We report continuous loading of ${}^{\text{87}}\mathrm{Rb}$ atoms in a magneto-optical trap (MOT) at narrow linewidth, 420-nm $5S_{1/2}$, $F=2\to 6P_{3/2}$, $F=3$ blue transition (blue MOT). Continuous loading of the blue MOT is achieved by superimposing the blue laser beam inside a hollow core of infrared (IR) laser beam driving the broad $5S_{1/2}$, $F=2\to 5P_{3/2}$, $F=3$ transition at 780 nm. We typically load $\sim {10}^8$ atoms in the blue MOT in 2.5 s. We characterize the continuous loading of the blue MOT with various parameters such as magnetic-field gradient, detuning, power and diameter of the blue MOT beam, and diameter of the hollow core (spot) inside the IR MOT beam. We observe that the blue laser beam should overfill the spot of the IR laser beam. This is because the blue laser cools the atoms to a lower temperature even in the presence of the broad IR laser, i.e., in the overlapped region, and hence helps in loading. To support the experimental result, we also present the theoretical framework for cooling atoms in the presence of two lasers driving two different transitions simultaneously. This method of continuous loading of the blue MOT can be useful to produce a continuous atomic beam of cold Rb atoms.

We used the monochromatic soft-x-ray beamline P04 at the synchrotron-radiation facility PETRA III to resonantly excite the strongest $2p\text{−}3d$ transitions in neonlike $\mathrm{Ni}$xix ions, ${\left[2p^6\right]}_{J=0}\to {\left[{\left(2p^5\right)}_{1/2}3d_{3/2}\right]}_{J=1}$ and ${\left[2p^6\right]}_{J=0}\to {\left[{\left(2p^5\right)}_{3/2}3d_{5/2}\right]}_{J=1}$, respectively dubbed $3C$ and $3D$, achieving a resolving power of 15 000 and signal-to-background ratio of 30. We obtain their natural linewidths, with an accuracy of better than 10%, as well as the oscillator-strength ratio $f\left(3C\right)/f\left(3D\right)=2.51\left(11\right)$ from analysis of the resonant fluorescence spectra. These results agree with those of previous experiments, earlier predictions, and our own advanced calculations.

We study intraband high-order harmonic generation arising from a band-gap material driven by a linearly polarized laser field. We factorize the intraband high-order harmonic-generation signal into intracycle and intercycle terms. The intracycle term uniquely determines the spectral characteristics, whereas the intercycle term merely modulates the spectral features by imposing energy conservation in the long-pulse limit. Through analysis of the intracycle interference, the cutoff is identified, and the origin of the harmonic selection rules is revealed. Further, it is outlined how different components of the band structure contribute to different regions of the harmonic spectrum, giving rise to nontrivial intensity scaling of individual harmonics in the plateau region.

We perform a systematic analysis of single-channel quantum interference in laser-induced nonsequential double ionization with few-cycle pulses, using the strong-field approximation. We focus on a below-threshold intensity for which the recollision-excitation with subsequent ionization (RESI) mechanism is prevalent. We derive and classify several analytic interference conditions for single-channel RESI in arbitrary driving fields and address specific issues for few-cycle pulses. Since the cycles in a short pulse are no longer equivalent, there are several events whose dominance varies. We quantify this dominance for single excitation channels by proposing a dominance parameter. Moreover, there will be many types of superimposed interference fringes that must be taken into consideration. We find an intricate tapestry of patterns arising from phase differences related to symmetrization, temporal shifts and a combination of exchange and event interference.

We employ the equation-of-motion coupled-cluster (EOM-CC) method in the four-component relativistic theory framework to understand the roles of electron correlation effects in the ab initio estimations of electric dipole polarizabilities ($\alpha$) of the states engaged in the clock transition (${}^{1}S_0\to {}^{3}P_0$) of the zinc atom. The roles of basis size, inclusion of higher-level excitations, and higher-order relativistic effects in the evaluation of both excitation energies of a few low-lying excited states and $\alpha$ are analyzed systematically. Our EOM-CC values are compared with the earlier reported theoretical and experimental results. This demonstrates the capability of the EOM-CC method to ascertain the preciseness of the blackbody radiation shift in a clock transition, which holds paramount importance for optical clock-based experiments.

