Orbit-based methods are widespread in strong-field laser-matter interaction. They provide a framework in which photoelectron momentum distributions can be interpreted as the quantum interference between different semiclassical pathways the electron can take on its way to the detector, which brings with it great predictive power. The transition amplitude of an electron going from a bound state to a final continuum state is often written as multiple integrals, which can be computed either numerically or by employing the saddle-point method. If one computes the momentum distribution via a saddle-point method, then the obtained distribution is highly dependent on the time window from which the saddle points are selected for inclusion in the “sum over paths.” In many cases, this leads to the distributions not even satisfying the basic symmetry requirements and often containing many more oscillations and interference fringes than their numerically integrated counterparts. Using the strong-field approximation, we find that the manual enforcement of the energy-conservation condition on the momentum distribution calculated via the saddle-point method provides a unique momentum distribution which satisfies the symmetry requirements of the system and which is in a good agreement with the numerical results. We illustrate our findings using the example of the Ar atom ionized by a selection of monochromatic and bichromatic linearly polarized fields.

We present a theoretical study of the collective excitations of the supersolid annular stripe phase of a spin-orbital-angular-momentum-coupled (SOAM-coupled) spin-1 Bose-Einstein condensate. The annular stripe phase simultaneously breaks two continuous symmetries, namely rotational and $U\left(1\right)$ gauge symmetry, and is more probable in the condensates with a larger orbital angular momentum transfer imparted by a pair of Laguerre-Gaussian beams than what has been considered in the recent experiments. Accordingly, we consider a SOAM-coupled spin-1 condensate with a $4\hslash$ orbital angular momentum transferred by the lasers. Depending on the values of the Raman coupling strength and quadratic Zeeman term, the condensate with realistic antiferromagnetic interactions supports three ground-state phases: the annular stripe, the vortex necklace, and the zero angular momentum phase. We numerically calculate the collective excitations of the condensate as a function of coupling and quadratic Zeeman field strengths for a fixed ratio of spin-dependent and spin-independent interaction strengths. At low Raman coupling strengths, we observe a direct transition from the zero angular momentum to the annular stripe phase, characterized by the softening of a double symmetric roton mode, which serves as a precursor to supersolidity.

Bell nonlocality stems from quantum correlations effectively identified using inequalities. Spin chains, simulated with ultracold atoms in optical lattices, Rydberg atoms in tweezer arrays, trapped ions, or molecules allow single-spin control and measurement. Therefore, they are suitable for studying fundamental aspects of these correlations and nonlocality. Occupation defects, such as vacancies or multiple atoms occupying a single site due to imperfect system preparation, limit the detection of Bell correlations. We study their effects with the help of a simplified toy model parametrized by the probability $p$ of having a single occupation for a given site. Within this model, and for entangled systems obtained by one-axis twisting evolution from an initial factorized state, we derive two Bell inequalities, one based on many-site correlations and the other on two-site correlations, and identify the smallest probability $p$ that allows the Bell inequalities violation to be detected. We then consider two physical realizations using entangled ultracold atoms in optical lattices where the parameter $p$ is related to a nonunitary filling factor and nonzero temperature. We test the predictions of the toy model against exact numerical results.

We generalize the textbook Kronig-Penney model to realistic conditions for a quantum-particle moving in the quasi-one-dimensional (quasi-1D) waveguide, where motion in the transverse direction is confined by a harmonic trapping potential. Along the waveguide, the particle scatters on an infinite array of regularized delta potentials. Our starting point is the Lippmann-Schwinger equation, which for quasi-1D geometry can be solved exactly, based on the analytical formula for the quasi-1D Green's function. We study the properties of eigenenergies as a function of particle quasimomentum, which form band structure, as in standard Kronig-Penney model. We test our model by comparing it to the numerical calculations for an atom scattering on an infinite chain of ions in quasi-1D geometry. The agreement is fairly good and can be further improved by introducing energy-dependent scattering length in the regularized delta potential. The energy spectrum exhibits the presence of multiple overlapping bands resulting from excitations in the transverse direction. At large lattice constants, our model reduces to standard Kronig-Penney result with 1D coupling constant for quasi-1D scattering, exhibiting confinement-induced resonances. In the opposite limit, when lattice constant becomes comparable to harmonic oscillator length of the transverse potential, we calculate the correction to the quasi-1D coupling constant due to the quantum interference between scatterers. Finally, we calculate the effective mass for the lowest band and show that it becomes negative for large and positive scattering lengths.

