In this work, we demonstrate experimentally the efficient generation and tunability of energetic highly elliptical high harmonics in Ar gas, driven by intense two-color counter-rotating laser electric fields. A bichromatic beam tailored by a Mach-Zehnder-Less for Threefold Optical Virginia spiderwort (MAZEL-TOV) apparatus generates high-order harmonic generation (HHG), where the output spectrum of the highly elliptical HHG radiation can be tuned for an energy range of $\mathrm{\Delta }E\approx 150$ meV in the spectral range of $\sim 20$ eV with energy per pulse $E^{\mathrm{XUV}}\approx 400$ nJ at the source. Furthermore, we employ time-dependent density-functional simulations to probe the dependence of the harmonic ellipticity and the strength of the attosecond pulses on the driving-field parameters and demonstrate the robustness of the HHG with the bichromatic field. We show how, by properly tuning the central frequency of the second harmonic, the central frequency of the extreme ultraviolet (XUV) high-harmonic radiation is continuously tuned. The demonstrated energy values largely exceed the output energy from many other laser-driven attosecond sources reported so far and prove to be sufficient for inducing nonlinear processes in an atomic system. We envisage that such tunable energetic highly elliptical HHG spectra can remove the facility restrictions from requirements of few-cycle driving pulses for isolated circular attosecond-pulse generation.

We investigate the rotational properties of a two-component, two-dimensional self-bound quantum droplet, which is confined in a harmonic potential and compare them with the well-known problem of a single-component atomic gas with contact interactions. For a fixed value of the trap frequency, choosing some representative values of the atom number, we determine the lowest-energy state, as the angular momentum increases. For a sufficiently small number of atoms, the angular momentum is carried via center-of-mass excitation. For larger values, when the angular momentum is sufficiently small, we observe vortex excitation instead. Depending on the actual atom number, one or more vortices enter the droplet. Beyond some critical value of the angular momentum, however, the droplet does not accommodate more vortices and the additional angular momentum is carried via center-of-mass excitation in a “mixed” state. Finally, the excitation spectrum is also briefly discussed.

The optical lattice plays an important role in the stability and dynamics of quantum droplets. In this article, we investigate the Bogoliubov excitation spectrum of quantum droplets in an optical lattice in the thermodynamic limit. We classify the collective excitations as synchronous modes, Bloch phononic modes, and site-population-imbalanced modes. For synchronous modes, we measure the dipole oscillation frequencies by quench dynamics with a sudden shift of the optical lattice and the breathing frequencies by Floquet dynamics with a periodic change of the lattice depth. Bloch phononic modes are observable from the Landau critical velocity of the droplets. We further discuss the instability induced by site-dependent density fluctuations and calculate the critical filling of atoms where the growth of lattice vacancy breaks down the translational symmetry of the system. This work makes essential steps towards measuring the excitation spectrum and understanding the superfluid nature of quantum droplets in an optical lattice.

Numerical simulations of the full quantum properties of interacting many-body systems by means of field-theoretic Monte Carlo techniques are often limited due to a sign problem. Here we simulate properties of a dilute two-dimensional Bose gas in the vicinity of the Berezinskii-Kosterlitz-Thouless (BKT) transition by means of the complex Langevin (CL) algorithm, thereby extending our previous CL study of the three-dimensional Bose gas to the lower-dimensional case. The purpose of the paper is twofold. On the one hand, it adds to benchmarking of the CL method and thus contributes to further exploring the range of applicability of the method. With the respective results, the universality of the equation of state is recovered, as well as the long-wave-length power-law dependence of the single-particle momentum spectrum below the BKT transition. Analysis of the rotational part of the current density corroborates vortex unbinding in crossing the transition. Beyond these measures of consistency we compute quantum corrections to the critical density and chemical potential in the weakly coupled regime. Our results show a shift of these quantities to lower values as compared to those obtained from classical field theory. It points in the opposite direction as compared to the shift of the critical density found by means of the path-integral Monte Carlo method at larger values of the coupling. Our simulations widen the perspective for precision comparisons with experiment.

We study the SU(3) symmetric Fermi-Hubbard model (FHM) in the square lattice at $1/3$-filling using numerically exact determinant quantum Monte Carlo and numerical linked-cluster expansion techniques. We present the different regimes of the model in the $T\text{−}U$ plane, which are characterized by local and short-range correlations, and capture signatures of the metal-insulator transition and magnetic crossovers. These signatures are detected as the temperature scales characterizing the rise of the compressibility, and an interaction-dependent change in the sign of the diagonal spin-spin correlation function. The analysis of the compressibility estimates the location of the metal-insulator quantum critical point at $U_c/t\sim 6$, and provides a temperature scale for observing Mott physics at finite $T$. Furthermore, from the analysis of the spin-spin correlation function we observe that for $U/t\gtrsim 6$ and $T\sim J=4t^2/U$ there is a development of a short-range two-sublattice (2SL) antiferromagnetic structure, as well as an emerging three-sublattice (3SL) antiferromagnetic structure as the temperature is lowered below $T/J\lesssim 0.57$. This crossover from 2SL to 3SL magnetic ordering agrees with Heisenberg limit predictions, and has observable effects on the density of on-site pairs. Finally, we describe how the features of the regimes in the $T-U$ plane can be explored with alkaline-earth-like atoms in optical lattices with currently achieved experimental techniques and temperatures. The results discussed in this paper provide a starting point for the exploration of the SU(3) FHM upon doping.