We study the localization dynamics of a SU(2) fermionic wave packet launched in a (pseudo)random potential. We show that, in the limit of strong intercomponent repulsions, the total wave packet exhibits a boomeranglike dynamics, returning to near its initial position as expected for noninteracting particles, while separately each spin-component does not. This spin-charge separation effect occurs both in the infinite repulsive limit and at finite interactions. At infinite interactions, the system is integrable and thermalization cannot occur: the two spin-components push each other during the dynamics and their centers of mass stop further from each other than their initial positions. At finite interactions, integrability is broken, the two spin-components oscillate and mix, with their center-of-mass positions converging very slowly to the center of mass of the whole system. This is a signature that the final localized state is a fully spin-mixed thermalized state.

Adiabatic frequency conversion (AFC) in microring resonators is a promising alternative for fully integrable and tunable frequency shifting of optical signals. Nonetheless, the magnitude of the frequency shift via simple AFC in a single ring is fundamentally limited by the material platform. To overcome this limitation, we propose and analyze a scheme to induce cascaded AFC (CAFC) along a chain of coupled, yet initially detuned rings, without requiring unloading the optical signal into a bus waveguide between successive modulations. For concreteness, we examine thoroughly the simplest nontrivial case of a chain of two rings and briefly discuss the generalization to a chain of an arbitrary number of rings. We analyze the temporal dynamics of this CAFC process using temporal coupled mode theory. We examine the transformation of the input into the frequency-shifted output as a rank-one linear operator in the vector space of finite-energy pulses. In this way we show that the energy efficiency of CAFC depends on the input pulse shape through a Schwarz inequality, just as in single-ring AFC. We propose a numerical scheme to maximize the CAFC efficiency with respect to the process's timescales and discuss the physics involved. We show that the resulting CAFC efficiency converges in a polynomial manner to a maximum as the process becomes progressively idealized. Furthermore, we show that this maximum efficiency is identical to that for single-ring AFC, e.g., 0.7951 for a symmetric, single-lobe input pulse. Thus, we show that CAFC can become more efficient than multiple instances of single-ring AFC. We explain the polynomial convergence of the CAFC efficiency as a consequence of its real analyticity as a function of the process's timescales under our scheme. We leverage this polynomial convergence to model the CAFC efficiency as a simple polynomial in few normalized timescales. We then utilize this polynomial model to predict optimal values for the remaining free parameters and the scaling of the CAFC with the interring detuning.

Like quantum state teleportation, quantum energy teleportation (QET) can transmit energy through local operation and classical communication. This QET process even can extract energy from a strong local passive (SLP) state, from which any local operation can only inject energy. However, finding a SLP state is an important open question. In this paper, in the Hamiltonian of the minimal QET model, we obtain a set of SLP pure states by correlation matrix analysis. Our method is computable and easier to understand in certain Hamiltonians and states. Through the method of mathematic programming, we find an amazing phenomenon in which the energy teleportation efficiency can be greater than 1 and we reveal the essence of this phenomenon. It can be extended to the mixed state and spin chain model. Our result shows that quantum steering plays an important role in the QET protocol.

Quantum coherent feedback control (CFC) has been widely applied to prepare and manipulate nonclassical beams. In this paper, we utilize an optical beam splitter with tunable reflectivity $k$ as a feedback controller and theoretically construct a CFC system based on four-wave mixing (FWM) processes with a spatially structured pump. After considering absorption loss in the atoms, beam propagation loss in the feedback loop, and detection loss in the detectors, we derive an input-output relationship and theoretically study the manipulation of feedback strength, gain of the FWM processes, and phase to the degree of hexapartite quantum correlation of the system. We find that the relative intensity-difference squeezing degree of all the six output fields from this system is enhanced within a certain range of feedback ratio and phase. Furthermore, there exists an optimal feedback ratio and phase to maximize the intensity-difference squeezing. In addition, we also study the entanglement properties of this CFC system by the positivity under the partial transposition criterion and find that the feedback ratio and phase can tailor the entanglement structure of the hexapartite state. The proposed CFC scheme has realized quantum correlation enhancement and entanglement manipulation, which can be expanded to a large-scale quantum network for more practical applications in quantum information processing.

