We investigate the dual-frequency Doppler-free resonance in the ${\mathrm{D}}_1$ line of alkali-metal atoms for any accessible value of the nuclear spin $I$ by considering the symmetries of the dipole operator. We show that there is the absence of optical pumping in the scheme with parallel polarizations for the center of the crossover, resulting in its smallest width. Secondly, the growth in absorption for the center of the peak with $F_e=I-1/2$ and the decrease of its width with the two-photon detuning in the case of orthogonal polarizations is explained. Particular attention is paid to the special case of $I=3/2$, where this effect is the most pronounced. Experimental results with ${}^{85}\mathrm{Rb}$ and ${}^{133}\mathrm{Cs}$ atoms are in agreement with the analysis.

We investigate the interaction of high-energy vortex photons with atomic nuclei. By performing a multipole expansion of the optical-vortex field, we show that the selection rules arise from the intrinsic symmetries of the vortex field. This rule governs on-axis nuclear transitions and breaks down off-axis, where extrinsic orbital angular momentum (OAM) enables otherwise forbidden excitations. By incorporating the finite size of both the photon wave packet and the nucleus through a spatiotemporal luminosity function, we show that although the on-axis absorption cross section can be locally enhanced by several orders of magnitude, the total excitation probability remains low due to vanishing field intensity at the vortex core. As a result, most nuclear transitions show negligible selectivity in probability. In contrast, nuclear hyperfine transitions can be selectively driven by vortex photons, particularly in the nonparaxial regime, where photon OAM enables sublevel-specific excitation and external-field-sensitive control of nuclear states.

The spectral minimum in high-order harmonic generation (HHG) from aligned ${\text{CO}}_2$ molecules has attracted significant interest and has been extensively studied. However, experimental investigations have largely been limited to short laser wavelengths, and theoretical interpretations often focus solely on the single-molecule response. In this work, we present a systematic study of the wavelength-dependent spectral minimum at the macroscopic level, extending the driving-laser wavelength up to 1800 nm. We first simulate experimentally measured HHG spectra of aligned ${\text{CO}}_2$ molecules, and the good agreement between the simulation and experiment confirms the accuracy of the quantitative rescattering model and macroscopic propagation theory. Our simulations reveal that, at shorter wavelengths, the spectral minimum shifts with increasing laser intensity, whereas at longer wavelengths, its position remains stable regardless of intensity. This behavior arises from differences in the harmonic-intensity distributions of short and long electron trajectories associated with the highest occupied molecular orbital (HOMO) and HOMO-2 orbitals at the single-molecule level. After macroscopic propagation, phase-matching effects further amplify the intensity gap between the HOMO and HOMO-2 contributions as the wavelength increases, thereby reducing the influence of the HOMO-2 orbital on the spectral minimum. We also find that at 1800 nm, the spectral minimum remains stable when we vary both the gas-target position and the degree of molecular alignment. This study provides an effective and robust approach for controlling the spectral structure of molecular high harmonics and offers valuable insights into multielectron effects in molecules.

We simulate the attosecond transient absorption spectroscopy (ATAS) of monolayer hexagonal boron nitride (hBN) using the time-dependent density functional theory and the two-band density-matrix equations within the tight-binding approximation. The simulation results from the two methods are qualitatively consistent. We focus on the fishbone structure around the $\text{M}$ point, which exhibits a temporal period equal to that of the pump laser. To gain deeper insight into this structure, we simplify the two-band model to a single-electron model located at the $\mathrm{M}$ point, allowing us to derive an analytical expression that can qualitatively reproduce the numerical results. Our analytical results reveal that both the interband transition dipole moments (TDMs) and the Berry connection play important roles in the fishbone structure of the ATAS. Furthermore, by isolating the influence of the Berry connection in the ATAS, we obtain two spectra that are respectively dominated by the interband TDMs and the Berry connection. Moreover, we have also investigated the dependence of ATAS on the gap energy based on the tight-binding approximation. The results demonstrate that the ATAS intensity is enhanced as the gap energy increases, in agreement with our analytical prediction. Our study may shed light on the generation mechanism of the fishbone structure of the ATAS in hBN.

