomic transitions that are well separated in frequency. We numerically investigate the temporal properties of the photonic wave packet, two-photon interference visibility, and atom-atom entanglement fidelity under various cavity polarization-mode splittings and find that our scheme is less affected by cavity birefringence. Finally, we propose practical implementations in two trapped ion systems, using the fine-structure splitting in the metastable D state of ${}^{40}{\mathrm{Ca}}^{+}$ and the hyperfine splitting in the ground state of ${}^{225}{\mathrm{Ra}}^{+}$. Our study presents an alternative approach for cavity-based quantum networks that is less sensitive to birefringent effects and is applicable to a variety of atomic and solid-state emitter-cavity interfaces.

Coupled-perturbed (CP) computational methods, in particular those based on the density functional theory (DFT), are indispensable in interpretations of Raman scattering on molecules. The simulations are most often based on the Placzek approximation, ignoring the dependence of energies of the excited electronic states on nuclear coordinates. Thus practical CP DFT computations do not exactly correspond to the more general sum over state (SOS) formalism. We discuss this problem, investigate the SOS convergence, and formulate a distributed origin gauge enabling us to calculate origin-independent Raman optical activity intensities. The variations of the computed spectra caused by the energy gradient terms are investigated with practical examples. The results show that although the contribution of the gradient parts to Raman intensities is rather small in the far from resonance cases, their subtraction may still occasionally improve the simulated spectra. However, the effect is much more important for Raman optical activity and in resonance conditions. The possibility to correct the computed intensities for the gradient contribution thus opens a way for systematic improvements of quantum-mechanical simulations of the spectra.

The intriguing nonlinear optical response of monolayer transition-metal dichalcogenides offers exciting opportunities for both fundamental research and practical applications. In this work, terahertz-modulated harmonic generation in monolayer molybdenum disulfide is explored through numerical simulations and semiclassical trajectory analysis. We find that the yields of both odd- and even-order harmonics, along with their ratios, can be modulated by the intensity of terahertz pulse. We use the subcycle interference model to elucidate the underlying mechanism of these manipulations. The terahertz pulse alters the trajectory of the electrons excited by the midinfrared pulse, introducing additional dipole phases and enabling the change of the harmonic yields. By varying the delay between the midinfrared and terahertz pulses, the temporal distribution of the terahertz intensity can be reconstructed from the delay-dependent harmonic yields, highlighting the potential for ultrabroadband and ultrasensitive terahertz time-domain spectroscopy.

We compare the high-harmonic-generation (HHG) yield driven by a midinfrared laser field in three organic ring-shaped molecules, calculated using time-dependent density-functional theory (TDDFT). We average the yield over the relative orientation of the molecules and the linearly polarized, 1825-nm driving laser pulse to compare to experimental spectra obtained by Alharbi et al., [Phys. Rev. A 92, 041801 (2015)]. We find that the raw TDDFT-calculated HHG yield in cyclohexane is strongly overestimated compared to those of benzene and cyclohexene, and that this can be attributed to unphysically large contributions from cyclohexane orbitals lying well below the highest-occupied molecular orbital. We demonstrate this by implementing a simple orbital-resolved scaling factor that adjusts the tunneling-ionization contribution to the harmonic yield. The rescaled yields are in much better agreement with experimental results, and in particular, reproduce the correct relative-yield ordering of the three molecules. Our results are encouraging for the use of TDDFT in systematic computations of HHG in large molecules.

Cold atomic clouds constitute highly resonant nonlinear optical media, whose refractive index can easily be tuned via the light frequency. When subjected to a retroreflected laser beam and under appropriate conditions, the cloud undergoes spontaneous symmetry breaking, and spatial patterns develop in the transverse cross section of the beam. We investigate the impact of the sign of the light detuning from the atomic resonance on these patterns, thus directly comparing pattern formation for self-focusing and self-defocusing nonlinearities. Our observations emphasize the need for a “diffractively thick” medium description of the light-cloud interaction in which diffraction and nonlinear propagation inside the sample are taken into account.

