Quantum entanglement in high-dimensional Hilbert space offers broad prospects for quantum information science. Polar molecules have a rich internal structure and long coherence time, serving as an attractive candidate for quantum information processing. In this paper, we propose a theoretical scheme for creating high-dimensional entangled states based on ultracold SrO molecules. Qudits are encoded in the pendular states induced by an external electric field and coupled by the dipole-dipole interaction. With the assistance of optimal control theory, a series of optimal microwave fields is designed for the generation of an entangled qutrit-qubit state, qutrit-qutrit state, and ququart-ququart state through numerical iterations. We detail the relation of the fidelity and entanglement of the systemic final states with the number of iteration steps after the control fields are applied and analyze the population dynamics of the field-driven wave functions during time evolution. Moreover, three coupled SrO molecules arranged in an equilateral triangle configuration are employed as pendular qubits, and the trimolecular $W$ and Greenberger-Horne-Zeilinger states are realized with high fidelities by optimal control. The specific molecule and experimental parameters used in our theoretical studies are chosen for computational convenience, but we include a discussion of how our results can be applied to realistic situations as well. In principle, our results could be extended to a situation with more pendular qudits, providing a significant step toward the achievement of high-dimensional quantum information processing with arrays of polar molecules.

We study the formation of a chiral mode in a one-dimensional bipartite optical potential with a double-well dispersion. The geometry of the first excited state is tuned such that two degenerate energy minima are formed at the quasimomentum $q=\pm \hslash k_0/2$ of the first Brillouin zone. Based on the coupled-mode theory, an unconventional superfluid order $p_q\pm i{p}_{-q}$ is formed between the interacting condensate components within the double-well dispersion band under well-defined phase-locked steady states.

We propose a scheme to generate a persistent current in driven-dissipative systems which can be described by the generalized Gross-Pitaevskii (GP) equation. Our proposal consists of fabricating a rachet-potential shape of the loss-rate profile, which simultaneously breaks the time-reversal symmetry and the parity-inversion symmetry. Unlike existing schemes to generate a current using a rachet potential in Hermitian systems, no dynamic drive is needed. The basic physics of our scheme is discussed by a simple discrete driven-dissipative GP model, and the results are also verified by a realistic continuous model. Furthermore, we demonstrate the experimental feasibility of our scheme to generate the persistent current in exciton-polariton condensates in a semiconductor microcavity.

We develop a realistic protocol to observe the robust topological dynamics of two-particle bound states in a lattice model with on-site interactions and suitably designed time-dependent hoppings. This Floquet scheme can be realistically implemented on existing digital quantum computer platforms. Marked differences from the topological single-particle dynamics of two independent particles and clear signatures of the entanglement between the two constituent particles are highlighted.

We investigate the response to radio-frequency driving of an ultracold gas of attractively interacting fermions in a one-dimensional optical lattice. We study the system dynamics by monitoring the driving-induced population transfer to a third state, and the evolution of the momentum density and pair distributions. Depending on the frequency of the radio-frequency field, two different dynamical regimes emerge when considering the evolution of the third-level population. One regime exhibits (off-)resonant many-body oscillations reminiscent of Rabi oscillations in a discrete two-level system, while the other displays a strong linear rise. Within this second regime, we connect, via linear response theory, the extracted transfer rate to the system single-particle spectral function and infer the nature of the excitations from Bethe ansatz calculations. In addition, we show that this radio-frequency technique can be employed to gain insights into this many-body system coupling mechanism away from equilibrium. This is done by monitoring the momentum density redistributions and the evolution of the pair correlations during the drive. Capturing such nonequilibrium physics goes beyond a linear response treatment and is achieved here by conducting time-dependent matrix-product-state simulations.

Photoionization of atoms and molecules by intense femtosecond laser pulses is a fundamental process of strong-field physics. Using a three-color femtosecond laser scheme with attosecond phase control precision, we demonstrate coherently controlled ionization of nitrogen molecules with a modulation level up to 45% by varying the phase shifts between the fundamental laser frequency at 800 nm and its second and third harmonics. Furthermore, the phase dependence of the ionization degree qualitatively changes with the laser intensity ratios between the three colors. The observations are interpreted as a manifestation of the competition between different parametric channels contributing to the ionization process. Such coherent control of ionization opens different ways to finely tune and optimize various phenomena accompanying laser-material interactions: high-order harmonic and attosecond generation, nanofabrication, remote ablation of samples, and even guidance of discharge and control of lightning by lasers.