Quantum effects, such as entanglement, Einstein-Podolsky-Rosen steering, and Bell correlations, can enhance metrological sensitivity beyond the standard quantum limit. These correlations are typically generated through interactions between atoms or molecules, or during the passage of a laser pulse through a birefringent crystal. Here, we consider an alternative method of generating scalable, many-body entangled states, and demonstrate their usability for quantum-enhanced metrology. Our setup is a one-dimensional array of double-well potentials holding independent and uncorrelated Bose-Einstein condensates. The beam-splitting transformation mixes the signal between adjacent wells and yields a strongly entangled state through a many-body equivalent of the Hong-Ou-Mandel effect. We demonstrate that this entanglement can improve the sensitivity of quantum sensors. In our analysis, we account for the effects of atomic fluctuations and identify the optimal measurement that saturates the quantum Cramer-Rao bound.

Wave vortices constitute a large family of wave entities, closely related to phase singularities and orbital angular momentum (OAM). So far, two main classes of localized wave vortices have been explored: (i) transversely localized monochromatic vortex beams that carry well-defined longitudinal OAM and propagate and diffract along the longitudinal $z$ axis in space, and (ii) two-dimensional (2D)-localized spatiotemporal vortex pulses that carry the more elusive transverse (or tilted) OAM and propagate and diffract along both the $z$ axis and time. Here, we introduce another class of wave vortices which are localized in a 2D $(x,y)$ plane, do not propagate in space (apart from uniform radial deformations), and instead propagate and diffract solely along time. These vortices possess well-defined transverse OAM and can naturally appear in 2D wave systems, such as surface polaritons or water waves. We provide a general integral expression for time-diffracting 2D wave vortices, their underlying ray model, and examples of approximate and exact wave solutions. We also analyze the temporal Gouy phase closely related to the rotational evolution in such vortices. Finally, we show that time-diffracting 2D vortices can provide strong spatiotemporal concentration of energy and OAM at subwavelength and oscillation-period scales.

The photon blockade phenomenon, a promising tool for realizing efficient single-photon sources, is the central focus of our work. We study this phenomenon within the context of the multimode extension of the Jaynes-Cummings model, incorporating two-photon dissipation and external coherent driving. Operating in the weak-driving regime, we confine our analysis to the two-excitation sector of the Hilbert space, initially exploring the single-mode case and then focusing on the corresponding multimode problem. Our study calculates the second-order correlation function (both numerically and analytically) for zero and nonzero time delays in single- and multimode cases, to pinpoint and validate the conditions that lead to conventional and unconventional photon blockade. Our zero-delay findings reveal that photon antibunching is comparable in both cases; however, the multimode case offers a greater degree of control and applicability. Furthermore, for the non-zero-delay operation, we find that when one of the multiple modes is set at the optimal conventional photon blockade conditions, the behavior of the curve mimics the single-mode problem with an overall slower rate of reaching the $g^{\left(2\right)}\left(\tau \right)=1$ value. These results highlight the practical implications of our findings for building useful single-photon sources.

We propose an experimentally feasible optomechanical system that is dispersively driven and operates in the reversed-dissipation regime (RDR), where the mechanical damping rate far exceeds the cavity decay rate. We demonstrate that coherent, fast-time modulation of the driving laser frequency—on timescales longer than the mechanical decoherence time—allows for adiabatic elimination of the mechanical mode, resulting in strong parametric amplification of quantum vacuum fluctuations of the intracavity field. This mechanism, known as the parametric dynamical Casimir effect (DCE), leads to the generation of Casimir photons. In the dispersive RDR, we find that the total system Hamiltonian—including the DCE term—is intrinsically modified by a generalized optomechanical Kerr-type nonlinearity. This nonlinearity not only saturates the mean number of radiated Casimir photons on short timescales, even without dissipation, but also induces oscillatory behavior in their dynamics and quantum characteristics. Remarkably, the presence of the Kerr nonlinearity causes the generated DCE photons to exhibit nonclassical features, including simultaneous sub-Poissonian statistics and negative Wigner function, as well as quadrature squeezing which can be controlled by adjusting the system parameters. Surprisingly, the controllable simultaneous nonclassical dynamics in the same physical parameter regime which is induced by the optomechanical Kerr nonlinearity to the parametric DCE cannot occur in the standard DCE or Kerr-type systems. The proposed nonclassical microwave radiation source possesses the potential to be applied in quantum information processing and quantum computing, as well as in microwave quantum sensing.