We investigate theoretically the laser intensity dependent valley polarization in gapped graphene driven by the circularly polarized pulse. The results show that the valley polarization without considering decoherence displays a nonmonotonic behavior as a function of the laser intensity. By analyzing the conduction-band electron distribution, we demonstrate that the decrease of the valley polarization in low laser intensity is caused by the change of dominant physical mechanisms, i.e., from few-photon to diabatic tunneling transitions. While in high laser intensity, the analysis of electron dynamics trajectories indicates that the intercycle interference determines the different $k$-resolved electron distributions in K and ${\text{K}}^{\prime }$ valleys, influences the valley polarization, finally leads to the formation of a peak. Moreover, when the decoherence is considered, although the interference structure of the $k$-resolved electron distribution becomes blurred, the oscillation of valley polarization with the laser intensity is still present. Our work illustrates that the laser intensity can significantly influence the field-driven electron dynamics processes and can be as a knob to adjust the valley polarization in gapped graphene.

We investigate the Beliaev damping of gapped excitations in a homogenous two-component Bose-Einstein condensate with Rabi coupling and find different damping behavior of gapped excitations in two quantum phases. In the paramagnetic phase, due to constraints imposed by the ${\mathbb{Z}}_2$ symmetry and energy-momentum conservation, the only damping channel available, that of decaying into one phonon and one new gapped mode, cannot occur unless the group velocity of the gapped modes surpasses the sound speed. Conversely, in the ferromagnetic phase, three distinct decay channels exist. Once the momentum of the excitations falls below a critical threshold, only one of these channels, that of decaying into two gapless modes, is activated, and the damping rate remains finite even at zero momentum.

Acting as analog models of curved spacetime, surfaces of revolution employed for exploring novel optical effects are followed with great interest nowadays to enhance our comprehension of the universe. It is of general interest to understand the spectral effect of light propagating a long distance in the universe. Here we address the issue of how curved space affects the phenomenon of spectral switches, a spectral sudden change during propagation caused by the finite size of a light source. Based on the point spread function of curved space under the paraxial approximation, the expression of the on-axis output spectrum is derived and calculated numerically. A theoretical way to find on-axis spectral switches is also derived, which interprets the effect of spatial curvature of surfaces on spectral switches as a modification of the effective Fresnel number. We find that the spectral switches on surfaces with positive Gaussian curvature are closer to the source compared with the flat surface case, while the effect is opposite on surfaces with negative Gaussian curvature. We also find that the spectral switches farther away from the light source are more sensitive to the change in Gaussian curvature. This work deepens our understanding of the properties of fully and partially coherent lights propagating on two-dimensional curved space.

In this paper we propose two classes of multiparticle entangled states: multigraph and multihypergraph states. Two types of operations are designed to correspond to the edges and hyperedges to prepare the states. Furthermore, multigraph and multihypergraph states contain qudit graph and hypergraph states, respectively. More interestingly, we demonstrate the one-to-one correspondence between the proposed multihypergraph states and the generalized real equally weighted states when the dimension of the states is an odd prime.

As a consequence of Bell's theorem, the statistics of measurements on some entangled states cannot be simulated with local hidden variables alone. The amount of communication that must be supplied is an intuitive quantifier of nonclassicality. While it is obvious that this amount can be very large in general, it has been surprisingly difficult to find simple examples of quantum correlations, whose simulation requires more than one bit of communication. In this paper, we report the simplest example to date, which lives in the (5,2,5,5) Bell scenario [the previously known smallest case living in the (7,3,16,16) scenario]. The proof is built on the observation that finding the largest 1-bit score is equivalent to finding the bipartition of the inputs, in which the sum of the local scores of the two subgames is maximal.