Magnetoelectric materials have the interesting property of exhibiting polarization induced by a magnetic field or magnetization induced by an electric field. As a consequence, a multitude of effects can be produced by means of controllable external fields. A method for deriving phase velocities and its corresponding polarization vectors for light rays in nonlinear optical materials in a nondispersive regime is here revisited and used to study wave propagation in a certain class of second-order magnetoelectric media. In particular, the birefringence effect is theoretically examined, and it is shown that it can be used as a tool to obtain most of the second-order magnetoelectric coefficients ${\beta }_{ijk}$ of a material having an isotropic linear sector. Estimates of the effect are presented. Some optical properties of nonlinear materials presenting a natural optic axis are also discussed.

Non-Hermitian topological photonics possesses abundant and complicated physical effects and phenomena, which have great potential and important applications in integrated photonic devices. However, the light localization properties and approaches for active control of topological states have not been revealed sufficiently so far. Here we prove the phase transition exists in the interface between gain and loss domains of a parity-time symmetric $C_{6\mathrm{v}}$ topological photonic crystal from its projected bands. The non-Hermitian topological cavity formed by closed gain-loss boundary has unique resonant responses and better light localization properties compared with the Hermitian counterpart. The cavity modes are composed of edge states and corner states whose eigenfrequencies change with gain-loss quantity in the same way as the projected band. The maximum value of the quality factor of non-Hermitian topological cavity reaches ${10}^4$, which is enlarged by one order compared with the Hermitian counterpart. At the same time, the mode volume of non-Hermitian topological cavity is reduced by over tenfold compared with the Hermitian counterpart. The non-Hermitian induced edge states and corner states may open a new route for the design of the topologically protected and reconfigurable localized resonances. This work provides a new degree of freedom to control different topological states and offers a scheme for solving the contradiction between the flexibility of devices and the robustness of topological properties.

We describe some of the main external mechanisms that lead to a loss of antibunching, i.e., that spoil the character of a given quantum light to deliver its photons separated from each other. Namely, we consider contamination by noise, a time jitter in the photon detection, and the effect of frequency filtering (or detection with finite bandwidth). The formalism to describe time jitter is derived and connected to the already existing one for frequency filtering. The emission from a two-level system under both incoherent and coherent driving is taken as a particular case of special interest. The coherent case is further separated into its vanishing- (Heitler) and high- (Mollow) driving regimes. We provide analytical solutions which, in the case of filtering, reveal an unsuspected structure in the transitions from perfect antibunching to thermal (incoherent case) or uncorrelated (coherent case) emission. The experimental observations of these basic and fundamental transitions would provide additional compelling evidence of the correctness and importance of the theory of frequency-resolved photon correlations.

We propose a general and feasible approach to realize a large number of squeezed spatial modes. This is achieved by manipulation of paraxial diffraction such that the critical wave components with most significant squeezing contribute in-phase to the spatial squeezing. As an example, we then demonstrate that it is possible to achieve localized squeezing of $\sim -1.51\text{dB}$ at an area ${10}^2\mu {\text{m}}^2$ within a homogeneously squeezed spatial regime of $w_p=1{\text{mm}}^2$ using four-wave mixing (FWM) based on current experimental settings, corresponding to approximately ${10}^4$ squeezed spatial modes, which is $>{10}^2$ larger in number of squeezed modes and also $\sim 6$ times stronger in squeezing as compared to that obtained in the state-of-the-art experiment. We also show that the obtained extremely localized squeezed light can be directly applied to enhance the signal-to-noise ratio in quantum imaging of weakly absorbing objects by a factor of $\sim 3.5$ at a spatial resolution of $d\sim 1\text{mm}$ where $d$ is the detector size.