We show that any pseudoentangled state ensemble with a gap of $t$ bits of entropy requires $\mathrm{\Omega }\left(t\right)$ non-Clifford gates to prepare. This bound is tight up to constant factors if linear-time quantum-secure pseudorandom functions exist. Our result follows from a polynomial-time algorithm to estimate the entanglement entropy of a quantum state across any cut of qubits. When run on an $n$-qubit state that is stabilized by at least $2^{n-t}$ Pauli operators, our algorithm produces an estimate that is within an additive factor of $\frac{t}{2}$ bits of the true entanglement entropy.

In this work, we introduce a general form of a two-parameter family of local interactions between quantum walkers conditioned on the internal state of their coins. By choosing their particular case, we systematically study the impact of these interactions on the dynamics of two initially localized and noncorrelated walkers. Our general interaction framework, which reduces to several previously studied models as special cases, provides a versatile platform for engineering quantum correlations with applications in quantum simulation, state preparation, and sensing protocols. It also opens up the possibility of analyzing many-body interactions for larger numbers of walkers.

We explore using an ergodic Floquet quantum system on a torus to generate pseudorandom unitary operators. We choose a regime of the perturbed Harper model with strong perturbations and perturbation frequency exceeding the libration frequency to ensure that the system has an ergodic region that covers phase space and lacks resonant substructure. We generate a sample of unitary operators in a finite-dimensional space by computing Floquet propagators from a distribution of its control parameters. To compare the distribution of unitaries to that of a Haar-random distribution, we compute k-frame potentials from samples of numerically generated unitaries. We find that uniform distributions of four control parameters can generate an approximate 3-design. Distributions of fewer control parameters are required to create an approximate 3-design if the Floquet system parameters drift.

The remote entanglement of two atomic quantum memories through photonic interactions is accompanied by atomic momentum recoil. When the interactions occur at different times, such as from the random emission over the lifetime of the atomic excited state, the difference in recoil timing can expose “which-path” information and ultimately lead to decoherence. Time-bin encoded photonic qubits can be particularly sensitive to asynchronous recoil timing. In this paper we study the limits of entanglement fidelity in atomic systems due to recoil and other timing imbalances and show how these effects can be suppressed or even eliminated through proper experimental design.

Multiply charged molecules fragment under Coulomb repulsion, leaving characteristic fingerprints in ensuing electronic decay. Using K-shell ionization of HCl as a benchmark, we identify metastable intermediates that persist to internuclear separations beyond 100 a.u., where weakened Coulomb interactions enable the emergence of narrow atomic lines, while short-lived states decay at smaller separations, producing broad, red-shifted backgrounds. This work provides a previously missing dynamical description of coupled electronic and nuclear evolution in the Coulomb fragmentation regime.

Aligning light spots into arbitrary shapes is a fundamental challenge in holography, leading to various applications across diverse fields in science and engineering. However, as the spot interval approaches the wavelength of light, interference effects among the spots become prominent, which complicates the generation of a distortion-free alignment. Herein, we introduce a hologram design method based on the optimization of a nonlinear cost function using a holographic phase pattern as the optimization parameter. We confirmed generation of a $5\times 5$ multispot pattern, in which each spot is separated clearly by a distance of $0.952\left(1\right)\mu \mathrm{m}$, on the focal plane of a high-numerical-aperture (0.75) objective by observing the near-infrared (wavelength: 820 nm) holographic output light from a spatial light modulator device. This result breaks through the spot spacing of a few micrometers that is a typical value achieved so far under similar experimental conditions. Furthermore, we propose redefinition of the Rayleigh diffraction limit by taking account of the separation of spot as well as the spot spacing. The proposed method is expected to advance laser fabrication, scanning laser microscopy, and cold atom physics, among other fields.