We numerically study the filamentation of femtosecond vector beams with spatially varying polarization profiles in air. The vector beams include azimuthal, radial, and spiral cylindrical vector beams (CVBs), as well as low-order full Poincaré beams (FPBs) with star and lemon polarization topologies. Comparing the filamentation of CVBs to that of a circularly polarized Laguerre-Gaussian (LG) beam, we found that CVBs undergo filamentation more easily due to their more effective nonlinear focusing, in contrast with LG beams. In the case of low-order FPBs, we examined the role of the constituent orthogonally polarized spatial modes in their filamentation process. Our findings reveal that the Gaussian mode within these beams primarily contributes to their filamentation. Additionally, when compared with a linearly polarized Gaussian beam, star and lemon FPBs displayed greater resistance to filamentation. In all comparisons, the vector beams and their counterparts are initialized with equal physical powers to isolate the role of polarization and beam structure on filamentation dynamics. Furthermore, we investigated the evolution of polarization profiles of the beams during filamentation. We observed that the polarization profiles of CVBs remained largely unchanged. In contrast, the polarization profiles of the FPBs underwent significant nonuniform changes due to differences in accumulated nonlinear phase and Gouy phase among the constituent modes.

Hot atoms have been demonstrated to be a promising platform for the manipulation of optical fields. However, the thermal motion of atoms results in the broadening of spectral lines, which is accompanied by a reduction in the light-atom interaction. In this study we propose a nonreciprocal optical system that significantly enhances the strength of the light-atom interaction by unidirectionally compensating for the broadening associated with two-photon processes, employing inhomogeneous light shifts in a $\mathrm{\Lambda }$ system. Utilizing the density matrix formalism, we provide a detailed description of the dynamics and examine the nonreciprocity in two distinct detuned regions. In the far-detuned region, a room-temperature nonreciprocity with an isolation of $37.3\mathrm{dB}$ and a transmission of 0.99 can be achieved. Furthermore, in the near-resonance region, the ultrabroad bandwidth of the nonreciprocal window can reach gigahertz. Our findings pave the way for the realization of high-performance optical nonreciprocity and present opportunities for applications in integrated optics and quantum networks.

Quantum emitters trapped near photonic crystal waveguides have recently emerged as an exciting platform for realizing novel quantum matter-light interfaces. Here, we study tunable photon scattering in a photonic crystal waveguide coupled to an atom dimer with an arbitrary spatial separation. In the weak-excitation regime, we give the energy levels and their decay rates into the waveguide modes in the dressed basis, which both depend on the distance between the two atoms. We focus on the Bragg case and anti-Bragg case, where subradiant and superradiant states are produced and perfect transmission with a $\pi$ phase shift may occur on resonance. We observe quantum beats in the photon-photon correlation function of the reflected field in the anti-Bragg case. Moreover, the frequencies of quantum beats can be controlled due to the tunability of the bound states via the dispersion engineering of the structure. We also observe directional photon emission in the anti-Bragg case and give the dynamic mechanism of the perfect transmission. We quantify the effects of the system imperfections, including the deviation in the distance between the two atoms and the asymmetry in the atomic decay rates into the waveguide modes. With recent experimental advances in the superconducting microwave transmission lines, our results should soon be realizable.

In the ultrastrong-coupling regime, the quantum Rabi model can exhibit quantum phase transition (QPT) when the ratio of the qubit transition frequency to the frequency of the cavity field approaches infinity. However, it is challenging to control the QPT in few-body systems because of the limited coupling strength and the $A^2$ term. Here, we propose a practical scheme to manipulate the QPT of quantum Rabi model in the strong-coupling regime. By applying a periodic frequency modulation to the two-level system in a quantum Rabi model in the strong-coupling regime, an anisotropic quantum Rabi model with ultrastrong and tunable coupling strengths for rotating and counterrotating terms is obtained. The ground state and excitation energy of this model, in terms of the modulation parameters, are studied. We find that the QPT of the quantum Rabi model can be observed in the strong-coupling regime and externally controlled by the modulation.

