Direct laser cooling and trapping of molecules to temperature below the Doppler limit and density exceeding ${10}^8$ are challenging due to the sub-Doppler heating effects of a molecular magneto-optical trap (MOT). In our previous paper [S. Xu et al., Phys. Rev. Res. 4, L042036 (2022)], we presented a general approach to engineering the sub-Doppler force by tuning the AC Stark shift with the addition of a blue-detuned laser. Here, by employing the Bayesian optimization method to optical Bloch equations, we have identified multiple blue-detuned MOT schemes for the CaF molecule. From the three-dimensional Monte Carlo simulation, we obtained a MOT temperature and density of 14 $\mu \mathrm{K}$ and $4.5\times {10}^8 {\mathrm{cm}}^{-3}$, respectively. Our findings present a potential avenue for directly loading molecular MOTs into conservative traps, which can capitalize on the high density and low temperature of the MOTs.

We study strong-field double ionization in a three-electron atom by applying a simplified, reduced-dimensionality model with three active electrons. The influence of the spin-induced symmetry of the spatial part of the wave function on the final two-photoelectron momentum distribution is discussed. We identify partial momentum distributions originating from different sets of spins of outgoing electrons providing in this way a quantum support connection between the V-structure and direct ionization typically explained classically. Changes in the momentum distribution with increasing field amplitude obtained in our simplified model are shown to be well correlated with the experimental data known from the literature. The possible relation between the observed dependencies and different ionization mechanisms is discussed.

We investigate the appearance of a four-boson limit cycle in Hamiltonian systems at the unitarity limit. The model interaction incorporates two-, three-, and four-body short-range potentials, which allow us to disentangle the interwoven dynamics of three- and four-boson energy levels. Through numerical evidence, we observe a correlation between the energies of two successive universal tetramer levels for a fixed weakly bound trimer, which is found to be largely model independent. Interestingly, this correlation cycle is consistent with the findings of Hadizadeh et al. [Phys. Rev. Lett. 107, 135304 (2011)] using a zero-range model.

We investigate the phases of a Bose-Einstein condensate of dipolar atoms restricted to move in a two-dimensional plane. The dipole moments are all aligned in a direction tilted with respect to the plane normal. As a result of the attractive and repulsive components of the dipole-dipole interaction, the dipolar gas has a self-bound phase, which is stabilized by quantum fluctuations. Tilting the dipoles tunes the anisotropy of the dipole-dipole interaction, which can trigger a spatial density modulation. In this work we study these two aspects and investigate the conditions for the formation of a self-bound and striped phase, which has been realized in experiments with dipolar droplets. We use a variational method based on the hypernetted-chain Euler-Lagrange optimization of a Jastrow-Feenberg ansatz for the many-body wave function to study the ground-state properties. This method takes into account quantum fluctuations in a nonperturbative way and thus can be used also for strongly correlated systems.

Density shift plays an important role in the uncertainty of an optical lattice clock and thus has attracted a great deal of theoretical and experimental studies. However, most of the theoretical research has considered the single-band model and collective approximation, so the density shift of the system at higher temperatures cannot be analyzed accurately. Here we design a numerical algorithm that combines Monte Carlo sampling and exact diagonalization and name it multiband sampling exact diagonalization (MBSED). The MBSED method considers the collision of atoms between multiple bands, so it can provide the density shift of an optical lattice clock with higher precision. Such an algorithm will benefit the numerical simulation of an optical lattice clock and may also be used in other platforms of quantum metrology.

Quantum interference (QI) among different spontaneous decay channels has led to many important quantum optics applications. This effect can be strongly enhanced in an anisotropic quantum vacuum (AQV) provided by photonic nanostructures, which is promising to apply in integrated photonic quantum technologies. However, the QI effect in a dielectric Mie resonance system with a large AQV has not been studied. Here, we investigate the QI enhancement and fluorescence spectral linewidth narrowing of a four-level system with a high-index dielectric nanosphere. We find that the nearly maximum QI can be obtained due to the extreme AQV under Mie resonances. We also show that the degree of QI varies in a wide range when selecting different Mie resonances and the emitter's position. We further illustrate the subnatural-linewidth narrowing of the fluorescence spectra in the Mie resonance system, which results from large QI and a relatively small Purcell factor. These findings not only provide a scheme for quantum coherence manipulation, but also have potential applications in integrated quantum photonic devices.