We theoretically investigate the stable mechanism of a ring soliton in two-dimensional Fermi superfluids by solving the Bogoliubov–de Gennes equations and their time-dependent counterparts. In the uniform situation, we discover that the ring soliton is always driven away from its initial location and moves towards the boundary due to a curvature-induced effective potential. The ring soliton is impossible to remain static at any location in the uniform system. To balance the density difference between the ring soliton's two sides, a harmonic trap is introduced, which can exert an effect to counterbalance the curvature-induced effective potential. This enables the ring dark soliton to become a stable state at a particular equilibrium position $r_s$, where the free energy of the ring dark soliton just reaches the maximum value. Once the ring soliton is slightly deviated from $r_s$, some stable periodic oscillations of the ring soliton around $r_s$ will occur. Some dissipation will occur to the ring soliton once its minimum radius is comparable to the healing length of the soliton's Friedel oscillation. This dissipation will increase the oscillation amplitude and, finally, make the ring soliton decay into sound ripples. Our research lays the groundwork for a more in-depth understanding of the stable mechanism of a ring dark soliton in the future.

The Hong-Ou-Mandel (HOM) effect is a quintessential process in various quantum information technologies and quantum optics applications. In this work we investigate multiphoton interference, developing a model for the simultaneous characterization of polarization and spectrotemporal mode mismatch on the coincidence probabilities under the effects of realistic imperfections of devices used in HOM experiments and quantum-channel-induced mode transformations on the input states. We also study the coincidence probability for coherent states as a function of source intensity, as well as spectrotemporal and polarization mismatch of the incident beams. We apply our model to the case of multiphoton interference from independent sources and analyze the consequences of mode mismatch in various instances that occur in quantum networking including entanglement swapping and measurement-device-independent quantum key distribution (MDI-QKD).

The dissipative quantum Rabi model exhibits rich nonequilibrium physics, including a dissipative phase transition from the normal phase to the superradiant phase. In this work we investigate the stability of the superradiant phase in the presence of a weak spin relaxation. We find that even a weak spin relaxation can render the superradiant phase a superradiant metastable phase, in which symmetry-breaking states are stable only for a finite time. This arises because each spin jump induced by relaxation applies as a strong perturbation to the system, potentially driving the system from a symmetry-breaking state to the symmetry-preserving saddle point with finite probability, before it eventually relaxes back to a symmetry-breaking state. Such dynamical processes lead to a finite lifetime of the symmetry-breaking states and restore the symmetry in the steady state. To substantiate these results, we combine mean-field and cumulant expansion analyses with exact numerical simulations. The lifetimes of the symmetry-breaking states are analyzed in finite-size systems and the conclusions are extrapolated to the thermodynamic limit via finite-size scaling. Our findings establish the metastable nature of the symmetry-breaking states in the dissipative quantum Rabi model and reveal the complexity of the dissipative phase transition beyond their equilibrium counterpart. The mechanisms uncovered here can be generalized to a broad class of open quantum systems, highlighting fundamental distinctions between equilibrium phase transitions and steady-state phase transitions.

Apart from quantum squeezing and entanglement, criticality is considered as a valuable resource for quantum sensing. They have a close relationship since squeezing and entanglement can be generated when a quantum system approaches the critical point of phase transition. To understand and improve critical metrology, it is necessary to study the strategy with squeezing prepared in the initial state. Here, we study quantum-sensing strategies in the quantum Rabi model with the system initially prepared in a squeezed thermal state, and the coupling of the system to a squeezed thermal reservoir is also taken into account. We characterize the metrological capability with the quantum Fisher information (QFI). It is found that the QFI can be grouped into two terms relating to variations in amplitudes and phase of system second-order operator moments, respectively. The classifications of the QFI provide privileges for delineating weights of variations coming from different physical origins, and thereby have implications on optimal measurement strategies design. Both the noiseless case and that with dissipation are studied. The role of initial state and reservoir quantum properties, criticality, and their interplay in quantum metrology are investigated in detail. We also identify time scaling of the QFI and propose feasible measurement schemes achieving the sensitivity bound of parameter estimation set by the QFI. Our analysis can be generalized to a diversity of quantum-sensing tasks.