We show how the chiral properties of Bose-Einstein condensates subject to current-density interactions and loaded in optical lattices can be observed in the realization of nonlinear Bloch states, whose spectrum lacks the usual periodic structure. Chirality is also manifested by spatially localized states, or gap solitons, which are found for positive rotation rates of the lattice at the energy gaps between the linear energy bands, whereas for negative rotations they appear in the semi-infinite gap of the linear spectrum. The stability of extended and localized states is checked through the spectrum of linear excitations and nonlinear time evolution of perturbed states, and the phenomenon of Bloch oscillations is explored. Our results are obtained in quasi-one-dimensional ring geometries with feasible experimental parameters.

A programmable quantum simulator using Rydberg-atom array provides a promising route to demystifying quantum many-body physics in strongly correlated systems. Motivated by the recent realization of various quantum magnetic phases on the frustrated Rydberg-atom array, we perform numerically exact quantum Monte Carlo simulations to investigate the exotic states of matter emerging in the model describing the Rydberg atom on a triangular lattice. Our state-of-the-art simulation unveils the $\sqrt{3}\times \sqrt{3}$ triangular antiferromagnetic order exists at $1/3$ or $2/3$-Rydberg filling, consistent with the observation in experiments. Remarkably, $\sqrt{3}\times \sqrt{3}$ long-range order arising from order-by-disorder mechanism emerges at $1/2$ filling. At finite temperature, $\mathrm{U}\left(1\right)$ symmetry is emergent at $1/2$ filling and a Kosterlitz-Thouless phase transition occurs with increasing temperature. These intriguing phenomena are potentially detected in future Rydberg-atom experiments.

Bound states in the continuum (BICs) exist in a variety of physical systems where they appear as lossless propagating states surrounded by radiating modes. In the case of open systems, they coexist with continuous families of guided states, which may be modes or other BICs, located in different regions of the frequency-momentum parameter space. Here we report anisotropic waveguiding structures where guided modes and BICs protected by symmetry are not possible whatsoever, though an isolated, single interference BIC emerges as a lossless, solitary needle from a sea of radiating states. The needle BIC is the unique possible bound state, which originates from the interplay of the two different radiation channels present in the structure, and remarkably, it exists at a single frequency and a precise propagation direction as a spectrally discrete bound state for any practical range of frequencies.

In S. N. Balybin et al. [Phys. Rev. A 106, 013720 (2022)] the scheme of quantum nondemolition measurement of optical quanta that uses a resonantly enhanced Kerr nonlinearity in optical microresonators was analyzed theoretically. It was shown that using modern high-$Q$ microresonators, it is possible to achieve sensitivity several times better than the standard quantum limit. Here we propose and analyze in detail a significantly improved version of that scheme. We show that by using a squeezed quantum state of the probe beam and the antisqueezing (parametric amplification) of this beam at the output of the microresonator, it is possible to reduce the measurement imprecision by about one order of magnitude. The resulting sensitivity allows us to generate and verify multiphoton non-Gaussian quantum states of light, making the scheme considered here interesting for quantum information processing tasks.

We propose an efficient method to generate single-phonon Fock states by exploiting a previously unidentified phonon blockade mechanism in a hybrid spin-mechanical system. Specifically, this mechanism makes use of arbitrarily weak Duffing nonlinearity to modify matrix elements of an effective phonon excitation process, assisted by both one- and two-phonon drives. The weak Duffing nonlinearity of the phonons is induced by a nitrogen-vacancy (NV) center magnetically coupled to a cantilever, while the mechanical parametric driving is realized by modulating the spring constant of the cantilever using a pump. By implementing a time-dependent protocol for the linear and parametric drives, there exists a strict cutoff in the phonon number distribution, resulting in a pure Fock state. Furthermore, the nonclassical nature of the generated state remains robust against dissipations.