A superconducting qubit coupled to a readout resonator is currently the building block of multiple quantum computing as well as quantum optics experiments. A typical qubit-resonator system is coupled in the dispersive regime, where the detuning between qubit and resonator is much greater than the coupling between them. In this work, we fabricated and measured a superconducting transmon-resonator system in the nondispersive regime. The dressed states formed by the mixing of the bare qubit and resonator states can be further mixed by applying a drive on the qubit, leading to the formation of polariton states. We report experimental studies of transitions between polariton states at varying driving powers and frequencies and show how the nondispersive coupling of the higher levels of the qubit-resonator system modifies the polariton eigenstates and the corresponding transition frequencies. We also report close agreement with numerical results obtained from a driven Jaynes-Cummings model beyond the dispersive regime.

The central claim of Gavassino and Disconzi [Gavassino and Disconzi, Phys. Rev. A 107, 032209 (2023)] that relativistic quantum tunneling is an entirely subluminal process is shown to be incorrect. The Hartman effect can lead to superluminal tunneling, but not to superluminal signaling.

We show that the criticism raised by Dumont and Pollak [Dumont and Pollak, preceding Comment, Phys. Rev. A 108, 036201 (2023)] is based on a flawed understanding of our analysis.

Quantum state transfer through spin chains has been extensively investigated. Two schemes, the coupling set for perfect state transfer (PST) or adding a leakage elimination operator (LEO) Hamiltonian have been proposed to boost the transmission fidelity. However, these ideal schemes are only suitable for closed systems and will lose their effectiveness in open ones. In this work, we invoke a well-explored optimization algorithm, Adam, to expand the applicable range of PST couplings and LEO to the open systems. Our results show that, although the transmission fidelity decreases with increasing system-bath coupling strength, Markovianity and temperature for both ideal and optimized cases, the fidelities obtained by the optimized schemes always outweigh the ideal cases. The enhancement becomes bigger for a stronger bath, indicating a stronger bath provides more space for the Adam to optimize. This method will be useful for the realization of high-fidelity information transfer in the presence of environment.

We show how to implement the unidirectional topological excitation transmission in the Rice-Mele model. We find that the unidirectional excitation transmissions can be implemented via the asymmetric gap states. We reveal that the asymmetric gap states are essentially induced by the breaking of generalized chiral symmetry and the gap state without band crossing determines the transmission direction. In particular, we find that the direction of the unidirectional edge transmission can be reversed via setting the band inversion or mirror symmetry. The unidirectional excitation transmissions based on gap states can be equivalent to a topological diode with properties of unidirectional conduction, which may have potential applications in the implementation of new-type topological devices, e.g., a topological sensor or topological switch.

We consider the theoretical formalism presented by Otobe et al. [J. Phys. Soc. Jpn, 88, 024706 (2019)] to write the photoionization rate of a crystal irradiated by a linearly and a circularly polarized laser beam valid in low- and high-intensity regimes. We expand these analytical expressions in the multiphoton regime corresponding to the low laser intensity domain. This provides a theoretical formulation for the photoionization cross section valid for a linear and a circular laser polarization. Whereas the quartz crystal produces a larger charge density when it is irradiated with an 800-nm linearly polarized laser beam than a circularly polarized one, no difference in ionization rate is noticeable when the laser wavelength is equal to 351 nm.

We report the creation of dual-species Bose-Einstein condensates (BECs) of ${}^7\mathrm{Li}$ and ${}^{133}\mathrm{Cs}$. These BECs are formed in a bichromatic optical dipole trap created with 1550- and 780-nm laser beams. During the production process, an external magnetic field of 886 G is applied to adjust the scattering lengths to $a_{\mathrm{Cs}}=123a_0, a_{\mathrm{Li}}=484a_0$, and $a_{\mathrm{LiCs}}=248a_0$. These scattering lengths allow for efficient evaporation and sympathetic cooling. The dual-species BECs are typically produced with $1.5\times {10}^4$ Cs atoms and $6.0\times {10}^3$ Li atoms. This quantum degenerate mixture of Li and Cs provides an ideal platform for exploring phenomena such as polarons and Efimov trimers, as well as for creating ground-state LiCs molecules.