Synchronization in quantum systems has been largely driven by specific examples of frequency entrainment as well as mutual synchronization. Here we study quantum synchronization as a Liouville space perturbation theory. We begin by clarifying the role of centers, symmetries, and oscillating coherences in the context of quantum synchronization. We then analyze the eigenspectrum of the Liouville superoperator generating the dynamics of the quantum system and determine the conditions under which synchronization arises. We apply our framework to derive a powerful relationship between energy conservation, degeneracies, and synchronization in quantum systems. Finally, we demonstrate our approach by analyzing two mutually coupled thermal machines and prove that nondegenerate thermal networks cannot be simultaneously energy conserving and synchronous.

We consider tight-binding single-particle lattice Hamiltonians which are invariant under an antiunitary antisymmetry: the anti-$\mathcal{PT}$ symmetry. The Hermitian Hamiltonians are defined on $d$-dimensional non-Bravais lattices. For an odd number of sublattices, the anti-$\mathcal{PT}$ symmetry protects a flatband at energy $E=0$. We derive the anti-$\mathcal{PT}$ constraints on the Hamiltonian and use them to generate examples of generalized kagome networks in two and three lattice dimensions. Furthermore, we show that the anti-$\mathcal{PT}$ symmetry persists in the presence of uniform DC fields and ensures the presence of flatbands in the corresponding irreducible Wannier-Stark band structure. We provide examples of the Wannier-Stark band structure of generalized kagome networks in the presence of DC fields, and their implementation using Floquet engineering.

We investigate parity-time ($\mathcal{PT}$) phase transitions in open quantum systems and discuss a criterion of Liouvillian $\mathcal{PT}$ symmetry proposed recently by Huber et al. [SciPost Phys. 9, 52 (2020)]. Using the third quantization, which is a general method to solve the Lindblad equation for open quadratic systems, we show, with a proposed criterion of $\mathcal{PT}$ symmetry, that the eigenvalue structure of the Liouvillian clearly changes at the $\mathcal{PT}$-symmetry-breaking point for an open two-spin model with exactly balanced gain and loss if the total spin is large. In particular, in a $\mathcal{PT}$-unbroken phase, some eigenvalues are pure imaginary numbers, while in a $\mathcal{PT}$-broken phase, all the eigenvalues are real. From this result, it is analytically shown for an open quantum system including quantum jumps that the dynamics in the long-time limit changes from an oscillatory to an overdamped behavior at the proposed $\mathcal{PT}$-symmetry-breaking point. Furthermore, we show a direct relation between the criterion of Huber et al. of Liouvillian $\mathcal{PT}$ symmetry and the dynamics of the physical quantities for quadratic bosonic systems. Our results support the validity of the proposed criterion of Liouvillian $\mathcal{PT}$ symmetry.

We propose a level-resolved protocol in a hybrid architecture for connecting a superconducting qubit and a magnon mode contained within a microwave cavity (resonator) to generate local and global entangled states, enabling a wide range of applications in quantum communication, quantum metrology, and quantum information processing. Exploiting the high-degree controllability in such a hybrid qubit-photon-magnon system, we derive the effective Hamiltonians at the second-order resonant points by virtue of the strong general Rabi interactions between the resonator and the qubit and between the resonator and the magnon. Consequently, we can efficiently generate the Bell states of the photon-magnon and the qubit-magnon subsystems and the Greenberger-Horne-Zeilinger state of the whole hybrid system. The robustness of our protocol is checked against a nonvanishing tuning time of system frequency and the environmental noise by the Lindblad master equation. Our work makes this hybrid platform of high-degree controllability a high-fidelity candidate for preparing the multiple maximally entangled states.

Spin squeezing—a central resource for quantum metrology—can be generated via the nonlinear, entangling evolution of an initially factorized spin state. Here we show that robust (i.e., persistent) squeezing dynamics is generated by a very large class of $S=1/2$ spin Hamiltonians with axial symmetry, in relationship with the existence of a peculiar structure of the low-lying Hamiltonian eigenstates—the so-called Anderson's tower of states. Such states are fundamentally related to the appearance of spontaneous symmetry breaking in quantum systems; and, for models with sufficiently high connectivity, they are parametrically close to the eigenstates of a planar rotor (Dicke states), in that they feature an anomalously large value of the total angular momentum. Our central insight is that starting from a coherent spin state, a generic U(1)-symmetric Hamiltonian featuring the Anderson's tower of states generates the same squeezing evolution at short times as the one governed by the paradigmatic one-axis-twisting (or planar-rotor) model of squeezing dynamics. The full squeezing evolution of the planar-rotor model is seemingly reproduced for interactions decaying with distance $r$ as $r^{-\alpha }$ when $\alpha <5d/3$ in $d$ dimensions. Our results connect quantum simulation with quantum metrology by unveiling the squeezing power of a large variety of Hamiltonian dynamics that are currently implemented by different quantum simulation platforms.