As a most important feature of non-Hermitian systems, exceptional points (EPs) lead to a variety of unconventional phenomena and applications. Here, we study a generic model composed of two coupled non-Hermitian qubits; the EPs can be easily obtained in this system by adjusting the driving amplitude applied to the qubits. The diverse entanglement dynamics on the two sides of the original fourth-order EP ($\mathrm{EP}{4}^{\prime }$) and second-order EP (EP2) can be observed simultaneously in the weak coupling regime. With the increase of the coupling strength, the $\mathrm{EP}{4}^{\prime }$ is replaced by an additional EP2, leading to the disappearance of the entanglement dynamics change induced by $\mathrm{EP}{4}^{\prime }$ in the strong coupling regime. Considering the case of Ising-type interaction, we also realize EP-induced entanglement dynamics changing without the driving field. Our study paves the way for the investigation of EP-induced quantum effects and applications of EP-related quantum technologies.

We study the topological properties of the one-dimensional generalized quasiperiodic modulated Su-Schrieffer-Heeger model. The results reveal that the topological re-entrant phase transition emerges. Through the analysis of a real-space winding number, we divide the emergent topological re-entrant phase transitions into two types. The first is the re-entrant phase transition from the traditional topological insulator phase into the topological Anderson insulator phase, and the second is the re-entrant phenomenon from one topological Anderson insulator phase into another topological Anderson insulator phase. These two types of re-entrant phase transitions correspond to bounded and unbounded cases of quasiperiodic modulation, respectively. Furthermore, we verify the above topological re-entrant phase transitions by analyzing the Lyapunov exponent and bulk gap. Since Su-Schrieffer-Heeger models have been realized in various artificial systems (such as cold atoms, optical waveguide arrays, ion traps, Rydberg atom arrays, etc.), the two types of topological re-entrant phase transitions predicted in this paper are expected to be realized in the near future.

Nontrivial spectral properties of non-Hermitian systems can give rise to intriguing effects that lack counterparts in Hermitian systems. For instance, when dynamically varying system parameters along a path enclosing an exceptional point (EP), chiral mode conversion occurs. A recent study [Phys. Rev. Lett. 133, 113802 (2024)] demonstrates the achievability of pure adiabatic state transfer by specifically selecting a trajectory in the system parameter space where the corresponding evolution operator exhibits a real spectrum while winding around an EP. However, the intended adiabatic state transfer becomes fragile when taking into account the effect of the nonadiabatic transition. In this work, we propose a scheme for achieving robust and rapid adiabatic state transfer in time-modulated two-level non-Hermitian systems by appropriately modulating the system Hamiltonian and time-evolution trajectory. Numerical simulations confirm that a complete adiabatic transfer can always be achieved even under nonadiabatic conditions after one period for different initialized adiabatic states, and the scheme remains insensitive to moderate fluctuations in control parameters. Therefore, this scheme offers alternative approaches for quantum-state engineering in non-Hermitian systems.

Adiabatic transformation can be approximated as alternating unitary operators of a Hamiltonian and its parameter derivative as proposed in a gate-based approach to counterdiabatic driving [D. van Vreumingen, Phys. Rev. A 110, 052419 (2024)]. In this paper, we conduct numerical benchmarking of this alternating unitary method in a finite parameter range against adiabatic driving in a nonadiabatic timescale. We find that the alternating unitary method results in a broader distribution on energy eigenstates than that obtained by adiabatic driving, but it has the ability to sample low-energy eigenstates when an energy gap of a given Hamiltonian is small. It indicates that the alternating unitary method may be able to find good approximate solutions in quantum annealing applied to hard instances.

Quantum extreme learning machines (QELMs) have emerged as a potent tool for various quantum information processing tasks. We present a QELM protocol for estimating the amount of entanglement in Werner states. The protocol requires the generation of a sequence of random Werner states, which are then combined with a reservoir state and evolved using an Ising Hamiltonian. A set of observables based on the Bloch basis is constructed and employed to train the system to recognize unseen features. To assess the protocol's robustness, noise is introduced into the input states, and the system's performance under these noisy conditions is analyzed. Additionally, the influence of the magnetic-field parameter within the Ising Hamiltonian on the estimation accuracy is investigated.