Atoms are not two-level systems, and their rich internal structure often leads to complex phenomena in the presence of light. Here, we analyze off-resonant light scattering including the full hyperfine and magnetic structure. We find a set of frequency detunings where the induced atomic dipole is the same irrespective of the Zeeman state, and where two-photon transitions that alter the atomic state turn off. For alkali-metal atoms and alkaline-earth-metal ions, if the hyperfine splitting is dominated by the magnetic dipole moment contribution, these detunings approximately coincide. Therefore, at a given “magical” detuning, all Zeeman states in a hyperfine manifold behave almost identically, and can be traced out to good approximation. This feature prevents state decoherence due to light scattering, which impacts quantum optics experiments and quantum information applications.

Motivated by a recent prediction [Commun. Phys. 6, 195 (2023).] that time-of-flight experiments with ultracold atoms could test different interpretations of quantum mechanics, this work investigates the arrival times predicted by the stochastic interpretation, whereby quantum particles follow definite but nondeterministic and nondifferentiable trajectories. The distribution of arrival times is obtained from a Fokker-Planck equation and confirmed by direct simulation of trajectories. It is found to be, in general, different from the distribution predicted by the Bohmian interpretation, in which quantum particles follow definite deterministic and differentiable trajectories. This result suggests that trajectory-based interpretations of quantum mechanics could be experimentally discriminated.

We theoretically examine the finite-temperature spectral function of Fermi polarons in three dimensions by using a self-consistent many-body $T$-matrix theory in real frequency. In comparison with the previous results from a non-self-consistent many-body $T$-matrix approach, we show that the treatment of self-consistency in the impurity Green's function leads to notable changes in almost all the dynamical quantities, including the vertex function, impurity self-energy, and spectral function. Eventually, it gives rise to quantitatively different predictions for the measurable radio-frequency spectrum and Raman spectrum at finite temperature. Using the recent spectroscopic measurements as a benchmark, we find that the self-consistent many-body $T$-matrix theory somehow provides a better explanation for the experimental data. The notable difference in the predictions from the non-self-consistent and self-consistent theories suggests that more accurate theoretical descriptions are needed in order to fully account for the current spectroscopic observations on Fermi polarons.

Slowing down light in on-chip photonic devices strongly enhances the light-matter interaction, but typically also leads to increased backscattering and small-bandwidth operation. It was shown recently that, if one modifies the edge termination of a photonic Chern insulator such that the edge mode wraps many times around the Brillouin zone, light can be slowed to arbitrarily low group velocity over a large bandwidth, without being subject to backscattering. Here we study the robustness of these in-gap slow light modes against fabrication disorder, finding that disorder on scales significantly larger than the minigaps between edge bands is tolerable. We identify the mechanism for wavepacket breakup as disorder-induced velocity renormalization and calculate the associated breakup time.

Efficiently controlling linear Gaussian quantum (LGQ) systems is a significant task in both the study of fundamental quantum theory and the development of modern quantum technology. Here, we propose a general quantum-learning-control method for optimally controlling LGQ systems based on the gradient-descent algorithm. Our approach flexibly designs the loss function for diverse tasks by utilizing first- and second-order moments that completely describe the quantum state of LGQ systems. We demonstrate both deep optomechanical cooling and large optomechanical entanglement using this approach. Our approach enables the fast and deep ground-state cooling of a mechanical resonator within a short time, surpassing the limitations of sideband cooling in the continuous-wave driven strong-coupling regime. Furthermore, optomechanical entanglement could be generated remarkably fast and surpass several times the corresponding steady-state entanglement, even when the thermal phonon occupation reaches one hundred. This work will not only broaden the application of quantum learning control, but also open an avenue for optimal control of LGQ systems.