The integration of nonreciprocal non-Hermiticity and topological lattice systems can induce the emergence of the non-Hermitian skin effect, enabling a clear distinction between the behaviors of bulk states in non-Hermitian systems and those in Hermitian systems. Specifically, we focus on examining the localized directions of eigenstates in an odd-sized Su-Schrieffer-Heeger-type lattice with intracell nonreciprocal hopping, and identify a notable phenomenon of detachment between the edge and bulk states, which stands in stark contrast to the behaviors observed in even-sized cases. The localized directions of the bulk and edge states are respectively subjected to the non-Hermitian skin effect and isolated site induced by the odd size of the lattice, resulting in the detachment between the edge and skin states. To provide a comprehensive understanding of the detachment phenomenon, we present analytical solutions that illustrate the localized directions of the edge and skin states whose results are in perfect agreement with the numerical findings. Furthermore, we demonstrate that a pair of eigenstates exhibiting opposite localized directions to the skin states can appear in a trimer non-Hermitian lattice, indicating the possibility of detachment between the edge and skin states in multimer non-Hermitian lattices as well.

We consider the ratchet dynamics in a $\mathcal{PT}$-symmetric Floquet quantum system with symmetric temporal (harmonic) driving. In the exact $\mathcal{PT}$-symmetry phase, for a finite number of resonant frequencies, we show that the long-lasting resonant currents can be generated with the symmetric time-continuous driving, which would otherwise forbid the generation of directed currents in the Hermitian limit. Such a non-Hermitian resonant current can be enhanced by increasing the non-Hermitian level and, in particular, the resonant current peaks (reaches the largest negative value) under the condition that the imaginary part of the potential depth is equal to the real part, at which the stable asymptotic current occurs owing to exceptional points mechanism. Moreover, the directed currents originating from the symmetry breaking are reported, which increase linearly with the driving frequency, the mechanism behind which is that the cutoff of the momentum eigenstates for the Floquet state with maximum imaginary quasienergy increases as the driving frequency is continuously increased. We also present a non-Hermitian three-level model that can account for the resonant currents and gives surprisingly good agreement with direct numerical results for weak driving, even in the broken-$\mathcal{PT}$-symmetry regime for the first-order resonance. Our results provide a means of realizing the non-Hermiticity-controlled ratchet current by means of a smooth continuous driving, previously used only to generate currents in Hermitian systems.

The non-Hermiticity of the system gives rise to distinct knot topology that has no Hermitian counterpart. Here, we report a comprehensive study of the knot topology in gapped non-Hermitian systems based on the universal dilation method with a long-coherence-time nitrogen-vacancy center in a ${}^{12}\mathrm{C}$ isotope purified diamond. Both the braiding patterns of energy bands and the eigenstate topology are revealed. Furthermore, the global biorthogonal Berry phase related to the eigenstate topology has been successfully observed, which identifies the topological invariance for the non-Hermitian system. Our method paves the way for further exploration of the interplay among band braiding, eigenstate topology, and symmetries in non-Hermitian quantum systems.

The wave-packet convergent close-coupling (WP-CCC) approach is applied to calculate the energy spectrum of electrons ejected in $p+{\mathrm{H}}_2$ collisions as a function of the scattering angle of the projectile. The calculations are performed for projectile energies of 75, 100, and 200 keV. At these incident energies there are many competing reaction channels that play an essential role in the collision dynamics. The target is modeled as an orientationally averaged effective one-electron system. The results are compared with available perturbative calculations and experimental data. Good agreement between the WP-CCC results and experimental data is found for small emission energies, especially when the projectile is scattered at small angles. However, when the electron is emitted with a speed comparable to or greater than the projectile speed we find that our method predicts smaller cross sections near zero scattering angles and a slower fall off than the experimental data. This is in agreement with other calculations. Furthermore, the structure observed in the experimental data at large scattering angles is not supported by our results. Interestingly, we find very good agreement with the continuum-distorted-wave eikonal-initial-state molecular-orbital calculations that use a two-effective center approximation, though our method describes the target as an effective one-electron spherically symmetric system. This suggests that in these models two-center interference effects may have a small effect on this particular cross section. Furthermore, we find that the experimentally observed decrease in average scattering angle in proton collisions with ${\mathrm{H}}_2$ near the electron-projectile matching speed is not reproduced by our results. We also present the doubly differential cross section for ionization as a function of the scattering angle of the projectile at select emission angles.

Nuclear electric resonance (NER) spectroscopy is currently experiencing a revival as a tool for nuclear spin-based quantum computing. Compared to magnetic or electric fields, local electron density fluctuations caused by changes in the atomic environment provide a much higher spatial resolution for the addressing of nuclear spins in qubit registers or within a single molecule. In this article we investigate the possibility of coherent spin control in atoms or molecules via nuclear quadrupole resonance from first principles. An abstract, time-dependent description is provided which entails and reflects on commonly applied approximations. This formalism is then used to propose a method we refer to as “optical” nuclear electric resonance (ONER). It employs pulsed optical excitations in the UV-visible light spectrum to modulate the electric field gradient at the position of a specific nucleus of interest by periodic changes of the surrounding electron density. Possible realizations and limitations of ONER for atomically resolved spin manipulation are discussed and tested on ${}^9\mathrm{Be}$ as an atomic benchmark system via electronic structure theory.