Continuous variables (CV) offer a promising platform for the development of various applications, such as quantum communication, computing, and sensing, and CV graph states represent a family of powerful entangled resource states for all these areas. In many of these protocols, a crucial aspect is the certification of the quantum state subsequently used. While numerous protocols exist, most rely on assumptions unrealistic for physical continuous-variable states, such as infinite precision in quadrature measurement or the use of states requiring infinite squeezing. In this work, we adapt existing protocols to deal with these unavoidable considerations and use them to certify their application for different quantum information tasks. More specifically, we show how CV graph states can be efficiently verified and certified even in a noisy and imperfect setting. We then discuss how our findings impact the usability of states obtained after the protocol for different applications, including quantum teleportation, computing, and sensing.

The ground-state degeneracy of topologically ordered gapped Hamiltonians is the bedrock for self-correcting quantum memories, which are unfortunately not stable away from equilibrium even at zero temperature. This plague precludes practical robust self-correction since stability at zero temperature is a prerequisite for finite-temperature robustness. In this work, we show that the emergence of a bounded light cone renders the unitary time evolution a quasiadiabatic continuation that preserves topological order, with the initial ground space retaining its macroscopic distance at all times as a quantum code. We also show how bounded light cones can emerge through suitable perturbations in Kitaev's toric code and honeycomb model. Our results suggest that topological orders and self-correcting quantum memories can be dynamically robust at zero temperature.

We construct a general family of quantum codes that protect against all emission, absorption, dephasing, and raising or lowering errors up to an arbitrary fixed order. Such codes are known in the literature as absorption-emission (AE) codes. We derive simplified error correction conditions for a general AE code and show that any permutation-invariant code that corrects $t$ or fewer errors can be mapped to an AE code that corrects up to order-$t$ transitions. Carefully tuning the parameters of permutationally invariant codes, we construct several examples of efficient AE codes, hosted in systems with low total angular momentum. Our results also imply that spin codes can be mapped to AE codes, enabling us to characterize logical operators for certain subclasses of such codes.

Quantum nonlocality, rooted in the quantum state discrimination task, contributes to the development of the theory of local distinguishability of quantum states. Recently, Zhu et al. [Phys. A (Amsterdam, Neth.) 624, 128956 (2023)] found that a nonlocal set of orthogonal product states (OPSs) can be perfectly identified by local operations and classical communication after removing a specific quantum state in a bipartite quantum system. They called this phenomenon minimal nonlocality. It is obvious that minimal nonlocality effectively reflects the lower boundary of the elements contained in a nonlocal set of OPSs. In this paper, we generalize the methods of constructing nonlocal sets with minimal nonlocality from bipartite systems to multipartite systems and from orthogonal product states to entangled states. First, we give a method to construct a set of OPSs with minimal nonlocality in the ${\mathbb{C}}^d\otimes {\mathbb{C}}^d\otimes {\mathbb{C}}^d$ quantum system, and then we extend it to the ${\otimes }_{i=1}^n{\mathbb{C}}^d$ quantum system for $d\ge 4$ and $n\ge 3$. In addition, we give a method to construct a nonlocal set of orthogonal states with entanglement relations in the ${\mathbb{C}}^d\otimes {\mathbb{C}}^d$ quantum system, and we prove that this set has minimal nonlocality, which can also be extended to the ${\otimes }_{i=1}^n{\mathbb{C}}^d$ quantum system.

The two-photon Hong-Ou-Mandel (HOM) interferometry sensor has attracted much attention in recent decades due to its numerous advantages, such as robustness against background noise, group velocity dispersion, and phase perturbations. However, the maximum measurement precision of a single measurement is limited by the Heisenberg uncertainty at the two-photon level, which constrains the development of a HOM-based sensor. In this paper, we combine the multipass approach of a sequential scheme for quantum metrology with the traditional HOM-based sensor, and provide a general theoretical scheme of a multipass HOM-based sensor. For $N$ passes of the probe through the sample, our proposed scheme offers an $N$-fold improvement in the precision of parameter estimation compared to a traditional HOM-based sensor under the same number of measurement repetitions. Furthermore, we present the simulation results of decoherence models for the multipass HOM-based sensor by introducing phase diffusion and photon loss, which are significantly enhanced due to the multipass approach. Last, we present the predictions of achievable precision for the multipass HOM-based sensor, considering measurement repetitions of the order of ${10}^4$ in a sample with a thickness of $100\mu \mathrm{m}$.