Unidirectional photon emission is crucial for constructing quantum networks and realizing scalable quantum information processing. In the present work an efficient scheme is developed for the unidirectional emission of a tunable squeezed microwave field. Our scheme is based on a chiral cavity magnonic system, where a magnon mode in a single-crystalline yttrium iron garnet (YIG) sphere is selectively coupled to one of the two degenerate rotating microwave modes in a torus-shaped cavity with the same chirality. With the YIG sphere driven by a two-color Floquet field to induce sidebands in the magnon-photon coupling, we show that the unidirectional emission of a tunable squeezed microwave field can be generated via the assistance of the dissipative magnon mode and a waveguide. Moreover, the direction of the proposed one-way emitter can be controlled on demand by reversing the biased magnetic field. Our work opens up an avenue to create and manipulate one-way nonclassical microwave radiation field and could find potential quantum technological applications.

Assessing whether a quantum state $\widehat{\rho }$ is nonclassical (i.e., incompatible with a mixture of coherent states) is a ubiquitous question in quantum optics, yet a nontrivial experimental task because many nonclassicality witnesses are nonlinear in $\widehat{\rho }$. In particular, if we want to witness or measure the nonclassicality of a state by evaluating its quadrature coherence scale, this a priori requires full state tomography. Here, we provide an experimental procedure for directly accessing this quantity with a simple linear interferometer involving two replicas (independent and identical copies) of the state $\widehat{\rho }$ supplemented with photon-number-resolving measurements. This finding, which we interpret as an extension of the Hong-Ou-Mandel effect, illustrates the wide applicability of the multicopy interferometric technique in order to circumvent state tomography in quantum optics.

Controlled deposition of high laser-power density at remote distances still remains a challenge. Previous experimental works using terawatt peak-power laser systems to initiate filamentation at distances of hundreds or even thousands of meters in the atmosphere have documented limited control on laser energy deposition due to the effects of diffraction and turbulence. In this work, we demonstrate a promising scenario for the projection of high power densities at kilometric distances in air, which requires multiple-gigawatt laser pulses reshaped into ring-Airy beams. We show that a power distribution over a relatively large primary ring accompanied by the inward power flux characterizing a ring-Airy beam limits the occurrence of nonlinear effects to the vicinity of the focal point. Close to focus, self-focusing sets in, which accelerates the convergence process of the beam. We quantify the influence of self-focusing on the focal shift and propose an empirical law that fits our numerical simulation results for the position of the nonlinear focus as a function of the input power and the apodization factor of the ring-Airy beam. We show that once the intensity of the beam exceeds the ionization threshold of air, a short filament is generated whose power content rather accurately corresponds to the critical power for self-focusing, independent of the input parameters.

Simultaneous observation of the wave and particle natures of a quantum object under one experimental arrangement represents the latest development regarding wave-particle duality. In this work, we propose a simple scheme using linear-optics devices to observe either the particle nature or wave nature or the wave-particle superposition (WPS) of photons by controlling a relevant classical parameter. Our setup consists of two stages, the preparation and measurement stages, with the latter stage containing an additional classical variable whose value will be given only after the first stage in order to avoid any possible communication between the two stages and thus rule out the classical hidden-variable model. We not only develop the theory but also perform the corresponding experiments. The experimental observations perfectly agree with the theoretical predictions. Our findings are the following. The frequency (wavelength) of both single- and two-photon systems is independent of whether the photon(s) is wavelike or in a WPS state, with the common fact being that the frequency (wavelength) of two photons as a whole entity is twice (half) that of a single photon. As for the interference-pattern visibility, it is always lower in the case of WPS than in the case of a wavelike state for a single photon as well as for two photons. Remarkably, with a given value of the control parameter, the interference visibility in the case of WPS is always higher in the single-photon scenario than in the two-photon one. Although here we are dealing with photons, the obtained results apply equally well to matter particles.