Recently, a different form of quantum steering, i.e., certification of quantum steering in a one-sided semi-device-independent way, has been formulated [C. Jebarathinam et al., Phys. Rev. A 108, 042211 (2023)]. In this work, we use this phenomenon to provide operational simultaneous correlations in mutually unbiased bases as quantified by the measures in [S. Wu et al., Sci. Rep. 4, 4036 (2014)]. First, we show that for any bipartite state, such measure of simultaneous correlations in two mutually unbiased bases can be operationally identified as exhibiting one-sided semi-device-independent steering in a two-setting scenario. Next, we demonstrate that for two-qubit Bell-diagonal states, quantifying one-sided semi-device-independent steerability provides an operational quantification of information-theoretic quantification of simultaneous correlations in mutually unbiased bases. Then, we provide a different classification of two-qubit separable states with the above-mentioned information-theoretic quantification of simultaneous correlations in mutually unbiased bases and the quantification of one-sided semi-device-independent steerability. Finally, we invoke quantum steering ellipsoid formalism to shed intuitions on the operational characterization of simultaneous correlations in complementary bases of two-qubit states via one-sided semi-device-independent steerability. This provides us with a geometric visualization of the results.

Differential equations (DEs) serve as the cornerstone for a wide range of scientific endeavors, their solutions weaving through the core of diverse fields such as structural engineering, fluid dynamics, and financial modeling. DEs are notoriously hard to solve, due to their intricate nature, and finding solutions to DEs often exceeds the capabilities of traditional computational approaches. Recent advances in quantum computing have triggered a growing interest from researchers for the design of quantum algorithms for solving DEs. In this work, we introduce two different architectures of a novel variational quantum algorithm (VQA) with Lagrange polynomial encoding in combination with derivative quantum circuits using the Hadamard test differentiation to approximate the solution of DEs. To demonstrate the potential of our new VQA, two well-known ordinary differential equations are used: the damped mass-spring system from a given initial condition and the Poisson equation for periodic, Dirichlet, and Neumann boundary conditions. It is shown that the proposed new VQA has a reduced gate complexity compared to previous variational quantum algorithms, for a similar or better quality of the solution.

Precise qubit control in the presence of spatiotemporally correlated noise is pivotal for transitioning to fault-tolerant quantum computing. Generically, such noise can also have non-Gaussian statistics, which hampers existing non-Markovian noise spectroscopy protocols. By utilizing frame-based characterization and a control-based symmetry analysis, we show how to achieve higher-order spectral estimation for noise-optimized circuit design. Remarkably, we find that the digitally driven qubit dynamics can be solely determined by the complexity of the applied control rather than the nonperturbative nature of the non-Gaussian environment. This enables us to address certain nonperturbative qubit dynamics more simply. We delineate several complexity bounds for learning such high-complexity noise and demonstrate our single- and two-qubit digital characterization and control using a series of numerical simulations. Our results not only provide insights into the exact solvability of (small-size) open quantum dynamics but also highlight a resource-efficient approach for optimal control and possible error-reduction techniques for current qubit devices.

Rydberg atoms are used in a wide range of applications due to their peculiar properties like strong dipolar and van der Waals interactions. The choice of Rydberg state has a huge impact on the strength and angular dependence of the interactions, and so a detailed understanding of the underlying processes and resulting properties of the interactions is therefore key to select the most suitable states for experiments. We study the van der Waals interactions in alkali-metal atoms in detail and highlight the structures which allow an understanding and exploitation of the various interaction properties. A particular theme is the identification of Förster resonances with $n_1\ne n_2$, which offer interaction potentials with a wide range of properties that make them particularly interesting for experimental applications. A second theme is a focus on the underlying structures that shape the angular dependency and sign of the interactions. This understanding — instead of brute-force calculations — allows for a much simpler and more systematic search for suitable pair states. These insights can be used for the selection of tailored interaction potentials subject to experimental constraints and requirements. We use rubidium as an example species in this work and also provide data for cesium and pair states that are coupled via two- or three-photon transitions, i.e. up to $F$ states, in the Appendix.