In spinor Bose Einstein condensates (BEC) gases, a fraction of its thermally excited atoms can still interact with the condensate ground state, leading to spin-spin interactions that can modify the main features of its spin-phase diagrams. In this work we study the spin-phase diagram of a BEC of general spin-$f$ and fully characterize its noncondensate thermal fraction. The latter provided that the condensate ground state lies within a spin phase with rotational symmetry. The study is based in the Hartree-Fock approximation in conjunction with the Majorana stellar representation approach for pure and mixed quantum states and the use of point-group symmetry arguments. The method allows us to study the phase diagram of spinorial BECs with usual point-group symmetries, including those with some exotic phases associated to the platonic solids ($f=2,3,4,$ and 6), which are known to lead to non-Abelian topological excitations. In addition, we explore the temperature effects on the admissible spin-phase domains for general spin values, as well as its physical implications on their multipolar magnetic moments.

Optical chirality plays an essential role in chiral light-matter interactions with broad applications in sensing and spectroscopy. Conventional methods of generating optical chirality usually employ chiral structures or chiral excitations. Here, we propose to use an achiral metasurface consisting of a gold disk array excited by a linearly polarized light to generate optical chirality. Using full-wave numerical simulations, we show that the metasurface can give rise to large-area optical chirality of the same sign for a wavelength ranging from 1.2 to $1.5\mu \mathrm{m}$. The magnitude of the chirality is comparable to that of circularly polarized plane waves. The emergence of optical chirality can be attributed to the asymmetric polarization singularity lines (C lines) in the near fields of the metasurface. We further explore the application of the proposed metasurface in chiral discriminations by simulating the absorption of chiral helix particles immersed in the near fields, and demonstrate that the left-handed and right-handed helix particles give rise to different absorptions. The phenomenon can be understood using an analytical theory based on the dipole approximation, which predicts differential absorption quantitatively agreeing with the numerical simulation results. Our study uncovers the subtle relationship between near-field optical chirality, polarization singularities, and symmetry. The results can find applications in optical sensing, chiral quantum optics, and optical manipulations of small particles.

The interplay between the dispersion and spectral filtering effect in Kerr-ring resonators allows the formation of dispersion-less Kerr solitons, an alternative class of Kerr solitons featuring with ultrabroad spectrum and zero chirp. In contrast to a ring resonator, the nonlinear interactions between the counterpropagating light waves in a F-P resonator introduce an additional phase shift, which modifies the system-effective detuning, hence impacting the soliton existence range and its formation dynamics. In this article, we investigate numerically the formation of dissipative Kerr solitons (DKSs) in a 10-mm-long Fabry-Pérot (FP) microresonator with a super-Gaussian spectral filter in the normal dispersion regime. Simulation results found that the spectral loss provided by the filter plays a significant role in the formation of DKS. Without the filter, the FP microresonator yields platicons featuring a flat-top pulse with oscillating tails on both sides. For a super-Gaussian spectral filter with an order of $n\to \infty$, the resonator approaches the transform-limited DKS regime with ultraflat spectrum. An intermediate state of chaotic states of multipulses was observed in the transition from the platicon to the transform-limited DKSs when using the detuning scan technology, the dependence of the appearance of the chaotic state on the spectral filtering effect. Finally, the energy-width scaling behavior and the enhancement of the performances of the transform-limited DKS are studied. Simulation results deepen our understanding of nonlinear dynamics of transform-limited DKS combs and point out a way for achieving a high-energy DKS comb with ultraflat spectrum in Kerr resonators.

Recent technological advancements have enabled strong light-matter interaction in highly dissipative cavity-emitter systems. However, in these systems, which are well described by the Tavis-Cummings model, the considerable loss rates render the realization of many desirable nonlinear effects, such as saturation and photon blockade, problematic. Here we present another effect occurring within the Tavis-Cummings model: A nonlinear response of the cavity for resonant external driving of intermediate strength, which makes use of large cavity dissipation rates. In this regime, $(N+1)$-photon absorption processes dominate when the cavity couples to $N$ emitters. We explore and characterize this effect in detail, and provide a picture of how the effect occurs due to destructive interference between the emitter ensemble and the external drive. We find that a central condition for the observed effect is large cooperativity; i.e., the product of the cavity and emitter decay rates is much smaller than the collective cavity-emitter interaction strength squared. Importantly, this condition does not require strong coupling. We also find an analytical expression for the critical drive strength at which the effect appears. Our results have potential for quantum state engineering, e.g., photon filtering, and could be used for the characterization of cavity-emitter systems where the number of emitters is unknown. In particular, our results open the way for investigations of unique quantum-optics applications in a variety of platforms that require neither high-quality cavities nor strong coupling.