Since the ${}^4\mathrm{He}$ dimer supports only one weakly bound state with an average interatomic distance much larger than the van der Waals length and no deeply bound states, ${}^4\mathrm{He}_N$ clusters with $N>2$ are a paradigmatic model system with which to explore foundational concepts such as large $s$-wave scattering length universality, van der Waals universality, Efimov physics, and effective field theories. This work presents structural properties such as the pair and triple distribution functions, the hyper-radial density, the probability to find the $N\mathrm{th}$ particle at a given distance from the center of mass of the other $N-1$ atoms, and selected contacts. The kinetic energy release, which can be measured via Coulomb explosion in dedicated size-selected molecular beam experiments—at least for small $N$ – is also presented. The structural properties are determined for three different realistic ${}^4\mathrm{He}-{}^4\mathrm{He}$ interaction potentials and contrasted with those for an effective low-energy potential model from the literature that reproduces the energies of ${}^4\mathrm{He}_N$ clusters in the ground state for $N=2$ to $N=\infty$ at the $\gtrsim$95% level with just four input parameters. The study is extended to unitarity (infinite $s$-wave scattering length) by artificially weakening the interaction potentials. In addition to contributing to the characterization of small bosonic helium quantum droplets, our study provides insights into the effective low-energy theory's predictability of various structural properties.

The ${\mathrm{AcOH}}^{+}$ molecular ion is identified as a prospective system to search for $\mathcal{CP}$-violation effects. According to our study ${\mathrm{AcOH}}^{+}$ belongs to the class of laser-coolable polyatomic molecular cations implying a large coherence time in the experiments to study symmetry-violating effects of fundamental interactions. We perform both nuclear and high-level relativistic coupled cluster electronic structure calculations to express the experimentally measurable $\mathcal{T},\mathcal{P}$-violating energy shift in terms of fundamental quantities such as the nuclear magnetic quadrupole moment (MQM), electron electric dipole moment ($e\mathrm{EDM}$) and dimensionless scalar-pseudoscalar nuclear-electron interaction constant. We further express nuclear MQM in terms of the strength constants of $\mathcal{CP}$-violating nuclear forces: quantum chromodynamics vacuum angle $\overline{\theta }$ and quark chromo-EDMs. The equilibrium structure of ${\mathrm{AcOH}}^{+}$ in the ground and the four lowest excited electronic states was found to be linear. The calculated Franck-Condon factors and transition dipole moments indicate that the laser cooling using an optical cycle involving the first excited state is possible for the trapped ${\mathrm{AcOH}}^{+}$ ions with the Doppler limit estimated to be $\sim$4 nK. The lifetime of the ($0,1^1,0$) excited vibrational state considered as a working one for MQM and $e\mathrm{EDM}$ search experiments is estimated to be $\sim 0.4$ s.

We study multilevel fermions in an optical lattice described by the Hubbard model with on-site $\mathrm{SU}\left(n\right)$-symmetric interactions. We show that in an appropriate parameter regime this system can be mapped onto a spin model with all-to-all $\mathrm{SU}\left(n\right)$-symmetric couplings. Raman pulses that address internal spin states modify the atomic dispersion relation and induce spin-orbit coupling, which can act as a synthetic inhomogeneous magnetic field that competes with the $\mathrm{SU}\left(n\right)$ exchange interactions. We investigate the mean-field dynamical phase diagram of the resulting model as a function of $n$ and different initial configurations that are accessible with Raman pulses. Consistent with previous studies for $n=2$, we find that for some initial states the spin model exhibits two distinct dynamical phases that obey simple scaling relations with $n$. Moreover, for $n>2$ we find that dynamical behavior can be highly sensitive to initial intraspin coherences. Our predictions are readily testable in current experiments with ultracold alkaline-earth-metal(-like) atoms.